Re: [PATCH 2/2] tracing/function-graph-tracer: use the more lightweight local clock

Ingo Molnar
2009-03-05T11:38:52Z

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> > There will be one problem though: function trace elapsed
> > time measurements across idle. Those are not correctly
> > measured by the local clock.
>
> Oh, why? If I'm not wrong it uses sched_clock() which uses tsc
> on x86. And this register always goes forward at the same
> rate. Unless some Cpu decrease their frequency while beeing
> idle for some time and then become unreliable?

TSC stops in idle on many CPUs.

Ingo