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authorJuri Lelli <juri.lelli@gmail.com>2013-06-12 12:03:18 +0200
committerSteven Rostedt <rostedt@goodmis.org>2013-06-19 23:32:09 -0400
commit52d85d763086594f139bf7d3a5641abeb91d9f57 (patch)
treeef1e85c2ac60076ecc3a31a60391653acff97e0b /kernel/trace
parent195a84d91e92ee3fe571a2086a6db7e17bf5bc7c (diff)
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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/trace')
-rw-r--r--kernel/trace/ftrace.c10
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);