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| author | Juri Lelli <juri.lelli@gmail.com> | 2013-06-12 12:03:18 +0200 |
|---|---|---|
| committer | Steven Rostedt <rostedt@goodmis.org> | 2013-06-19 23:32:09 -0400 |
| commit | 52d85d763086594f139bf7d3a5641abeb91d9f57 (patch) | |
| tree | ef1e85c2ac60076ecc3a31a60391653acff97e0b /kernel | |
| parent | 195a84d91e92ee3fe571a2086a6db7e17bf5bc7c (diff) | |
| download | linux-52d85d763086594f139bf7d3a5641abeb91d9f57.tar.gz linux-52d85d763086594f139bf7d3a5641abeb91d9f57.tar.bz2 linux-52d85d763086594f139bf7d3a5641abeb91d9f57.zip | |
ftrace: Fix stddev calculation in function profiler
When FUNCTION_GRAPH_TRACER is enabled, ftrace can profile kernel functions
and print basic statistics about them. Unfortunately, running stddev
calculation is wrong. This patch corrects it implementing Welford’s method:
s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2) .
Link: http://lkml.kernel.org/r/1371031398-24048-1-git-send-email-juri.lelli@gmail.com
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/trace/ftrace.c | 10 |
1 files changed, 8 insertions, 2 deletions
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index 800a8a2fbddb..26e19105cdcc 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -641,12 +641,18 @@ static int function_stat_show(struct seq_file *m, void *v) if (rec->counter <= 1) stddev = 0; else { - stddev = rec->time_squared - rec->counter * avg * avg; + /* + * Apply Welford's method: + * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2) + */ + stddev = rec->counter * rec->time_squared - + rec->time * rec->time; + /* * Divide only 1000 for ns^2 -> us^2 conversion. * trace_print_graph_duration will divide 1000 again. */ - do_div(stddev, (rec->counter - 1) * 1000); + do_div(stddev, rec->counter * (rec->counter - 1) * 1000); } trace_seq_init(&s); |