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

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Date:   Tue, 6 Dec 2016 15:47:40 +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 Tue, Dec 06, 2016 at 03:02:20PM +0000, Liang, Kan wrote:
> >
> >
> > > 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.
> 
> It doesn't record anything, it generates the output. And it doesn't explain
> why that needs to be in pmu::del(), in general that's a horrible thing to do.

Yes, it only generate/log the output. Sorry for the confused wording.

The NMI overhead is pmu specific overhead. So the NMI overhead output
should be generated in pmu code.  
I assume that the pmu:del is the last called pmu function when perf finish.
Is it a good place for logging?

Thanks,
Kan