linux-kernel - Re: [PATCH v4 1/3] perf: Use monotonic clock as a source for timestamps

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Date:	Mon, 5 Jan 2015 14:00:35 +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 v4 1/3] perf: Use monotonic clock as a source for
 timestamps

On Thu, Nov 06, 2014 at 04:51:56PM +0000, Pawel Moll wrote:
>  Documentation/kernel-parameters.txt |  9 +++++++++
>  kernel/events/core.c                | 37 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+)

> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 4c81a86..8ead8d8 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt

> @@ -2763,6 +2764,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  			allocator.  This parameter is primarily	for debugging
>  			and performance comparison.
>  
> +	perf_use_local_clock
> +			[PERF]
> +			Use local_clock() as a source for perf timestamps
> +			generation. This was be the default behaviour and
> +			this parameter can be used to maintain backward
> +			compatibility or on older hardware with expensive
> +			monotonic clock source.
> +
>  	pf.		[PARIDE]
>  			See Documentation/blockdev/paride.txt.

So I'm always terminally confused on the naming of kernel parameters,
sometimes things are modules (even when they're not actually =m capable)
and get a module::foo naming or so and sometimes they're not.

So we want to use the module naming thing or not?

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 2b02c9f..5d0aa03 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c

> @@ -322,8 +323,41 @@ extern __weak const char *perf_pmu_name(void)
>  	return "pmu";
>  }
>  
> +static bool perf_use_local_clock;
> +static int __init perf_use_local_clock_setup(char *__unused)
> +{
> +	perf_use_local_clock = true;
> +	return 1;
> +}
> +__setup("perf_use_local_clock", perf_use_local_clock_setup);

>  static inline u64 perf_clock(void)
>  {
> +	if (likely(!perf_use_local_clock))
> +		return ktime_get_mono_fast_ns();
> +
>  	return local_clock();
>  }

Since this all is boot time, should we not use things like static_key
and avoid the 'pointless' conditional at runtime?
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