linux-kernel - RE: [PATCH V2 03/13] perf/x86: output sampling overhead

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Date:   Tue, 6 Dec 2016 15:02:20 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "wangnan0@...wei.com" <wangnan0@...wei.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "andi@...stfloor.org" <andi@...stfloor.org>
Subject: RE: [PATCH V2 03/13] perf/x86: output sampling overhead



> On Fri, Dec 02, 2016 at 04:19:11PM -0500, kan.liang@...el.com wrote:
> > From: Kan Liang <kan.liang@...el.com>
> >
> > On x86, NMI handler is the most important part which brings overhead
> > for sampling. Adding a pmu specific overhead type
> > PERF_PMU_SAMPLE_OVERHEAD for it.
> >
> > For other architectures which may don't have NMI, the overhead type
> > can be reused.
> >
> > Signed-off-by: Kan Liang <kan.liang@...el.com>
> > ---
> >  arch/x86/events/core.c          | 17 ++++++++++++++++-
> >  arch/x86/events/perf_event.h    |  2 ++
> >  include/uapi/linux/perf_event.h |  1 +
> >  3 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index
> > 9d4bf3a..de40f96 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -1397,6 +1397,9 @@ static void x86_pmu_del(struct perf_event
> > *event, int flags)
> >
> >  	perf_event_update_userpage(event);
> >
> > +	if ((flags & PERF_EF_LOG) && cpuc->nmi_overhead.nr)
> > +		perf_log_overhead(event, PERF_PMU_SAMPLE_OVERHEAD,
> > +&cpuc->nmi_overhead);
> > +
> >  do_del:
> >  	if (x86_pmu.del) {
> >  		/*
> 
> That's not at all mentioned in the changelog, and it clearly isn't
> nmi_overhead.

Here it only records the overhead, not calculate.
The calculation is in nmi_hanlder as below. I will make it clear in the changelog.

@@ -1492,8 +1505,10 @@ perf_event_nmi_handler(unsigned int cmd, struct pt_regs *regs)
 	start_clock = sched_clock();
 	ret = x86_pmu.handle_irq(regs);
 	finish_clock = sched_clock();
+	clock = finish_clock - start_clock;
 
-	perf_sample_event_took(finish_clock - start_clock);
+	perf_calculate_nmi_overhead(clock);
+	perf_sample_event_took(clock);
 
 	return ret;
 }

Thanks,
Kan