Commit 52d85d76 authored Jun 12, 2013 by Juri Lelli Committed by Steven Rostedt Jun 19, 2013

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>

parent 195a84d9

kernel/trace/ftrace.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -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);

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