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

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Date:	Thu, 6 May 2010 08:44:53 +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 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
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