Event Histograms Documentation written by Tom Zanussi 1. Introduction =============== Histogram triggers are special event triggers that can be used to aggregate trace event data into histograms. For information on trace events and event triggers, see Documentation/trace/events.txt. 2. Histogram Trigger Command ============================ A histogram trigger command is an event trigger command that aggregates event hits into a hash table keyed on one or more trace event format fields (or stacktrace) and a set of running totals derived from one or more trace event format fields and/or event counts (hitcount). The format of a hist trigger is as follows: hist:keys=[:values=] [:sort=][:size=#entries][:pause][:continue] [:clear][:name=histname1] [if ] When a matching event is hit, an entry is added to a hash table using the key(s) and value(s) named. Keys and values correspond to fields in the event's format description. Values must correspond to numeric fields - on an event hit, the value(s) will be added to a sum kept for that field. The special string 'hitcount' can be used in place of an explicit value field - this is simply a count of event hits. If 'values' isn't specified, an implicit 'hitcount' value will be automatically created and used as the only value. Keys can be any field, or the special string 'stacktrace', which will use the event's kernel stacktrace as the key. The keywords 'keys' or 'key' can be used to specify keys, and the keywords 'values', 'vals', or 'val' can be used to specify values. Compound keys consisting of up to two fields can be specified by the 'keys' keyword. Hashing a compound key produces a unique entry in the table for each unique combination of component keys, and can be useful for providing more fine-grained summaries of event data. Additionally, sort keys consisting of up to two fields can be specified by the 'sort' keyword. If more than one field is specified, the result will be a 'sort within a sort': the first key is taken to be the primary sort key and the second the secondary key. If a hist trigger is given a name using the 'name' parameter, its histogram data will be shared with other triggers of the same name, and trigger hits will update this common data. Only triggers with 'compatible' fields can be combined in this way; triggers are 'compatible' if the fields named in the trigger share the same number and type of fields and those fields also have the same names. Note that any two events always share the compatible 'hitcount' and 'stacktrace' fields and can therefore be combined using those fields, however pointless that may be. 'hist' triggers add a 'hist' file to each event's subdirectory. Reading the 'hist' file for the event will dump the hash table in its entirety to stdout. If there are multiple hist triggers attached to an event, there will be a table for each trigger in the output. The table displayed for a named trigger will be the same as any other instance having the same name. Each printed hash table entry is a simple list of the keys and values comprising the entry; keys are printed first and are delineated by curly braces, and are followed by the set of value fields for the entry. By default, numeric fields are displayed as base-10 integers. This can be modified by appending any of the following modifiers to the field name: .hex display a number as a hex value .sym display an address as a symbol .sym-offset display an address as a symbol and offset .syscall display a syscall id as a system call name .execname display a common_pid as a program name .log2 display log2 value rather than raw number .usecs display a common_timestamp in microseconds Note that in general the semantics of a given field aren't interpreted when applying a modifier to it, but there are some restrictions to be aware of in this regard: - only the 'hex' modifier can be used for values (because values are essentially sums, and the other modifiers don't make sense in that context). - the 'execname' modifier can only be used on a 'common_pid'. The reason for this is that the execname is simply the 'comm' value saved for the 'current' process when an event was triggered, which is the same as the common_pid value saved by the event tracing code. Trying to apply that comm value to other pid values wouldn't be correct, and typically events that care save pid-specific comm fields in the event itself. A typical usage scenario would be the following to enable a hist trigger, read its current contents, and then turn it off: # echo 'hist:keys=skbaddr.hex:vals=len' > \ /sys/kernel/debug/tracing/events/net/netif_rx/trigger # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist # echo '!hist:keys=skbaddr.hex:vals=len' > \ /sys/kernel/debug/tracing/events/net/netif_rx/trigger The trigger file itself can be read to show the details of the currently attached hist trigger. This information is also displayed at the top of the 'hist' file when read. By default, the size of the hash table is 2048 entries. The 'size' parameter can be used to specify more or fewer than that. The units are in terms of hashtable entries - if a run uses more entries than specified, the results will show the number of 'drops', the number of hits that were ignored. The size should be a power of 2 between 128 and 131072 (any non- power-of-2 number specified will be rounded up). The 'sort' parameter can be used to specify a value field to sort on. The default if unspecified is 'hitcount' and the default sort order is 'ascending'. To sort in the opposite direction, append .descending' to the sort key. The 'pause' parameter can be used to pause an existing hist trigger or to start a hist trigger but not log any events until told to do so. 'continue' or 'cont' can be used to start or restart a paused hist trigger. The 'clear' parameter will clear the contents of a running hist trigger and leave its current paused/active state. Note that the 'pause', 'cont', and 'clear' parameters should be applied using 'append' shell operator ('>>') if applied to an existing trigger, rather than via the '>' operator, which will cause the trigger to be removed through truncation. - enable_hist/disable_hist The enable_hist and disable_hist triggers can be used to have one event conditionally start and stop another event's already-attached hist trigger. Any number of enable_hist and disable_hist triggers can be attached to a given event, allowing that event to kick off and stop aggregations on a host of other events. The format is very similar to the enable/disable_event triggers: enable_hist::[:count] disable_hist::[:count] Instead of enabling or disabling the tracing of the target event into the trace buffer as the enable/disable_event triggers do, the enable/disable_hist triggers enable or disable the aggregation of the target event into a hash table. A typical usage scenario for the enable_hist/disable_hist triggers would be to first set up a paused hist trigger on some event, followed by an enable_hist/disable_hist pair that turns the hist aggregation on and off when conditions of interest are hit: # echo 'hist:keys=skbaddr.hex:vals=len:pause' > \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \ /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \ /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger The above sets up an initially paused hist trigger which is unpaused and starts aggregating events when a given program is executed, and which stops aggregating when the process exits and the hist trigger is paused again. The examples below provide a more concrete illustration of the concepts and typical usage patterns discussed above. 