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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>
This commit is contained in:
Juri Lelli 2013-06-12 12:03:18 +02:00 committed by Steven Rostedt
parent 195a84d91e
commit 52d85d7630
1 changed files with 8 additions and 2 deletions
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10
kernel/trace/ftrace.c
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@ -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);