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

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Date:	Fri, 8 May 2015 09:05:24 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Drew Richardson <drew.richardson@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Wade Cherry <Wade.Cherry@....com>,
	Pawel Moll <Pawel.Moll@....com>
Subject: Re: [PATCHv3] ftrace: Provide trace clock monotonic raw

On Fri, May 8, 2015 at 7:41 AM, Steven Rostedt <rostedt@...dmis.org> 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?

Yea. No objections from me wrt the functionality of the patch.

I'd just again maybe caution folks to not go too far in assuming
CLOCK_MONOTONIC_RAW doesn't have frequency changes, as the underlying
hardware will drift due to varying thermal conditions.

thanks
-john
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