'special' event fields ------------------------ There are a number of 'special event fields' available for use as keys or values in a hist trigger. These look like and behave as if they were actual event fields, but aren't really part of the event's field definition or format file. They are however available for any event, and can be used anywhere an actual event field could be. They are: common_timestamp u64 - timestamp (from ring buffer) associated with the event, in nanoseconds. May be modified by .usecs to have timestamps interpreted as microseconds. cpu int - the cpu on which the event occurred. Extended error information -------------------------- For some error conditions encountered when invoking a hist trigger command, extended error information is available via the corresponding event's 'hist' file. Reading the hist file after an error will display more detailed information about what went wrong, if information is available. This extended error information will be available until the next hist trigger command for that event. If available for a given error condition, the extended error information and usage takes the following form: # echo xxx > /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger echo: write error: Invalid argument # cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/hist ERROR: Couldn't yyy: zzz Last command: xxx 6.2 'hist' trigger examples --------------------------- The first set of examples creates aggregations using the kmalloc event. The fields that can be used for the hist trigger are listed in the kmalloc event's format file: # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format name: kmalloc ID: 374 format: field:unsigned short common_type; offset:0; size:2; signed:0; field:unsigned char common_flags; offset:2; size:1; signed:0; field:unsigned char common_preempt_count; offset:3; size:1; signed:0; field:int common_pid; offset:4; size:4; signed:1; field:unsigned long call_site; offset:8; size:8; signed:0; field:const void * ptr; offset:16; size:8; signed:0; field:size_t bytes_req; offset:24; size:8; signed:0; field:size_t bytes_alloc; offset:32; size:8; signed:0; field:gfp_t gfp_flags; offset:40; size:4; signed:0; We'll start by creating a hist trigger that generates a simple table that lists the total number of bytes requested for each function in the kernel that made one or more calls to kmalloc: # echo 'hist:key=call_site:val=bytes_req' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger This tells the tracing system to create a 'hist' trigger using the call_site field of the kmalloc event as the key for the table, which just means that each unique call_site address will have an entry created for it in the table. The 'val=bytes_req' parameter tells the hist trigger that for each unique entry (call_site) in the table, it should keep a running total of the number of bytes requested by that call_site. We'll let it run for awhile and then dump the contents of the 'hist' file in the kmalloc event's subdirectory (for readability, a number of entries have been omitted): # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist # trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active] { call_site: 18446744072106379007 } hitcount: 1 bytes_req: 176 { call_site: 18446744071579557049 } hitcount: 1 bytes_req: 1024 { call_site: 18446744071580608289 } hitcount: 1 bytes_req: 16384 { call_site: 18446744071581827654 } hitcount: 1 bytes_req: 24 { call_site: 18446744071580700980 } hitcount: 1 bytes_req: 8 { call_site: 18446744071579359876 } hitcount: 1 bytes_req: 152 { call_site: 18446744071580795365 } hitcount: 3 bytes_req: 144 { call_site: 18446744071581303129 } hitcount: 3 bytes_req: 144 { call_site: 18446744071580713234 } hitcount: 4 bytes_req: 2560 { call_site: 18446744071580933750 } hitcount: 4 bytes_req: 736 . . . { call_site: 18446744072106047046 } hitcount: 69 bytes_req: 5576 { call_site: 18446744071582116407 } hitcount: 73 bytes_req: 2336 { call_site: 18446744072106054684 } hitcount: 136 bytes_req: 140504 { call_site: 18446744072106224230 } hitcount: 136 bytes_req: 19584 { call_site: 18446744072106078074 } hitcount: 153 bytes_req: 2448 { call_site: 18446744072106062406 } hitcount: 153 bytes_req: 36720 { call_site: 18446744071582507929 } hitcount: 153 bytes_req: 37088 { call_site: 18446744072102520590 } hitcount: 273 bytes_req: 10920 { call_site: 18446744071582143559 } hitcount: 358 bytes_req: 716 { call_site: 18446744072106465852 } hitcount: 417 bytes_req: 56712 { call_site: 18446744072102523378 } hitcount: 485 bytes_req: 27160 { call_site: 18446744072099568646 } hitcount: 1676 bytes_req: 33520 Totals: Hits: 4610 Entries: 45 Dropped: 0 The output displays a line for each entry, beginning with the key specified in the trigger, followed by the value(s) also specified in the trigger. At the beginning of the output is a line that displays the trigger info, which can also be displayed by reading the 'trigger' file: # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active] At the end of the output are a few lines that display the overall totals for the run. The 'Hits' field shows the total number of times the event trigger was hit, the 'Entries' field shows the total number of used entries in the hash table, and the 'Dropped' field shows the number of hits that were dropped because the number of used entries for the run exceeded the maximum number of entries allowed for the table (normally 0, but if not a hint that you may want to increase the size of the table using the 'size' parameter). Notice in the above output that there's an extra field, 'hitcount', which wasn't specified in the trigger. Also notice that in the trigger info output, there's a parameter, 'sort=hitcount', which wasn't specified in the trigger either. The reason for that is that every trigger implicitly keeps a count of the total number of hits attributed to a given entry, called the 'hitcount'. That hitcount information is explicitly displayed in the output, and in the absence of a user-specified sort parameter, is used as the default sort field. The value 'hitcount' can be used in place of an explicit value in the 'values' parameter if you don't really need to have any particular field summed and are mainly interested in hit frequencies. To turn the hist trigger off, simply call up the trigger in the command history and re-execute it with a '!' prepended: # echo '!hist:key=call_site:val=bytes_req' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger Finally, notice that the call_site as displayed in the output above isn't really very useful. It's an address, but normally addresses are displayed in hex. To have a numeric field displayed as a hex value, simply append '.hex' to the field name in the trigger: # echo 'hist:key=call_site.hex:val=bytes_req' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist # trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active] { call_site: ffffffffa026b291 } hitcount: 1 bytes_req: 433 { call_site: ffffffffa07186ff } hitcount: 1 bytes_req: 176 { call_site: ffffffff811ae721 } hitcount: 1 bytes_req: 16384 { call_site: ffffffff811c5134 } hitcount: 1 bytes_req: 8 { call_site: ffffffffa04a9ebb } hitcount: 1 bytes_req: 511 { call_site: ffffffff8122e0a6 } hitcount: 1 bytes_req: 12 { call_site: ffffffff8107da84 } hitcount: 1 bytes_req: 152 { call_site: ffffffff812d8246 } hitcount: 1 bytes_req: 24 { call_site: ffffffff811dc1e5 } hitcount: 3 bytes_req: 144 { call_site: ffffffffa02515e8 } hitcount: 3 bytes_req: 648 { call_site: ffffffff81258159 } hitcount: 3 bytes_req: 144 { call_site: ffffffff811c80f4 } hitcount: 4 bytes_req: 544 . . . { call_site: ffffffffa06c7646 } hitcount: 106 bytes_req: 8024 { call_site: ffffffffa06cb246 } hitcount: 132 bytes_req: 31680 { call_site: ffffffffa06cef7a } hitcount: 132 bytes_req: 2112 { call_site: ffffffff8137e399 } hitcount: 132 bytes_req: 23232 { call_site: ffffffffa06c941c } hitcount: 185 bytes_req: 171360 { call_site: ffffffffa06f2a66 } hitcount: 185 bytes_req: 26640 { call_site: ffffffffa036a70e } hitcount: 265 bytes_req: 10600 { call_site: