Messages in this thread |  | | | Date | Fri, 7 Aug 2009 12:31:27 +0200 | | From | Ingo Molnar <> | | Subject | Re: [patch] x86, perf_counter, bts: add bts to perf_counter |
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* 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
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