Re: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()

[Don Zickus <dzickus@redhat.com>]

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