ffffffff81325447 } hitcount: 292 bytes_req: 584 { call_site: ffffffffa072da3c } hitcount: 446 bytes_req: 60656 { call_site: ffffffffa036b1f2 } hitcount: 526 bytes_req: 29456 { call_site: ffffffffa0099c06 } hitcount: 1780 bytes_req: 35600 Totals: Hits: 4775 Entries: 46 Dropped: 0 Even that's only marginally more useful - while hex values do look more like addresses, what users are typically more interested in when looking at text addresses are the corresponding symbols instead. To have an address displayed as symbolic value instead, simply append '.sym' or '.sym-offset' to the field name in the trigger: # echo 'hist:key=call_site.sym:val=bytes_req' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active] { call_site: [ffffffff810adcb9] syslog_print_all } hitcount: 1 bytes_req: 1024 { call_site: [ffffffff8154bc62] usb_control_msg } hitcount: 1 bytes_req: 8 { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid] } hitcount: 1 bytes_req: 7 { call_site: [ffffffff8154acbe] usb_alloc_urb } hitcount: 1 bytes_req: 192 { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid] } hitcount: 1 bytes_req: 7 { call_site: [ffffffff811e3a25] __seq_open_private } hitcount: 1 bytes_req: 40 { call_site: [ffffffff8109524a] alloc_fair_sched_group } hitcount: 2 bytes_req: 128 { call_site: [ffffffff811febd5] fsnotify_alloc_group } hitcount: 2 bytes_req: 528 { call_site: [ffffffff81440f58] __tty_buffer_request_room } hitcount: 2 bytes_req: 2624 { call_site: [ffffffff81200ba6] inotify_new_group } hitcount: 2 bytes_req: 96 { call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211] } hitcount: 2 bytes_req: 464 { call_site: [ffffffff81672406] tcp_get_metrics } hitcount: 2 bytes_req: 304 { call_site: [ffffffff81097ec2] alloc_rt_sched_group } hitcount: 2 bytes_req: 128 { call_site: [ffffffff81089b05] sched_create_group } hitcount: 2 bytes_req: 1424 . . . { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915] } hitcount: 1185 bytes_req: 123240 { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm] } hitcount: 1185 bytes_req: 104280 { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915] } hitcount: 1402 bytes_req: 190672 { call_site: [ffffffff812891ca] ext4_find_extent } hitcount: 1518 bytes_req: 146208 { call_site: [ffffffffa029070e] drm_vma_node_allow [drm] } hitcount: 1746 bytes_req: 69840 { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 2021 bytes_req: 792312 { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm] } hitcount: 2592 bytes_req: 145152 { call_site: [ffffffffa0489a66] intel_ring_begin [i915] } hitcount: 2629 bytes_req: 378576 { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915] } hitcount: 2629 bytes_req: 3783248 { call_site: [ffffffff81325607] apparmor_file_alloc_security } hitcount: 5192 bytes_req: 10384 { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid] } hitcount: 5529 bytes_req: 110584 { call_site: [ffffffff8131ebf7] aa_alloc_task_context } hitcount: 21943 bytes_req: 702176 { call_site: [ffffffff8125847d] ext4_htree_store_dirent } hitcount: 55759 bytes_req: 5074265 Totals: Hits: 109928 Entries: 71 Dropped: 0 Because the default sort key above is 'hitcount', the above shows a the list of call_sites by increasing hitcount, so that at the bottom we see the functions that made the most kmalloc calls during the run. If instead we we wanted to see the top kmalloc callers in terms of the number of bytes requested rather than the number of calls, and we wanted the top caller to appear at the top, we can use the 'sort' parameter, along with the 'descending' modifier: # echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active] { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915] } hitcount: 2186 bytes_req: 3397464 { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 1790 bytes_req: 712176 { call_site: [ffffffff8125847d] ext4_htree_store_dirent } hitcount: 8132 bytes_req: 513135 { call_site: [ffffffff811e2a1b] seq_buf_alloc } hitcount: 106 bytes_req: 440128 { call_site: [ffffffffa0489a66] intel_ring_begin [i915] } hitcount: 2186 bytes_req: 314784 { call_site: [ffffffff812891ca] ext4_find_extent } hitcount: 2174 bytes_req: 208992 { call_site: [ffffffff811ae8e1] __kmalloc } hitcount: 8 bytes_req: 131072 { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915] } hitcount: 859 bytes_req: 116824 { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm] } hitcount: 1834 bytes_req: 102704 { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915] } hitcount: 972 bytes_req: 101088 { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm] } hitcount: 972 bytes_req: 85536 { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid] } hitcount: 3333 bytes_req: 66664 { call_site: [ffffffff8137e559] sg_kmalloc } hitcount: 209 bytes_req: 61632 . . . { call_site: [ffffffff81095225] alloc_fair_sched_group } hitcount: 2 bytes_req: 128 { call_site: [ffffffff81097ec2] alloc_rt_sched_group } hitcount: 2 bytes_req: 128 { call_site: [ffffffff812d8406] copy_semundo } hitcount: 2 bytes_req: 48 { call_site: [ffffffff81200ba6] inotify_new_group } hitcount: 1 bytes_req: 48 { call_site: [ffffffffa027121a] drm_getmagic [drm] } hitcount: 1 bytes_req: 48 { call_site: [ffffffff811e3a25] __seq_open_private } hitcount: 1 bytes_req: 40 { call_site: [ffffffff811c52f4] bprm_change_interp } hitcount: 2 bytes_req: 16 { call_site: [ffffffff8154bc62] usb_control_msg } hitcount: 1 bytes_req: 8 { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid] } hitcount: 1 bytes_req: 7 { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid] } hitcount: 1 bytes_req: 7 Totals: Hits: 32133 Entries: 81 Dropped: 0 To display the offset and size information in addition to the symbol name, just use 'sym-offset' instead: # echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist # trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active] { call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915] } hitcount: 4569 bytes_req: 3163720 { call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915] } hitcount: 4569 bytes_req: 657936 { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915] } hitcount: 1519 bytes_req: 472936 { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915] } hitcount: 3050 bytes_req: 211832 { call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50 } hitcount: 34 bytes_req: 148384 { call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915] } hitcount: 1385 bytes_req: 144040 { call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0 } hitcount: 8 bytes_req: 131072 { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm] } hitcount: 1385 bytes_req: 121880 { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm] } hitcount: 1848 bytes_req: 103488 { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915] } hitcount: 461 bytes_req: 62696 { call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm] } hitcount: 1541 bytes_req: 61640 { call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0 } hitcount: 57 bytes_req: 57456 . . . { call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0 } hitcount: 2 bytes_req: 128 { call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm] } hitcount: 3 bytes_req: 96 { call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0 } hitcount: 8 bytes_req: 96 { call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650 } hitcount: 3 bytes_req: 84 { call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110 } hitcount: 1 bytes_req: 8 { call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid] } hitcount: 1 bytes_req: 7 { call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid] } hitcount: 1 bytes_req: 7 Totals: Hits: 26098 Entries: 64 Dropped: 0 We can also add multiple fields to the 'values' parameter. For example, we might want to see the total number of bytes allocated alongside bytes requested, and display the result sorted by bytes allocated in a descending order: # echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist # trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active] { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915] } hitcount: 7403 bytes_req: 4084360 bytes_alloc: 5958016 { call_site: [ffffffff811e2a1b] seq_buf_alloc } hitcount: 541 bytes_req: 2213968 bytes_alloc: 2228224 { call_site: [ffffffffa0489a66] intel_ring_begin [i915] } hitcount: 7404 bytes_req: 1066176 bytes_alloc: 1421568 { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 1565 bytes_req: 557368 bytes_alloc: 1037760 { call_site: [ffffffff8125847d] ext4_htree_store_dirent } hitcount: 9557 bytes_req: 595778 bytes_alloc: 695744 { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 5839 bytes_req: 430680 bytes_alloc: 470400 { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915] } hitcount: 2388 bytes_req: 324768 bytes_alloc: 458496 { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm] } hitcount: 3911 bytes_req: 219016 bytes_alloc: 250304 { call_site: [ffffffff815f8d7b] sk_prot_alloc } hitcount: 235 bytes_req: 236880 bytes_alloc: 240640 { call_site: [ffffffff8137e559] sg_kmalloc } hitcount: 557 bytes_req: 169024 bytes_alloc: 221760 { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid] } hitcount: 9378 bytes_req: 187548 bytes_alloc: 206312 { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915] } hitcount: 1519 bytes_req: 157976 bytes_alloc: 194432 . . . { call_site: [ffffffff8109bd3b] sched_autogroup_create_attach } hitcount: 2 bytes_req: 144 bytes_alloc: 192 { call_site: [ffffffff81097ee8] alloc_rt_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128 { call_site: [ffffffff8109524a] alloc_fair_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128 { call_site: [ffffffff81095225] alloc_fair_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128 { call_site: [ffffffff81097ec2] alloc_rt_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128 { call_site: [ffffffff81213e80] load_elf_binary } hitcount: 3 bytes_req: 84 bytes_alloc: 96 { call_site: [ffffffff81079a2e] kthread_create_on_node } hitcount: 1 bytes_req: 56 bytes_alloc: 64 { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid] } hitcount: 1 bytes_req: 7 bytes_alloc: 8 { call_site: [ffffffff8154bc62] usb_control_msg } hitcount: 1 bytes_req: 8 bytes_alloc: 8 { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid] } hitcount: 1 bytes_req: 7 bytes_alloc: 8 Totals: Hits: 66598 Entries: 65 Dropped: 0 Finally, to finish off our kmalloc example, instead of simply having the hist trigger display symbolic call_sites, we can have the hist trigger additionally display the complete set of kernel stack traces that led to each call_site. To do that, we simply use the special value 'stacktrace' for the key parameter: # echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \ /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger The above trigger will use the kernel stack trace in effect when an event is triggered as the key for the hash table. This allows the enumeration of every kernel callpath that led up to a particular event, along with a running total of any of the event fields for that event. Here we tally bytes requested and bytes allocated for every callpath in the system that led up to a kmalloc (in this case every callpath to a kmalloc for a kernel compile): # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist # trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active] { stacktrace: __kmalloc_track_caller+0x10b/0x1a0 kmemdup+0x20/0x50 hidraw_report_event+0x8a/0x120 [hid] hid_report_raw_event+0x3ea/0x440 [hid] hid_input_report+0x112/0x190 [hid] hid_irq_in+0xc2/0x260 [usbhid] __usb_hcd_giveback_urb+0x72/0x120 usb_giveback_urb_bh+0x9e/0xe0 tasklet_hi_action+0xf8/0x100 __do_softirq+0x114/0x2c0 irq_exit+0xa5/0xb0 do_IRQ+0x5a/0xf0 ret_from_intr+0x0/0x30 cpuidle_enter+0x17/0x20 cpu_startup_entry+0x315/0x3e0 rest_init+0x7c/0x80 } hitcount: 3 bytes_req: 21 bytes_alloc: 24 { stacktrace: __kmalloc_track_caller+0x10b/0x1a0 kmemdup+0x20/0x50 hidraw_report_event+0x8a/0x120 [hid] hid_report_raw_event+0x3ea/0x440 [hid] hid_input_report+0x112/0x190 [hid] hid_irq_in+0xc2/0x260 [usbhid] __usb_hcd_giveback_urb+0x72/0x120 usb_giveback_urb_bh+0x9e/0xe0 tasklet_hi_action+0xf8/0x100 __do_softirq+0x114/0x2c0 irq_exit+0xa5/0xb0 do_IRQ+0x5a/0xf0 ret_from_intr+0x0/0x30 } hitcount: 3 bytes_req: 21 bytes_alloc: 24 { stacktrace: kmem_cache_alloc_trace+0xeb/0x150 aa_alloc_task_context+0x27/0x40 apparmor_cred_prepare+0x1f/0x50 security_prepare_creds+0x16/0x20 prepare_creds+0xdf/0x1a0 SyS_capset+0xb5/0x200 system_call_fastpath+0x12/0x6a } hitcount: 1 bytes_req: 32 bytes_alloc: 32 . . . { stacktrace: __kmalloc+0x11b/0x1b0 i915_gem_execbuffer2+0x6c/0x2c0 [i915] drm_ioctl+0x349/0x670 [drm] do_vfs_ioctl+0x2f0/0x4f0 SyS_ioctl+0x81/0xa0 system_call_fastpath+0x12/0x6a } hitcount: 17726 bytes_req: 13944120 bytes_alloc: 19593808 { stacktrace: __kmalloc+0x11b/0x1b0 load_elf_phdrs+0x76/0xa0 load_elf_binary+0x102/0x1650 search_binary_handler+0x97/0x1d0 do_execveat_common.isra.34+0x551/0x6e0 SyS_execve+0x3a/0x50 return_from_execve+0x0/0x23 } hitcount: 33348 bytes_req: 17152128 bytes_alloc: 20226048 { stacktrace: kmem_cache_alloc_trace+0xeb/0x150 apparmor_file_alloc_security+0x27/0x40 security_file_alloc+0x16/0x20 get_empty_filp+0x93/0x1c0 path_openat+0x31/0x5f0 do_filp_open+0x3a/0x90 do_sys_open+0x128/0x220 SyS_open+0x1e/0x20 system_call_fastpath+0x12/0x6a } hitcount: 4766422 bytes_req: 9532844 bytes_alloc: 38131376 { stacktrace: __kmalloc+0x11b/0x1b0 seq_buf_alloc+0x1b/0x50 seq_read+0x2cc/0x370 proc_reg_read+0x3d/0x80 __vfs_read+0x28/0xe0 vfs_read+0x86/0x140 SyS_read+0x46/0xb0 system_call_fastpath+0x12/0x6a } hitcount: 19133 bytes_req: 78368768 bytes_alloc: 78368768 Totals: Hits: 6085872 Entries: 253 Dropped: 0 If you key a hist trigger on common_pid, in order for example to gather and display sorted totals for each process, you can use the special .execname modifier to display the executable names for the processes in the table rather than raw pids. The example below keeps a per-process sum of total bytes read: # echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \ /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist # trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active] { common_pid: gnome-terminal [ 3196] } hitcount: 280 count: 1093512 { common_pid: Xorg [ 1309] } hitcount: 525 count: 256640 { common_pid: compiz [ 2889] } hitcount: 59 count: 254400 { common_pid: bash [ 8710] } hitcount: 3 count: 66369 { common_pid: dbus-daemon-lau [ 8703] } hitcount: 49 count: 47739 { common_pid: irqbalance [ 1252] } hitcount: 27 count: 27648 { common_pid: 01ifupdown [ 8705] } hitcount: 3 count: 17216 { common_pid: dbus-daemon [ 772] } hitcount: 10 count: 12396 { common_pid: Socket Thread [ 8342] } hitcount: 11 count: 11264 { common_pid: nm-dhcp-client. [ 8701] } hitcount: 6 count: 7424 { common_pid: gmain [ 1315] } hitcount: 18 count: 6336 . . . { common_pid: postgres [ 1892] } hitcount: 2 count: 32 { common_pid: postgres [ 1891] } hitcount: 2 count: 32 { common_pid: gmain [ 8704] } hitcount: 2 count: 32 { common_pid: upstart-dbus-br [ 2740] } hitcount: 21 count: 21 { common_pid: nm-dispatcher.a [ 8696] } hitcount: 1 count: 16 { common_pid: indicator-datet [ 2904] } hitcount: 1 count: 16 { common_pid: gdbus [ 2998] } hitcount: 1 count: 16 { common_pid: rtkit-daemon [ 2052] } hitcount: 1 count: 8 { common_pid: init [ 1] } hitcount: 2 count: 2 Totals: Hits: 2116 Entries: 51 Dropped: 0 Similarly, if you key a hist trigger on syscall id, for example to gather and display a list of systemwide syscall hits, you can use the special .syscall modifier to display the syscall names rather than raw ids. The example below keeps a running total of syscall counts for the system during the run: # echo 'hist:key=id.syscall:val=hitcount' > \ /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist # trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active] { id: sys_fsync [ 74] } hitcount: 1 { id: sys_newuname [ 63] } hitcount: 1 { id: sys_prctl [157] } hitcount: 1 { id: sys_statfs [137] } hitcount: 1 { id: sys_symlink [ 88] } hitcount: 1 { id: sys_sendmmsg [307] } hitcount: 1 { id: sys_semctl [ 66] } hitcount: 1 { id: sys_readlink [ 89] } hitcount: 3 { id: sys_bind [ 49] } hitcount: 3 { id: sys_getsockname [ 51] } hitcount: 3 { id: sys_unlink [ 87] } hitcount: 3 { id: sys_rename [ 82] } hitcount: 4 { id: unknown_syscall [ 58] } hitcount: 4 { id: sys_connect [ 42] } hitcount: 4 { id: sys_getpid [ 39] } hitcount: 4 . . . { id: sys_rt_sigprocmask [ 14] } hitcount: 952 { id: sys_futex [202] } hitcount: 1534 { id: sys_write [ 1] } hitcount: 2689 { id: sys_setitimer [ 38] } hitcount: 2797 { id: sys_read [ 0] } hitcount: 3202 { id: sys_select [ 23] } hitcount: 3773 { id: sys_writev [ 20] } hitcount: 4531 { id: sys_poll [ 7] } hitcount: 8314 { id: sys_recvmsg [ 47] } hitcount: 13738 { id: sys_ioctl [ 16] } hitcount: 21843 Totals: Hits: 67612 Entries: 72 Dropped: 0 The syscall counts above provide a rough overall picture of system call activity on the system; we can see for example that the most popular system call on this system was the 'sys_ioctl' system call. We can use 'compound' keys to refine that number and provide some further insight as to which processes exactly contribute to the overall ioctl count. The command below keeps a hitcount for every unique combination of system call id and pid - the end result is essentially a table that keeps a per-pid sum of system call hits. The results are sorted using the system call id as the primary key, and the hitcount sum as the secondary key: # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \ /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active] { id: sys_read [ 0], common_pid: rtkit-daemon [ 1877] } hitcount: 1 { id: sys_read [ 0], common_pid: gdbus [ 2976] } hitcount: 1 { id: sys_read [ 0], common_pid: console-kit-dae [ 3400] } hitcount: 1 { id: sys_read [ 0], common_pid: postgres [ 1865] } hitcount: 1 { id: sys_read [ 0], common_pid: deja-dup-monito [ 3543] } hitcount: 2 { id: sys_read [ 0], common_pid: NetworkManager [ 890] } hitcount: 2 { id: sys_read [ 0], common_pid: evolution-calen [ 3048] } hitcount: 2 { id: sys_read [ 0], common_pid: postgres [ 1864] } hitcount: 2 { id: sys_read [ 0], common_pid: nm-applet [ 3022] } hitcount: 2 { id: sys_read [ 0], common_pid: whoopsie [ 1212] } hitcount: 2 . . . { id: sys_ioctl [ 16], common_pid: bash [ 8479] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: bash [ 3472] } hitcount: 12 { id: sys_ioctl [ 16], common_pid: gnome-terminal [ 3199] } hitcount: 16 { id: sys_ioctl [ 16], common_pid: Xorg [ 1267] } hitcount: 1808 { id: sys_ioctl [ 16], common_pid: compiz [ 2994] } hitcount: 5580 . . . { id: sys_waitid [247], common_pid: upstart-dbus-br [ 2690] } hitcount: 3 { id: sys_waitid [247], common_pid: upstart-dbus-br [ 2688] } hitcount: 16 { id: sys_inotify_add_watch [254], common_pid: gmain [ 975] } hitcount: 2 { id: sys_inotify_add_watch [254], common_pid: gmain [ 3204] } hitcount: 4 { id: sys_inotify_add_watch [254], common_pid: gmain [ 2888] } hitcount: 4 { id: sys_inotify_add_watch [254], common_pid: gmain [ 3003] } hitcount: 4 { id: sys_inotify_add_watch [254], common_pid: gmain [ 2873] } hitcount: 4 { id: sys_inotify_add_watch [254], common_pid: gmain [ 3196] } hitcount: 6 { id: sys_openat [257], common_pid: java [ 2623] } hitcount: 2 { id: sys_eventfd2 [290], common_pid: ibus-ui-gtk3 [ 2760] } hitcount: 4 { id: sys_eventfd2 [290], common_pid: compiz [ 2994] } hitcount: 6 Totals: Hits: 31536 Entries: 323 Dropped: 0 The above list does give us a breakdown of the ioctl syscall by pid, but it also gives us quite a bit more than that, which we don't really care about at the moment. Since we know the syscall id for sys_ioctl (16, displayed next to the sys_ioctl name), we can use that to filter out all the other syscalls: # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \ /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active] { id: sys_ioctl [ 16], common_pid: gmain [ 2769] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: evolution-addre [ 8571] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: gmain [ 3003] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: gmain [ 2781] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: gmain [ 2829] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: bash [ 8726] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: bash [ 8508] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: gmain [ 2970] } hitcount: 1 { id: sys_ioctl [ 16], common_pid: gmain [ 2768] } hitcount: 1 . . . { id: sys_ioctl [ 16], common_pid: pool [ 8559] } hitcount: 45 { id: sys_ioctl [ 16], common_pid: pool [ 8555] } hitcount: 48 { id: sys_ioctl [ 16], common_pid: pool [ 8551] } hitcount: 48 { id: sys_ioctl [ 16], common_pid: avahi-daemon [ 896] } hitcount: 66 { id: sys_ioctl [ 16], common_pid: Xorg [ 1267] } hitcount: 26674 { id: sys_ioctl [ 16], common_pid: compiz [ 2994] } hitcount: 73443 Totals: Hits: 101162 Entries: 103 Dropped: 0 The above output shows that 'compiz' and 'Xorg' are far and away the heaviest ioctl callers (which might lead to questions about whether they really need to be making all those calls and to possible avenues for further investigation.) The compound key examples used a key and a sum value (hitcount) to sort the output, but we can just as easily use two keys instead. Here's an example where we use a compound key composed of the the common_pid and size event fields. Sorting with pid as the primary key and 'size' as the secondary key allows us to display an ordered summary of the recvfrom sizes, with counts, received by each process: # echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \ /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist # trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active] { common_pid: smbd [ 784], size: 4 } hitcount: 1 { common_pid: dnsmasq [ 1412], size: 4096 } hitcount: 672 { common_pid: postgres [ 1796], size: 1000 } hitcount: 6 { common_pid: postgres [ 1867], size: 1000 } hitcount: 10 { common_pid: bamfdaemon [ 2787], size: 28 } hitcount: 2 { common_pid: bamfdaemon [ 2787], size: 14360 } hitcount: 1 { common_pid: compiz [ 2994], size: 8 } hitcount: 1 { common_pid: compiz [ 2994], size: 20 } hitcount: 11 { common_pid: gnome-terminal [ 3199], size: 4 } hitcount: 2 { common_pid: firefox [ 8817], size: 4 } hitcount: 1 { common_pid: firefox [ 8817], size: 8 } hitcount: 5 { common_pid: firefox [ 8817], size: 588 } hitcount: 2 { common_pid: firefox [ 8817], size: 628 } hitcount: 1 { common_pid: firefox [ 8817], size: 6944 } hitcount: 1 { common_pid: firefox [ 8817], size: 408880 } hitcount: 2 { common_pid: firefox [ 8822], size: 8 } hitcount: 2 { common_pid: firefox [ 8822], size: 160 } hitcount: 2 { common_pid: firefox [ 8822], size: 320 } hitcount: 2 { common_pid: firefox [ 8822], size: 352 } hitcount: 1 . . . { common_pid: pool [ 8923], size: 1960 } hitcount: 10 { common_pid: pool [ 8923], size: 2048 } hitcount: 10 { common_pid: pool [ 8924], size: 1960 } hitcount: 10 { common_pid: pool [ 8924], size: 2048 } hitcount: 10 { common_pid: pool [ 8928], size: 1964 } hitcount: 4 { common_pid: pool [ 8928], size: 1965 } hitcount: 2 { common_pid: pool [ 8928], size: 2048 } hitcount: 6 { common_pid: pool [ 8929], size: 1982 } hitcount: 1 { common_pid: pool [ 8929], size: 2048 } hitcount: 1 Totals: Hits: 2016 Entries: 224 Dropped: 0 The above example also illustrates the fact that although a compound key is treated as a single entity for hashing purposes, the sub-keys it's composed of can be accessed independently. The next example uses a string field as the hash key and demonstrates how you