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SubjectRe: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()
On Sat, Oct 26, 2013 at 12:36:52PM +0200, Ingo Molnar wrote:
>
> * Don Zickus <dzickus@redhat.com> wrote:
>
> > On Thu, Oct 24, 2013 at 12:52:06PM +0200, Peter Zijlstra wrote:
> > > On Wed, Oct 23, 2013 at 10:48:38PM +0200, Peter Zijlstra wrote:
> > > > I'll also make sure to test we actually hit the fault path
> > > > by concurrently running something like:
> > > >
> > > > while :; echo 1 > /proc/sys/vm/drop_caches ; done
> > > >
> > > > while doing perf top or so..
> > >
> > > So the below appears to work; I've ran:
> > >
> > > while :; do echo 1 > /proc/sys/vm/drop_caches; sleep 1; done &
> > > while :; do make O=defconfig-build/ clean; perf record -a -g fp -e cycles:pp make O=defconfig-build/ -s -j64; done
> > >
> > > And verified that the if (in_nmi()) trace_printk() was visible in the
> > > trace output verifying we indeed took the fault from the NMI code.
> > >
> > > I've had this running for ~ 30 minutes or so and the machine is still
> > > healthy.
> > >
> > > Don, can you give this stuff a spin on your system?
> >
> > Hi Peter,
> >
> > I finally had a chance to run this on my machine. From my
> > testing, it looks good. Better performance numbers. I think my
> > longest latency went from 300K cycles down to 150K cycles and very
> > few of those (most are under 100K cycles).
>
> Btw., do we know where those ~100k-150k cycles are spent
> specifically? 100k cycles is still an awful lot of time to spend in
> NMI context ...

I agree, there is still a bunch of latency in the nmi path. I believe it
is still in the pebs code. I share the machine with a colleague right
now, so I haven't been able to isolate it.

But going from a few hundred samples over a million cycles to about a
couple dozen over 100K was a big step. :-)

I still see perf throttling and people are complaining about it here so I
still plan to keep investigating. Just taking me a while. Sorry about
that.

Cheers,
Don


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