Messages in this thread Patch in this message | | | From | Juri Lelli <> | Subject | [PATCH 3/3] ftrace: fix stddev calculation | Date | Tue, 11 Jun 2013 11:08:48 +0200 |
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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) .
Signed-off-by: Juri Lelli <juri.lelli@gmail.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Ingo Molnar <mingo@redhat.com> --- kernel/trace/ftrace.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index 6caaa0e..073a328 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -593,13 +593,17 @@ void function_stat_calc(struct ftrace_profile *rec, if (rec->counter <= 1) *stddev = 0; else { - *stddev = rec->time_squared - rec->counter * (*avg) * (*avg); + /* + * Apply Welford's method. + */ + *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); } } -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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