can manually pause and continue a hist trigger. In this example, we'll aggregate fork counts and don't expect a large number of entries in the hash table, so we'll drop it to a much smaller number, say 256: # echo 'hist:key=child_comm:val=hitcount:size=256' > \ /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active] { child_comm: dconf worker } hitcount: 1 { child_comm: ibus-daemon } hitcount: 1 { child_comm: whoopsie } hitcount: 1 { child_comm: smbd } hitcount: 1 { child_comm: gdbus } hitcount: 1 { child_comm: kthreadd } hitcount: 1 { child_comm: dconf worker } hitcount: 1 { child_comm: evolution-alarm } hitcount: 2 { child_comm: Socket Thread } hitcount: 2 { child_comm: postgres } hitcount: 2 { child_comm: bash } hitcount: 3 { child_comm: compiz } hitcount: 3 { child_comm: evolution-sourc } hitcount: 4 { child_comm: dhclient } hitcount: 4 { child_comm: pool } hitcount: 5 { child_comm: nm-dispatcher.a } hitcount: 8 { child_comm: firefox } hitcount: 8 { child_comm: dbus-daemon } hitcount: 8 { child_comm: glib-pacrunner } hitcount: 10 { child_comm: evolution } hitcount: 23 Totals: Hits: 89 Entries: 20 Dropped: 0 If we want to pause the hist trigger, we can simply append :pause to the command that started the trigger. Notice that the trigger info displays as [paused]: # echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \ /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused] { child_comm: dconf worker } hitcount: 1 { child_comm: kthreadd } hitcount: 1 { child_comm: dconf worker } hitcount: 1 { child_comm: gdbus } hitcount: 1 { child_comm: ibus-daemon } hitcount: 1 { child_comm: Socket Thread } hitcount: 2 { child_comm: evolution-alarm } hitcount: 2 { child_comm: smbd } hitcount: 2 { child_comm: bash } hitcount: 3 { child_comm: whoopsie } hitcount: 3 { child_comm: compiz } hitcount: 3 { child_comm: evolution-sourc } hitcount: 4 { child_comm: pool } hitcount: 5 { child_comm: postgres } hitcount: 6 { child_comm: firefox } hitcount: 8 { child_comm: dhclient } hitcount: 10 { child_comm: emacs } hitcount: 12 { child_comm: dbus-daemon } hitcount: 20 { child_comm: nm-dispatcher.a } hitcount: 20 { child_comm: evolution } hitcount: 35 { child_comm: glib-pacrunner } hitcount: 59 Totals: Hits: 199 Entries: 21 Dropped: 0 To manually continue having the trigger aggregate events, append :cont instead. Notice that the trigger info displays as [active] again, and the data has changed: # echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \ /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active] { child_comm: dconf worker } hitcount: 1 { child_comm: dconf worker } hitcount: 1 { child_comm: kthreadd } hitcount: 1 { child_comm: gdbus } hitcount: 1 { child_comm: ibus-daemon } hitcount: 1 { child_comm: Socket Thread } hitcount: 2 { child_comm: evolution-alarm } hitcount: 2 { child_comm: smbd } hitcount: 2 { child_comm: whoopsie } hitcount: 3 { child_comm: compiz } hitcount: 3 { child_comm: evolution-sourc } hitcount: 4 { child_comm: bash } hitcount: 5 { child_comm: pool } hitcount: 5 { child_comm: postgres } hitcount: 6 { child_comm: firefox } hitcount: 8 { child_comm: dhclient } hitcount: 11 { child_comm: emacs } hitcount: 12 { child_comm: dbus-daemon } hitcount: 22 { child_comm: nm-dispatcher.a } hitcount: 22 { child_comm: evolution } hitcount: 35 { child_comm: glib-pacrunner } hitcount: 59 Totals: Hits: 206 Entries: 21 Dropped: 0 The previous example showed how to start and stop a hist trigger by appending 'pause' and 'continue' to the hist trigger command. A hist trigger can also be started in a paused state by initially starting the trigger with ':pause' appended. This allows you to start the trigger only when you're ready to start collecting data and not before. For example, you could start the trigger in a paused state, then unpause it and do something you want to measure, then pause the trigger again when done. Of course, doing this manually can be difficult and error-prone, but it is possible to automatically start and stop a hist trigger based on some condition, via the enable_hist and disable_hist triggers. For example, suppose we wanted to take a look at the relative weights in terms of skb length for each callpath that leads to a netif_receieve_skb event when downloading a decent-sized file using wget. First we set up an initially paused stacktrace trigger on the netif_receive_skb event: # echo 'hist:key=stacktrace:vals=len:pause' > \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger Next, we set up an 'enable_hist' trigger on the sched_process_exec event, with an 'if filename==/usr/bin/wget' filter. The effect of this new trigger is that it will 'unpause' the hist trigger we just set up on netif_receive_skb if and only if it sees a sched_process_exec event with a filename of '/usr/bin/wget'. When that happens, all netif_receive_skb events are aggregated into a hash table keyed on stacktrace: # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \ /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger The aggregation continues until the netif_receive_skb is paused again, which is what the following disable_hist event does by creating a similar setup on the sched_process_exit event, using the filter 'comm==wget': # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \ /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger Whenever a process exits and the comm field of the disable_hist trigger filter matches 'comm==wget', the netif_receive_skb hist trigger is disabled. The overall effect is that netif_receive_skb events are aggregated into the hash table for only the duration of the wget. Executing a wget command and then listing the 'hist' file will display the output generated by the wget command: $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused] { stacktrace: __netif_receive_skb_core+0x46d/0x990 __netif_receive_skb+0x18/0x60 netif_receive_skb_internal+0x23/0x90 napi_gro_receive+0xc8/0x100 ieee80211_deliver_skb+0xd6/0x270 [mac80211] ieee80211_rx_handlers+0xccf/0x22f0 [mac80211] ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211] ieee80211_rx+0x31d/0x900 [mac80211] iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm] iwl_rx_dispatch+0x8e/0xf0 [iwldvm] iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi] irq_thread_fn+0x20/0x50 irq_thread+0x11f/0x150 kthread+0xd2/0xf0 ret_from_fork+0x42/0x70 } hitcount: 85 len: 28884 { stacktrace: __netif_receive_skb_core+0x46d/0x990 __netif_receive_skb+0x18/0x60 netif_receive_skb_internal+0x23/0x90 napi_gro_complete+0xa4/0xe0 dev_gro_receive+0x23a/0x360 napi_gro_receive+0x30/0x100 ieee80211_deliver_skb+0xd6/0x270 [mac80211] ieee80211_rx_handlers+0xccf/0x22f0 [mac80211] ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211] ieee80211_rx+0x31d/0x900 [mac80211] iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm] iwl_rx_dispatch+0x8e/0xf0 [iwldvm] iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi] irq_thread_fn+0x20/0x50 irq_thread+0x11f/0x150 kthread+0xd2/0xf0 } hitcount: 98 len: 664329 { stacktrace: __netif_receive_skb_core+0x46d/0x990 __netif_receive_skb+0x18/0x60 process_backlog+0xa8/0x150 net_rx_action+0x15d/0x340 __do_softirq+0x114/0x2c0 do_softirq_own_stack+0x1c/0x30 do_softirq+0x65/0x70 __local_bh_enable_ip+0xb5/0xc0 ip_finish_output+0x1f4/0x840 ip_output+0x6b/0xc0 ip_local_out_sk+0x31/0x40 ip_send_skb+0x1a/0x50 udp_send_skb+0x173/0x2a0 udp_sendmsg+0x2bf/0x9f0 inet_sendmsg+0x64/0xa0 sock_sendmsg+0x3d/0x50 } hitcount: 115 len: 13030 { stacktrace: __netif_receive_skb_core+0x46d/0x990 __netif_receive_skb+0x18/0x60 netif_receive_skb_internal+0x23/0x90 napi_gro_complete+0xa4/0xe0 napi_gro_flush+0x6d/0x90 iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi] irq_thread_fn+0x20/0x50 irq_thread+0x11f/0x150 kthread+0xd2/0xf0 ret_from_fork+0x42/0x70 } hitcount: 934 len: 5512212 Totals: Hits: 1232 Entries: 4 Dropped: 0 The above