Re: [patch] x86, perf_counter, bts: add bts to perf_counter

[Ingo Molnar <mingo@elte.hu>]

* Metzger, Markus T <markus.t.metzger@intel.com> wrote:

> >-----Original Message-----
> >From: Peter Zijlstra [mailto:a.p.zijlstra@chello.nl]
> >Sent: Friday, August 07, 2009 10:21 AM
> >To: Metzger, Markus T
> >Cc: Ingo Molnar; tglx@linutronix.de; hpa@zytor.com; markus.t.metzger@gmail.com; linux-
> >kernel@vger.kernel.org
> >Subject: RE: [patch] x86, perf_counter, bts: add bts to perf_counter
> >
> >On Fri, 2009-08-07 at 08:29 +0100, Metzger, Markus T wrote:
> >
> >> I incorporated Peter's review comments, except that I would not enforce sample_period == 1
> >> when branch tracing is requested. There might be users who want to sample the IP every 100.000'th
> >> branch for profiling reasons.
> >
> >But in case you don't set sample_period==1 then you won't be able to
> >match the BTS counter:
> >
> >+       if (unlikely((event ==
> >+                     x86_pmu.event_map(PERF_COUNT_HW_BRANCH_INSTRUCTIONS)) &&
> >+                    (hwc->sample_period == 1)))
> >+               return X86_PMC_IDX_FIXED_BTS;
> >
> >Also,
> >
> >+               /*
> >+                * Try to use BTS for branch tracing. If that is not
> >+                * available, try to get a generic counter.
> >+                */
> >+               if (unlikely(!cpuc->ds))
> >+                       goto try_generic;
> >
> >How will be use a generic counter for BTS, will it generate an NMI for
> >every encountered branch? That might very will hit the throttle as that
> >might be many.
> >
> >Would it not be better to force sample_period==1 usage onto the BTS and
> >simply fail if its not available?
> 
> In case someone requests a bigger sample-period, we would use the 
> normal counter - as we do without this patch. We would also use 
> the normal counter in case BTS is not available.
> 
> In that case, we won't have BTS, we will have normal performance 
> monitoring. It will be throttled just like any other sampling 
> request with small sample_period.
> 
> If we forced sample_period = 1 for 
> PERF_COUNT_HW_BRANCH_INSTRUCTIONS, we would remove functionality.

i'm inclined to agree with you.

period=1 branch execution sampling without BTS might be seriously 
limited, but it could still be (borderline) applied to user mode 
tasks and the result ought to be usable as a crude poor man's trace 
of execution. So no matter how sucky it may be, we shouldnt limit 
it.

	Ingo