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

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Message-ID: <20141104072308.GE10501@worktop.programming.kicks-ass.net>
Date:	Tue, 4 Nov 2014 08:23:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pawel Moll <pawel.moll@....com>
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-api@...r.kernel.org
Subject: Re: [PATCH v3 1/3] perf: Use monotonic clock as a source for
 timestamps

On Tue, Nov 04, 2014 at 12:28:36AM +0000, Pawel Moll wrote:

> +int sysctl_perf_sample_time_clk_id = CLOCK_MONOTONIC;

const ?

>  /*
>   * 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?

Everybody using Core2 and older will very much not want to have this
unless they've got a very good reason for wanting it.
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