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

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Date:	Fri, 8 May 2015 10:41:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Drew Richardson <drew.richardson@....com>
Cc:	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>,
	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, 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?

-- Steve

> Signed-off-by: Drew Richardson <drew.richardson@....com>
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> ---
>  kernel/trace/trace.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 05330494a0df..458031c31a37 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -876,6 +876,7 @@ static struct {
>  	{ trace_clock_jiffies,		"uptime",	0 },
>  	{ trace_clock,			"perf",		1 },
>  	{ ktime_get_mono_fast_ns,	"mono",		1 },
> +	{ ktime_get_raw_fast_ns,	"mono_raw",	1 },
>  	ARCH_TRACE_CLOCKS
>  };
>  

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