linux-kernel - Re: [PATCH 1/2] perf: Add sampling of the raw monotonic clock

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Date:	Mon, 29 Sep 2014 17:28:32 +0200
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>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] perf: Add sampling of the raw monotonic clock

On Thu, Sep 18, 2014 at 03:34:32PM +0100, Pawel Moll wrote:
> @@ -4456,6 +4459,13 @@ static void __perf_event_header__init_id(struct perf_event_header *header,
>  		data->cpu_entry.cpu	 = raw_smp_processor_id();
>  		data->cpu_entry.reserved = 0;
>  	}
> +
> +	if (sample_type & PERF_SAMPLE_CLOCK_RAW_MONOTONIC) {
> +		struct timespec now;
> +
> +		getrawmonotonic(&now);
> +		data->clock_raw_monotonic = timespec_to_ns(&now);
> +	}
>  }
>  

This cannot work, getrawmonotonic() isn't NMI-safe and there's
nothing stopping this being used from NMI context.

Also getrawmonotonic() + timespec_to_ns() will make tglx sad, he's just
done a tree-wide eradication of silly conversions and now you're adding
a ns -> timespec -> ns dance right back.

I _think_ you want ktime_get_mono_fast_ns(), but this does bring us
right back to the question/discussion on which timebase you'd want to
sync again. MONO does make sense for most cases, but I think we've had
fairly sane stories for people wanting to sync against other clocks.

A well.. 
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