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

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Message-ID: <1412005518.3817.23.camel@hornet>
Date:	Mon, 29 Sep 2014 16:45:18 +0100
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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...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 Mon, 2014-09-29 at 16:28 +0100, Peter Zijlstra wrote:
> 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.

Last thing I want is to make Thomas sad... For obvious reasons ;-)

> I _think_ you want ktime_get_mono_fast_ns(), 

With pleasure, it's exactly what I need.

> 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.

Yes. I've asked the same question somewhere in the thread.

ftrace has got a switch and a selection of trace_clocks in
kernel/trace/trace.c - do we want something similar (in integer form
probably, though) in perf_events.h with an additional "flag" in struct
perf_event_attr? It could be used to pick a time source for
PERF_SAMPLE_CLOCK (PERF_SAMPLE_TRACE_CLOCK?) sample.

Pawel

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