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Subject: Re: [GIT PULL] perf crash fix
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Paul Mackerras <paulus@samba.org>,
       Stephane Eranian <eranian@google.com>
In-Reply-To: <1275552159.27810.34944.camel@twins>
References: <1275534810-1837-1-git-send-regression-fweisbec@gmail.com>
	 <1275552159.27810.34944.camel@twins>
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Date: Thu, 03 Jun 2010 11:08:04 +0200
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On Thu, 2010-06-03 at 10:02 +0200, Peter Zijlstra wrote:
> On Thu, 2010-06-03 at 05:13 +0200, Frederic Weisbecker wrote:
> 
> > diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> > index 858f56f..b666d7d 100644
> > --- a/kernel/perf_event.c
> > +++ b/kernel/perf_event.c
> > @@ -1510,20 +1510,16 @@ do {					\
> >  	return div64_u64(dividend, divisor);
> >  }
> >  
> > -static void perf_event_stop(struct perf_event *event)
> > +static void perf_event_stop_hwevent(struct perf_event *event)
> >  {
> > -	if (!event->pmu->stop)
> > -		return event->pmu->disable(event);
> > -
> > -	return event->pmu->stop(event);
> > +	if (event->pmu->stop && !is_software_event(event))
> > +		return event->pmu->stop(event);
> >  }
> >  
> > -static int perf_event_start(struct perf_event *event)
> > +static int perf_event_start_hwevent(struct perf_event *event)
> >  {
> > -	if (!event->pmu->start)
> > -		return event->pmu->enable(event);
> > -
> > -	return event->pmu->start(event);
> > +	if (event->pmu->start && !is_software_event(event))
> > +		return event->pmu->start(event);
> >  }
> >  
> >  static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
> > @@ -1546,9 +1542,9 @@ static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)
> >  
> >  	if (atomic64_read(&hwc->period_left) > 8*sample_period) {
> >  		perf_disable();
> > -		perf_event_stop(event);
> > +		perf_event_stop_hwevent(event);
> >  		atomic64_set(&hwc->period_left, 0);
> > -		perf_event_start(event);
> > +		perf_event_start_hwevent(event);
> >  		perf_enable();
> >  	}
> >  }

Your patch actually breaks some hardware pmu implementations.

> Urhm,. isn't is much easier to simply give the software events a NOP
> stop/start callback?
> 
>  kernel/perf_event.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index 858f56f..16c99e1 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -4276,9 +4276,15 @@ static void perf_swevent_disable(struct perf_event *event)
>  	hlist_del_rcu(&event->hlist_entry);
>  }
>  
> +static void perf_swevent_nop(struct perf_event *event)
> +{
> +}
> +
>  static const struct pmu perf_ops_generic = {
>  	.enable		= perf_swevent_enable,
>  	.disable	= perf_swevent_disable,
> +	.start		= perf_swevent_nop,
> +	.stop		= perf_swevent_nop,
>  	.read		= perf_swevent_read,
>  	.unthrottle	= perf_swevent_unthrottle,
>  };
>