shows all the netif_receive_skb callpaths and their total lengths for the duration of the wget command. The 'clear' hist trigger param can be used to clear the hash table. Suppose we wanted to try another run of the previous example but this time also wanted to see the complete list of events that went into the histogram. In order to avoid having to set everything up again, we can just clear the histogram first: # echo 'hist:key=stacktrace:vals=len:clear' >> \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger Just to verify that it is in fact cleared, here's what we now see in the hist file: # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused] Totals: Hits: 0 Entries: 0 Dropped: 0 Since we want to see the detailed list of every netif_receive_skb event occurring during the new run, which are in fact the same events being aggregated into the hash table, we add some additional 'enable_event' events to the triggering sched_process_exec and sched_process_exit events as such: # echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \ /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger # echo 'disable_event:net:netif_receive_skb if comm==wget' > \ /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger If you read the trigger files for the sched_process_exec and sched_process_exit triggers, you should see two triggers for each: one enabling/disabling the hist aggregation and the other enabling/disabling the logging of events: # cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget # cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger enable_event:net:netif_receive_skb:unlimited if comm==wget disable_hist:net:netif_receive_skb:unlimited if comm==wget In other words, whenever either of the sched_process_exec or sched_process_exit events is hit and matches 'wget', it enables or disables both the histogram and the event log, and what you end up with is a hash table and set of events just covering the specified duration. Run the wget command again: $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz Displaying the 'hist' file should show something similar to what you saw in the last run, but this time you should also see the individual events in the trace file: # cat /sys/kernel/debug/tracing/trace # tracer: nop # # entries-in-buffer/entries-written: 183/1426 #P:4 # # _-----=> irqs-off # / _----=> need-resched # | / _---=> hardirq/softirq # || / _--=> preempt-depth # ||| / delay # TASK-PID CPU# |||| TIMESTAMP FUNCTION # | | | |||| | | wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60 wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60 dnsmasq-1382 [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130 dnsmasq-1382 [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138 ##### CPU 2 buffer started #### irq/29-iwlwifi-559 [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948 irq/29-iwlwifi-559 [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500 irq/29-iwlwifi-559 [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948 irq/29-iwlwifi-559 [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948 irq/29-iwlwifi-559 [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500 . . . The following example demonstrates how multiple hist triggers can be attached to a given event. This capability can be useful for creating a set of different summaries derived from the same set of events, or for comparing the effects of different filters, among other things. # echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger # echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger # echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger # echo 'hist:keys=skbaddr.hex:vals=len' >> \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger # echo 'hist:keys=len:vals=common_preempt_count' >> \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger The above set of commands create four triggers differing only in their filters, along with a completely different though fairly nonsensical trigger. Note that in order to append multiple hist triggers to the same file, you should use the '>>' operator to append them ('>' will also add the new hist trigger, but will remove any existing hist triggers beforehand). Displaying the contents of the 'hist' file for the event shows the contents of all five histograms: # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist # event histogram # # trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active] # { len: 176 } hitcount: 1 common_preempt_count: 0 { len: 223 } hitcount: 1 common_preempt_count: 0 { len: 4854 } hitcount: 1 common_preempt_count: 0 { len: 395 } hitcount: 1 common_preempt_count: 0 { len: 177 } hitcount: 1 common_preempt_count: 0 { len: 446 } hitcount: 1 common_preempt_count: 0 { len: 1601 } hitcount: 1 common_preempt_count: 0 . . . { len: 1280 } hitcount: 66 common_preempt_count: 0 { len: 116 } hitcount: 81 common_preempt_count: 40 { len: 708 } hitcount: 112 common_preempt_count: 0 { len: 46 } hitcount: 221 common_preempt_count: 0 { len: 1264 } hitcount: 458 common_preempt_count: 0 Totals: Hits: 1428 Entries: 147 Dropped: 0 # event histogram # # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active] # { skbaddr: ffff8800baee5e00 } hitcount: 1 len: 130 { skbaddr: ffff88005f3d5600 } hitcount: 1 len: 1280 { skbaddr: ffff88005f3d4900 } hitcount: 1 len: 1280 { skbaddr: ffff88009fed6300 } hitcount: 1 len: 115 { skbaddr: ffff88009fe0ad00 } hitcount: 1 len: 115 { skbaddr: ffff88008cdb1900 } hitcount: 1 len: 46 { skbaddr: ffff880064b5ef00 } hitcount: 1 len: 118 { skbaddr: ffff880044e3c700 } hitcount: 1 len: 60 { skbaddr: ffff880100065900 } hitcount: 1 len: 46 { skbaddr: ffff8800d46bd500 } hitcount: 1 len: 116 { skbaddr: ffff88005f3d5f00 } hitcount: 1 len: 1280 { skbaddr: ffff880100064700 } hitcount: 1 len: 365 { skbaddr: ffff8800badb6f00 } hitcount: 1 len: 60 . . . { skbaddr: ffff88009fe0be00 } hitcount: 27 len: 24677 { skbaddr: ffff88009fe0a400 } hitcount: 27 len: 23052 { skbaddr: ffff88009fe0b700 } hitcount: 31 len: 25589 { skbaddr: ffff88009fe0b600 } hitcount: 32 len: 27326 { skbaddr: ffff88006a462800 } hitcount: 68 len: 71678 { skbaddr: ffff88006a463700 } hitcount: 70 len: 72678 { skbaddr: ffff88006a462b00 } hitcount: 71 len: 77589 { skbaddr: ffff88006a463600 } hitcount: 73 len: 71307 { skbaddr: ffff88006a462200 } hitcount: 81 len: 81032 Totals: Hits: 1451 Entries: 318 Dropped: 0 # event histogram # # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active] # Totals: Hits: 0 Entries: 0 Dropped: 0 # event histogram # # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len > 4096 [active] # { skbaddr: ffff88009fd2c300 } hitcount: 1 len: 7212 { skbaddr: ffff8800d2bcce00 } hitcount: 1 len: 7212 { skbaddr: ffff8800d2bcd700 } hitcount: 1 len: 7212 { skbaddr: ffff8800d2bcda00 } hitcount: 1 len: 21492 { skbaddr: ffff8800ae2e2d00 } hitcount: 1 len: 7212 { skbaddr: ffff8800d2bcdb00 } hitcount: 1 len: 7212 { skbaddr: ffff88006a4df500 } hitcount: 1 len: 4854 { skbaddr: ffff88008ce47b00 } hitcount: 1 len: 18636 { skbaddr: ffff8800ae2e2200 } hitcount: 1 len: 12924 { skbaddr: ffff88005f3e1000 } hitcount: 1 len: 4356 { skbaddr: ffff8800d2bcdc00 } hitcount: 2 len: 24420 { skbaddr: ffff8800d2bcc200 } hitcount: 2 len: 12996 Totals: Hits: 14 Entries: 12 Dropped: 0 # event histogram # # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active] # Totals: Hits: 0 Entries: 0 Dropped: 0 Named triggers can be used to have triggers share a common set of histogram data. This capability is mostly useful for combining the output of events generated by tracepoints contained inside inline functions, but names can be used in a hist trigger on any event. For example, these two triggers when hit will update the same 'len' field in the shared 'foo' histogram data: # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \ /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \ /sys/kernel/debug/tracing/events/net/netif_rx/trigger You can see that they're updating common histogram data by reading each event's hist files at the same time: # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist; cat /sys/kernel/debug/tracing/events/net/netif_rx/hist # event histogram # # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active] # { skbaddr: ffff88000ad53500 } hitcount: 1 len: 46 { skbaddr: ffff8800af5a1500 } hitcount: 1 len: 76 { skbaddr: ffff8800d62a1900 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bccb00 } hitcount: 1 len: 468 { skbaddr: ffff8800d3c69900 } hitcount: 1 len: 46 { skbaddr: ffff88009ff09100 } hitcount: 1 len: 52 { skbaddr: ffff88010f13ab00 } hitcount: 1 len: 168 { skbaddr: ffff88006a54f400 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bcc500 } hitcount: 1 len: 260 { skbaddr: ffff880064505000 } hitcount: 1 len: 46 { skbaddr: ffff8800baf24e00 } hitcount: 1 len: 32 { skbaddr: ffff88009fe0ad00 } hitcount: 1 len: 46 { skbaddr: ffff8800d3edff00 } hitcount: 1 len: 44 { skbaddr: ffff88009fe0b400 } hitcount: 1 len: 168 { skbaddr: ffff8800a1c55a00 } hitcount: 1 len: 40 { skbaddr: ffff8800d2bcd100 } hitcount: 1 len: 40 { skbaddr: ffff880064505f00 } hitcount: 1 len: 174 { skbaddr: ffff8800a8bff200 } hitcount: 1 len: 160 { skbaddr: ffff880044e3cc00 } hitcount: 1 len: 76 { skbaddr: ffff8800a8bfe700 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bcdc00 } hitcount: 1 len: 32 { skbaddr: ffff8800a1f64800 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bcde00 } hitcount: 1 len: 988 { skbaddr: ffff88006a5dea00 } hitcount: 1 len: 46 { skbaddr: ffff88002e37a200 } hitcount: 1 len: 44 { skbaddr: ffff8800a1f32c00 } hitcount: 2 len: 676 { skbaddr: ffff88000ad52600 } hitcount: 2 len: 107 { skbaddr: ffff8800a1f91e00 } hitcount: 2 len: 92 { skbaddr: ffff8800af5a0200 } hitcount: 2 len: 142 { skbaddr: ffff8800d2bcc600 } hitcount: 2 len: 220 { skbaddr: ffff8800ba36f500 } hitcount: 2 len: 92 { skbaddr: ffff8800d021f800 } hitcount: 2 len: 92 { skbaddr: ffff8800a1f33600 } hitcount: 2 len: 675 { skbaddr: ffff8800a8bfff00 } hitcount: 3 len: 138 { skbaddr: ffff8800d62a1300 } hitcount: 3 len: 138 { skbaddr: ffff88002e37a100 } hitcount: 4 len: 184 { skbaddr: ffff880064504400 } hitcount: 4 len: 184 { skbaddr: ffff8800a8bfec00 } hitcount: 4 len: 184 { skbaddr: ffff88000ad53700 } hitcount: 5 len: 230 { skbaddr: ffff8800d2bcdb00 } hitcount: 5 len: 196 { skbaddr: ffff8800a1f90000 } hitcount: 6 len: 276 { skbaddr: ffff88006a54f900 } hitcount: 6 len: 276 Totals: Hits: 81 Entries: 42 Dropped: 0 # event histogram # # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active] # { skbaddr: ffff88000ad53500 } hitcount: 1 len: 46 { skbaddr: ffff8800af5a1500 } hitcount: 1 len: 76 { skbaddr: ffff8800d62a1900 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bccb00 } hitcount: 1 len: 468 { skbaddr: ffff8800d3c69900 } hitcount: 1 len: 46 { skbaddr: ffff88009ff09100 } hitcount: 1 len: 52 { skbaddr: ffff88010f13ab00 } hitcount: 1 len: 168 { skbaddr: ffff88006a54f400 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bcc500 } hitcount: 1 len: 260 { skbaddr: ffff880064505000 } hitcount: 1 len: 46 { skbaddr: ffff8800baf24e00 } hitcount: 1 len: 32 { skbaddr: ffff88009fe0ad00 } hitcount: 1 len: 46 { skbaddr: ffff8800d3edff00 } hitcount: 1 len: 44 { skbaddr: ffff88009fe0b400 } hitcount: 1 len: 168 { skbaddr: ffff8800a1c55a00 } hitcount: 1 len: 40 { skbaddr: ffff8800d2bcd100 } hitcount: 1 len: 40 { skbaddr: ffff880064505f00 } hitcount: 1 len: 174 { skbaddr: ffff8800a8bff200 } hitcount: 1 len: 160 { skbaddr: ffff880044e3cc00 } hitcount: 1 len: 76 { skbaddr: ffff8800a8bfe700 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bcdc00 } hitcount: 1 len: 32 { skbaddr: ffff8800a1f64800 } hitcount: 1 len: 46 { skbaddr: ffff8800d2bcde00 } hitcount: 1 len: 988 { skbaddr: ffff88006a5dea00 } hitcount: 1 len: 46 { skbaddr: ffff88002e37a200 } hitcount: 1 len: 44 { skbaddr: ffff8800a1f32c00 } hitcount: 2 len: 676 { skbaddr: ffff88000ad52600 } hitcount: 2 len: 107 { skbaddr: ffff8800a1f91e00 } hitcount: 2 len: 92 { skbaddr: ffff8800af5a0200 } hitcount: 2 len: 142 { skbaddr: ffff8800d2bcc600 } hitcount: 2 len: 220 { skbaddr: ffff8800ba36f500 } hitcount: 2 len: 92 { skbaddr: ffff8800d021f800 } hitcount: 2 len: 92 { skbaddr: ffff8800a1f33600 } hitcount: 2 len: 675 { skbaddr: ffff8800a8bfff00 } hitcount: 3 len: 138 { skbaddr: ffff8800d62a1300 } hitcount: 3 len: 138 { skbaddr: ffff88002e37a100 } hitcount: 4 len: 184 { skbaddr: ffff880064504400 } hitcount: 4 len: 184 { skbaddr: ffff8800a8bfec00 } hitcount: 4 len: 184 { skbaddr: ffff88000ad53700 } hitcount: 5 len: 230 { skbaddr: ffff8800d2bcdb00 } hitcount: 5 len: 196 { skbaddr: ffff8800a1f90000 } hitcount: 6 len: 276 { skbaddr: ffff88006a54f900 } hitcount: 6 len: 276 Totals: Hits: 81 Entries: 42 Dropped: 0 And here's an example that shows how to combine histogram data from any two events even if they don't share any 'compatible' fields other than 'hitcount' and 'stacktrace'. These commands create a couple of triggers named 'bar' using those fields: # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \ /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \ /sys/kernel/debug/tracing/events/net/netif_rx/trigger And displaying the output of either shows some interesting if somewhat confusing output: # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist # event histogram # # trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active] # { stacktrace: _do_fork+0x18e/0x330 kernel_thread+0x29/0x30 kthreadd+0x154/0x1b0 ret_from_fork+0x3f/0x70 } hitcount: 1 { stacktrace: netif_rx_internal+0xb2/0xd0 netif_rx_ni+0x20/0x70 dev_loopback_xmit+0xaa/0xd0 ip_mc_output+0x126/0x240 ip_local_out_sk+0x31/0x40 igmp_send_report+0x1e9/0x230 igmp_timer_expire+0xe9/0x120 call_timer_fn+0x39/0xf0 run_timer_softirq+0x1e1/0x290 __do_softirq+0xfd/0x290 irq_exit+0x98/0xb0 smp_apic_timer_interrupt+0x4a/0x60 apic_timer_interrupt+0x6d/0x80 cpuidle_enter+0x17/0x20 call_cpuidle+0x3b/0x60 cpu_startup_entry+0x22d/0x310 } hitcount: 1 { stacktrace: netif_rx_internal+0xb2/0xd0 netif_rx_ni+0x20/0x70 dev_loopback_xmit+0xaa/0xd0 ip_mc_output+0x17f/0x240 ip_local_out_sk+0x31/0x40 ip_send_skb+0x1a/0x50 udp_send_skb+0x13e/0x270 udp_sendmsg+0x2bf/0x980 inet_sendmsg+0x67/0xa0 sock_sendmsg+0x38/0x50 SYSC_sendto+0xef/0x170 SyS_sendto+0xe/0x10 entry_SYSCALL_64_fastpath+0x12/0x6a } hitcount: 2 { stacktrace: netif_rx_internal+0xb2/0xd0 netif_rx+0x1c/0x60 loopback_xmit+0x6c/0xb0 dev_hard_start_xmit+0x219/0x3a0 __dev_queue_xmit+0x415/0x4f0 dev_queue_xmit_sk+0x13/0x20 ip_finish_output2+0x237/0x340 ip_finish_output+0x113/0x1d0 ip_output+0x66/0xc0 ip_local_out_sk+0x31/0x40 ip_send_skb+0x1a/0x50 udp_send_skb+0x16d/0x270 udp_sendmsg+0x2bf/0x980 inet_sendmsg+0x67/0xa0 sock_sendmsg+0x38/0x50 ___sys_sendmsg+0x14e/0x270 } hitcount: 76 { stacktrace: netif_rx_internal+0xb2/0xd0 netif_rx+0x1c/0x60 loopback_xmit+0x6c/0xb0 dev_hard_start_xmit+0x219/0x3a0 __dev_queue_xmit+0x415/0x4f0 dev_queue_xmit_sk+0x13/0x20 ip_finish_output2+0x237/0x340 ip_finish_output+0x113/0x1d0 ip_output+0x66/0xc0 ip_local_out_sk+0x31/0x40 ip_send_skb+0x1a/0x50 udp_send_skb+0x16d/0x270 udp_sendmsg+0x2bf/0x980 inet_sendmsg+0x67/0xa0 sock_sendmsg+0x38/0x50 ___sys_sendmsg+0x269/0x270 } hitcount: 77 { stacktrace: netif_rx_internal+0xb2/0xd0 netif_rx+0x1c/0x60 loopback_xmit+0x6c/0xb0 dev_hard_start_xmit+0x219/0x3a0 __dev_queue_xmit+0x415/0x4f0 dev_queue_xmit_sk+0x13/0x20 ip_finish_output2+0x237/0x340 ip_finish_output+0x113/0x1d0 ip_output+0x66/0xc0 ip_local_out_sk+0x31/0x40 ip_send_skb+0x1a/0x50 udp_send_skb+0x16d/0x270 udp_sendmsg+0x2bf/0x980 inet_sendmsg+0x67/0xa0 sock_sendmsg+0x38/0x50 SYSC_sendto+0xef/0x170 } hitcount: 88 { stacktrace: _do_fork+0x18e/0x330 SyS_clone+0x19/0x20 entry_SYSCALL_64_fastpath+0x12/0x6a } hitcount: 244 Totals: Hits: 489 Entries: 7 Dropped: 0