linux-kernel - Re: [PATCH] ftrace: Provide trace clock monotonic raw

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Date:	Fri, 8 May 2015 01:27:56 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Drew Richardson <drew.richardson@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Wade Cherry <Wade.Cherry@....com>,
	Pawel Moll <Pawel.Moll@....com>
Subject: Re: [PATCH] ftrace: Provide trace clock monotonic raw

----- Original Message -----
> On Tue, 5 May 2015 07:54:46 -0700
> Drew Richardson <drew.richardson@....com> wrote:
> 
> > CLOCK_MONOTONIC_RAW will advance more constantly than CLOCK_MONOTONIC.
> > 
> > Imagine someone is trying to optimize a particular program to reduce
> > instructions executed for a given workload while minimizing the effect
> > on runtime. Also suppose that ntp is running and potentially making
> > larger adjustments to CLOCK_MONOTONIC. If ntp is adjusting
> > CLOCK_MONOTONIC to advance more rapidly, the program will appear to
> > use fewer instructions per second but run longer than it would if
> > CLOCK_MONOTONIC_RAW had been used. The total number of instructions
> > observed would be the same regardless of the clock source used, but
> > how it's attributed to time would be affected.
> > 
> > Conversely if ntp is adjusting CLOCK_MONOTONIC to advance more slowly,
> > the program will appear to use more instructions per second but run
> > more quickly. Of course there are many sources that can cause jitter
> > in performance measurements on modern processors, but I'd like to
> > remove ntp from the list.
> 
> What's the consensus on this patch? Everyone OK with it? If so, can you
> please post a new patch with the proper change log. And can everyone
> else give acks. I can take it in my tree.

I can see it being useful for tracing early boot, e.g.
when debugging issues with NTP. So adding it to ftrace
makes sense to me.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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