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Message-ID: <515DFE1F.3010106@linaro.org>
Date: Thu, 04 Apr 2013 15:26:39 -0700
From: John Stultz <john.stultz@linaro.org>
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To: Stephane Eranian <eranian@google.com>
CC: David Ahern <dsahern@gmail.com>, 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>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples
References: <CABPqkBQALeD=iO9x-N0nw+shhqa1kmUaj=sCvx+MvoAPGQ-y9A@mail.gmail.com> <1350408232.2336.42.camel@laptop> <1359728280.8360.15.camel@hornet> <CABPqkBSVeU_JP2KpVZLepKDJX=-g6A45Y5MoNphd6+DaU2PQzQ@mail.gmail.com> <51118797.9080800@linaro.org> <alpine.LFD.2.02.1302182132230.22263@ionos> <5123C3AF.8060100@linaro.org> <1361356160.10155.22.camel@laptop> <51285BF1.2090208@linaro.org> <1361801441.4007.40.camel@laptop> <CABPqkBS4TELhoUGrO+moca2fPUoqGjiYCdHJc=H8CjLomPWWDQ@mail.gmail.com> <1363291021.3100.144.camel@hornet> <51586315.7080006@gmail.com> <5159D221.70304@linaro.org> <515A0A3A.2040105@gmail.com> <CABPqkBStMd-VT8aoPhFV-rxiLOrH7h2HtS9O9FBkd+ggd5X+xQ@mail.gmail.com> <515C34BF.6060508@gmail.com> <CABPqkBS_z8zzSn=N_Q+AGbo1kV2EMOoF4dz2Aze=rPNB2n_n2g@mail.gmail.com> <515C393C.8030405@gmail.com> <CABPqkBQjur31gpPWTavjSP5nbc7u8mQwqsRRctYiHnPXz0d=_A@mail.gmail.com> <515C6D87.2060106@linaro.org> <CABPqkBRaCgb3Gp8_DT_S5rd9BcpTVeKgY2Ozk_yyAQP4W4SR1Q@mail.gmail.com>
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On 04/04/2013 01:12 AM, Stephane Eranian wrote:
> On Wed, Apr 3, 2013 at 7:57 PM, John Stultz <john.stultz@linaro.org> wrote:
>> I'm not sure I follow this. If perf exported data came with CLOCK_MONOTONIC
>> timestamps, no correlation would need to be exposed.  perf would just have
>> to do the extra overhead of doing the conversion on export.
> There is no explicit export operation in perf.  You record a sample when
> the counter overflows and generates an NMI interrupt. In the NMI interrupt
> handler, the sample record is written to the sampling buffer. That is when
> the timestamp is generated. The sampling buffer is directly accessible to
> users via mmap(). The perf tool just dumps the raw sampling buffer into
> a file, no sample record is modified or even looked at. The processing
> of the samples is done offline (via perf report) and could be done on
> another machine. In other words, the perf.data file is self-contained.
Ah. Ok, I didn't realize perfs buffers were directly mmaped.  I was 
thinking perf could do the translation not at NMI time but when the 
buffer was later read by the application.  That helps explain some of 
the constraints.

thanks
-john