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Diffstat (limited to 'Documentation/kernel-per-CPU-kthreads.txt')
| -rw-r--r-- | Documentation/kernel-per-CPU-kthreads.txt | 49 |
1 files changed, 48 insertions, 1 deletions
diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt index cbf7ae412da4..32351bfabf20 100644 --- a/Documentation/kernel-per-CPU-kthreads.txt +++ b/Documentation/kernel-per-CPU-kthreads.txt @@ -157,6 +157,53 @@ RCU_SOFTIRQ: Do at least one of the following: calls and by forcing both kernel threads and interrupts to execute elsewhere. +Name: kworker/%u:%d%s (cpu, id, priority) +Purpose: Execute workqueue requests +To reduce its OS jitter, do any of the following: +1. Run your workload at a real-time priority, which will allow + preempting the kworker daemons. +2. Do any of the following needed to avoid jitter that your + application cannot tolerate: + a. Build your kernel with CONFIG_SLUB=y rather than + CONFIG_SLAB=y, thus avoiding the slab allocator's periodic + use of each CPU's workqueues to run its cache_reap() + function. + b. Avoid using oprofile, thus avoiding OS jitter from + wq_sync_buffer(). + c. Limit your CPU frequency so that a CPU-frequency + governor is not required, possibly enlisting the aid of + special heatsinks or other cooling technologies. If done + correctly, and if you CPU architecture permits, you should + be able to build your kernel with CONFIG_CPU_FREQ=n to + avoid the CPU-frequency governor periodically running + on each CPU, including cs_dbs_timer() and od_dbs_timer(). + WARNING: Please check your CPU specifications to + make sure that this is safe on your particular system. + d. It is not possible to entirely get rid of OS jitter + from vmstat_update() on CONFIG_SMP=y systems, but you + can decrease its frequency by writing a large value to + /proc/sys/vm/stat_interval. The default value is HZ, + for an interval of one second. Of course, larger values + will make your virtual-memory statistics update more + slowly. Of course, you can also run your workload at + a real-time priority, thus preempting vmstat_update(). + e. If running on high-end powerpc servers, build with + CONFIG_PPC_RTAS_DAEMON=n. This prevents the RTAS + daemon from running on each CPU every second or so. + (This will require editing Kconfig files and will defeat + this platform's RAS functionality.) This avoids jitter + due to the rtas_event_scan() function. + WARNING: Please check your CPU specifications to + make sure that this is safe on your particular system. + f. If running on Cell Processor, build your kernel with + CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from + spu_gov_work(). + WARNING: Please check your CPU specifications to + make sure that this is safe on your particular system. + g. If running on PowerMAC, build your kernel with + CONFIG_PMAC_RACKMETER=n to disable the CPU-meter, + avoiding OS jitter from rackmeter_do_timer(). + Name: rcuc/%u Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels. To reduce its OS jitter, do at least one of the following: @@ -185,7 +232,7 @@ Purpose: Offload RCU callbacks from the corresponding CPU. To reduce its OS jitter, do at least one of the following: 1. Use affinity, cgroups, or other mechanism to force these kthreads to execute on some other CPU. -2. Build with CONFIG_RCU_NOCB_CPUS=n, which will prevent these +2. Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these kthreads from being created in the first place. However, please note that this will not eliminate OS jitter, but will instead shift it to RCU_SOFTIRQ. |