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

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Message-ID: <1415114727.24819.8.camel@arm.com>
Date:	Tue, 04 Nov 2014 15:25:27 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christopher Covington <cov@...eaurora.org>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tomeu Vizoso <tomeu@...euvizoso.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] perf: Use monotonic clock as a source for
 timestamps

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

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