linux-kernel - Re: [PATCH -tip] x86,perf: P4 PMU -- protect sensible procedures from preemption

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Date:	Thu, 6 May 2010 09:42:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH -tip] x86,perf: P4 PMU -- protect sensible procedures
 from preemption


* Cyrill Gorcunov <gorcunov@...il.com> wrote:

> On Thursday, May 6, 2010, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Cyrill Gorcunov <gorcunov@...il.com> wrote:
> >
> >> On Wed, May 05, 2010 at 06:57:34PM +0200, Frederic Weisbecker wrote:
> >> ...
> >> > > @@ -741,7 +743,7 @@ static int p4_pmu_schedule_events(struct
> >> > > ?{
> >> > > ? unsigned long used_mask[BITS_TO_LONGS(X86_PMC_IDX_MAX)];
> >> > > ? unsigned long escr_mask[BITS_TO_LONGS(ARCH_P4_TOTAL_ESCR)];
> >> > > - int cpu = raw_smp_processor_id();
> >> > > + int cpu = get_cpu();
> >> > > ? struct hw_perf_event *hwc;
> >> > > ? struct p4_event_bind *bind;
> >> > > ? unsigned int i, thread, num;
> >> > > @@ -777,6 +779,7 @@ reserve:
> >> > > ? }
> >> > >
> >> > > ?done:
> >> > > + put_cpu();
> >> > > ? return num ? -ENOSPC : 0;
> >> > > ?}
> >> >
> >> > That's no big deal. But I think the schedule_events() is called on
> >> > pmu::enable() time, when preemption is already disabled.
> >> >
> >>
> >> We'll be on a safe side using get/put_cpu here (ie in case
> >> if something get changed one day).
> >
> > hm, when 'something gets changed one day' we'll see a warning when using
> > unsafe primitives.
> >
> > So if preemption is always off here we really should not add extra runtime
> > overhead via get_cpu()/put_cpu().
> >
> > So wouldnt it be better (and faster) to disable preemption in
> > hw_perf_event_init(), which seems to be the bit missing?
> >
> >  ? ? ? ?Ingo
> >
> 
> the thing are that p4 is only snippet here which is sensible to preemtion, 
> and hw_perf_event_init is executing with preemtion off (but i could miss the 
> details here, dont have code under my hands at moment, so PeterZ help is 
> needed ;) but more important reason why i've saved get/put here is that 
> otherwise i would not have rights to put tested-by tag, since it would not 
> be the patch Steven has tested. We could make a patch on top of this one, or 
> we could drop this one, make new with explicit preemt off in caller and use 
> smp_processor_id in p4 schedule routine. What is preferred?

We want the one with the least runtime overhead. These are instrumentation 
routines, so we want to optimize them as much as possible.

Thanks,

	Ingo
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