Re: [PATCH v3 1/3] perf: Use monotonic clock as a source for timestamps

[Pawel Moll <pawel.moll@arm.com>]

On Tue, 2014-11-04 at 07:23 +0000, Peter Zijlstra wrote:
> On Tue, Nov 04, 2014 at 12:28:36AM +0000, Pawel Moll wrote:
> 
> > +int sysctl_perf_sample_time_clk_id = CLOCK_MONOTONIC;
> 
> const ?

Sure (unless we have to change it as mentioned below)

> >  /*
> >   * perf samples are done in some very critical code paths (NMIs).
> >   * If they take too much CPU time, the system can lock up and not
> > @@ -324,7 +326,7 @@ extern __weak const char *perf_pmu_name(void)
> >  
> >  static inline u64 perf_clock(void)
> >  {
> > -	return local_clock();
> > +	return ktime_get_mono_fast_ns();
> >  }
> 
> Do we maybe want to make it boot-time switchable back to local_clock for
> people with bad systems and or backwards compat issues?

Very good idea, should have came up with it myself :-)

Does __setup("perf_use_local_clock") sound reasonable? Then we have to
decide whether to hide the sysctl "perf_sample_time_clk_id" (my
preferred option, will see how difficult it is) or just provide an
invalid clock_id (eg. -1) in it.

Cheers!

Pawel