linux-kernel - Re: [PATCHv3] ftrace: Provide trace clock monotonic raw

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Message-ID: <20150508161102.GP27504@twins.programming.kicks-ass.net>
Date:	Fri, 8 May 2015 18:11:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Drew Richardson <drew.richardson@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <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: [PATCHv3] ftrace: Provide trace clock monotonic raw

On Fri, May 08, 2015 at 10:41:20AM -0400, Steven Rostedt wrote:
> On Fri, 8 May 2015 07:30:39 -0700
> Drew Richardson <drew.richardson@....com> wrote:
> 
> > Expose the NMI safe accessor to the monotonic raw clock to the
> > tracer. The mono clock was added with commit
> > 1b3e5c0936046e7e023149ddc8946d21c2ea20eb. The advantage of the
> > monotonic raw clock is that it will advance more constantly than the
> > monotonic clock.
> > 
> > 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 the monotonic clock. If NTP is adjusting the
> > monotonic clock to advance more rapidly, the program will appear to
> > use fewer instructions per second but run longer than if the monotonic
> > raw clock 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 the monotonic clock 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 let's
> > remove NTP from the list.
> > 
> > The monotonic raw clock can also be useful for tracing early boot,
> > e.g. when debugging issues with NTP.
> > 
> 
> Peter, Thomas, John, you OK with this?

Sure.
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