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Date: Wed, 3 Apr 2013 16:22:47 +0200
Message-ID: <CABPqkBQjur31gpPWTavjSP5nbc7u8mQwqsRRctYiHnPXz0d=_A@mail.gmail.com>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples
From: Stephane Eranian <eranian@google.com>
To: David Ahern <dsahern@gmail.com>
Cc: John Stultz <john.stultz@linaro.org>, Pawel Moll <pawel.moll@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, "mingo@elte.hu" <mingo@elte.hu>,
        Paul Mackerras <paulus@samba.org>, Anton Blanchard <anton@samba.org>,
        Will Deacon <Will.Deacon@arm.com>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        Pekka Enberg <penberg@gmail.com>, Steven Rostedt <rostedt@goodmis.org>
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On Wed, Apr 3, 2013 at 4:14 PM, David Ahern <dsahern@gmail.com> wrote:
> On 4/3/13 8:00 AM, Stephane Eranian wrote:
>>>
>>> What's the advantage of changing apps -- like the JIT compiler -- to emit
>>> perf based timestamps versus having perf emit existing timestamps? ie.,
>>> monotonic and realtime clocks already have vdso mappings for userspace
>>> with
>>> well known performance characteristics. Why not have perf convert its
>>> perf_clock timestamps into monotonic or realtime when dumping events?
>>>
>> Can monotonic timestamps be obtained from NMI context in the kernel?
>
>
> I don't understand the context of the question.
>
> I am not suggesting perf_clock be changed. I am working on correlating
> existing perf_clock timestamps to clocks typically used by apps (REALTIME
> and time-of-day but also applies to MONOTONIC).
>
But for that, you'd need to expose to users the correlation between
the two clocks.
And now you'd fixed two clock sources definitions not just one.

> You are wanting the reverse -- have apps emit perf_clock timestamps. I was
> just wondering what is the advantage of this approach?
>
Well, that's how I interpreted your question ;-<

If you could have perf_clock use monotonic then we would not have this
discussion.
The correlation would be trivial.