tracing: profile likely and unlikely annotations

Impact: new unlikely/likely profiler

Andrew Morton recently suggested having an in-kernel way to profile
likely and unlikely macros. This patch achieves that goal.

When configured, every(*) likely and unlikely macro gets a counter attached
to it. When the condition is hit, the hit and misses of that condition
are recorded. These numbers can later be retrieved by:

  /debugfs/tracing/profile_likely    - All likely markers
  /debugfs/tracing/profile_unlikely  - All unlikely markers.

# cat /debug/tracing/profile_unlikely | head
 correct incorrect  %        Function                  File              Line
 ------- ---------  -        --------                  ----              ----
    2167        0   0 do_arch_prctl                  process_64.c         832
       0        0   0 do_arch_prctl                  process_64.c         804
    2670        0   0 IS_ERR                         err.h                34
   71230     5693   7 __switch_to                    process_64.c         673
   76919        0   0 __switch_to                    process_64.c         639
   43184    33743  43 __switch_to                    process_64.c         624
   12740    64181  83 __switch_to                    process_64.c         594
   12740    64174  83 __switch_to                    process_64.c         590

# cat /debug/tracing/profile_unlikely | \
  awk '{ if ($3 > 25) print $0; }' |head -20
   44963    35259  43 __switch_to                    process_64.c         624
   12762    67454  84 __switch_to                    process_64.c         594
   12762    67447  84 __switch_to                    process_64.c         590
    1478      595  28 syscall_get_error              syscall.h            51
       0     2821 100 syscall_trace_leave            ptrace.c             1567
       0        1 100 native_smp_prepare_cpus        smpboot.c            1237
   86338   265881  75 calc_delta_fair                sched_fair.c         408
  210410   108540  34 calc_delta_mine                sched.c              1267
       0    54550 100 sched_info_queued              sched_stats.h        222
   51899    66435  56 pick_next_task_fair            sched_fair.c         1422
       6       10  62 yield_task_fair                sched_fair.c         982
    7325     2692  26 rt_policy                      sched.c              144
       0     1270 100 pre_schedule_rt                sched_rt.c           1261
    1268    48073  97 pick_next_task_rt              sched_rt.c           884
       0    45181 100 sched_info_dequeued            sched_stats.h        177
       0       15 100 sched_move_task                sched.c              8700
       0       15 100 sched_move_task                sched.c              8690
   53167    33217  38 schedule                       sched.c              4457
       0    80208 100 sched_info_switch              sched_stats.h        270
   30585    49631  61 context_switch                 sched.c              2619

# cat /debug/tracing/profile_likely | awk '{ if ($3 > 25) print $0; }'
   39900    36577  47 pick_next_task                 sched.c              4397
   20824    15233  42 switch_mm                      mmu_context_64.h     18
       0        7 100 __cancel_work_timer            workqueue.c          560
     617    66484  99 clocksource_adjust             timekeeping.c        456
       0   346340 100 audit_syscall_exit             auditsc.c            1570
      38   347350  99 audit_get_context              auditsc.c            732
       0   345244 100 audit_syscall_entry            auditsc.c            1541
      38     1017  96 audit_free                     auditsc.c            1446
       0     1090 100 audit_alloc                    auditsc.c            862
    2618     1090  29 audit_alloc                    auditsc.c            858
       0        6 100 move_masked_irq                migration.c          9
       1      198  99 probe_sched_wakeup             trace_sched_switch.c 58
       2        2  50 probe_wakeup                   trace_sched_wakeup.c 227
       0        2 100 probe_wakeup_sched_switch      trace_sched_wakeup.c 144
    4514     2090  31 __grab_cache_page              filemap.c            2149
   12882   228786  94 mapping_unevictable            pagemap.h            50
       4       11  73 __flush_cpu_slab               slub.c               1466
  627757   330451  34 slab_free                      slub.c               1731
    2959    61245  95 dentry_lru_del_init            dcache.c             153
     946     1217  56 load_elf_binary                binfmt_elf.c         904
     102       82  44 disk_put_part                  genhd.h              206
       1        1  50 dst_gc_task                    dst.c                82
       0       19 100 tcp_mss_split_point            tcp_output.c         1126

As you can see by the above, there's a bit of work to do in rethinking
the use of some unlikelys and likelys. Note: the unlikely case had 71 hits
that were more than 25%.

Note:  After submitting my first version of this patch, Andrew Morton
  showed me a version written by Daniel Walker, where I picked up
  the following ideas from:

  1)  Using __builtin_constant_p to avoid profiling fixed values.
  2)  Using __FILE__ instead of instruction pointers.
  3)  Using the preprocessor to stop all profiling of likely
       annotations from vsyscall_64.c.

Thanks to Andrew Morton, Arjan van de Ven, Theodore Tso and Ingo Molnar
for their feed back on this patch.

(*) Not ever unlikely is recorded, those that are used by vsyscalls
 (a few of them) had to have profiling disabled.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Theodore Tso <tytso@mit.edu>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 59 insertions, 2 deletions

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 98115d9d04da..935e30cfaf3c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -59,8 +59,65 @@ extern void __chk_io_ptr(const volatile void __iomem *);
  * specific implementations come from the above header files
  */
 
-#define likely(x)	__builtin_expect(!!(x), 1)
-#define unlikely(x)	__builtin_expect(!!(x), 0)
+#ifdef CONFIG_TRACE_UNLIKELY_PROFILE
+struct ftrace_likely_data {
+	const char *func;
+	const char *file;
+	unsigned line;
+	unsigned long correct;
+	unsigned long incorrect;
+};
+void ftrace_likely_update(struct ftrace_likely_data *f, int val, int expect);
+
+#define likely_notrace(x)	__builtin_expect(!!(x), 1)
+#define unlikely_notrace(x)	__builtin_expect(!!(x), 0)
+
+#define likely_check(x) ({						\
+			int ______r;					\
+			static struct ftrace_likely_data		\
+				__attribute__((__aligned__(4)))		\
+				__attribute__((section("_ftrace_likely"))) \
+				______f = {				\
+				.func = __func__,			\
+				.file = __FILE__,			\
+				.line = __LINE__,			\
+			};						\
+			______f.line = __LINE__;			\
+			______r = likely_notrace(x);			\
+			ftrace_likely_update(&______f, ______r, 1);	\
+			______r;					\
+		})
+#define unlikely_check(x) ({						\
+			int ______r;					\
+			static struct ftrace_likely_data		\
+				__attribute__((__aligned__(4)))		\
+				__attribute__((section("_ftrace_unlikely"))) \
+				______f = {				\
+				.func = __func__,			\
+				.file = __FILE__,			\
+				.line = __LINE__,			\
+			};						\
+			______f.line = __LINE__;			\
+			______r = unlikely_notrace(x);			\
+			ftrace_likely_update(&______f, ______r, 0);	\
+			______r;					\
+		})
+
+/*
+ * Using __builtin_constant_p(x) to ignore cases where the return
+ * value is always the same.  This idea is taken from a similar patch
+ * written by Daniel Walker.
+ */
+# ifndef likely
+#  define likely(x)	(__builtin_constant_p(x) ? !!(x) : likely_check(x))
+# endif
+# ifndef unlikely
+#  define unlikely(x)	(__builtin_constant_p(x) ? !!(x) : unlikely_check(x))
+# endif
+#else
+# define likely(x)	__builtin_expect(!!(x), 1)
+# define unlikely(x)	__builtin_expect(!!(x), 0)
+#endif
 
 /* Optimization barrier */
 #ifndef barrier