Re: [PATCH v2 1/2] arm: perf: Prevent wraparound during overflow

[Daniel Thompson <daniel.thompson@linaro.org>]

On Mon, Jan 05, 2015 at 03:57:39PM +0100, Peter Zijlstra wrote:
> On Fri, Nov 21, 2014 at 04:24:26PM +0000, Daniel Thompson wrote:
> > diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
> > index 266cba46db3e..ab68833c1e31 100644
> > --- a/arch/arm/kernel/perf_event.c
> > +++ b/arch/arm/kernel/perf_event.c
> > @@ -115,8 +115,14 @@ int armpmu_event_set_period(struct perf_event *event)
> >  		ret = 1;
> >  	}
> >  
> > -	if (left > (s64)armpmu->max_period)
> > -		left = armpmu->max_period;
> > +	/*
> > +	 * Limit the maximum period to prevent the counter value
> > +	 * from overtaking the one we are about to program. In
> > +	 * effect we are reducing max_period to account for
> > +	 * interrupt latency (and we are being very conservative).
> > +	 */
> > +	if (left > (armpmu->max_period >> 1))
> > +		left = armpmu->max_period >> 1;
> 
> On x86 we simply half max_period, why did you choose to do differently?

In truth because I didn't look at the x86 code... there is an existing
halving of max_period in the arm code and that was enough to satisfy me
that halving max_period was reasonable.

Predividing max_period looks to me like it would work for ARM too although I
don't think we could blame hardware insanity for doing so ;-).

Will: Do you want me to update this?

-- 
Daniel Thompson (STMicroelectronics) <daniel.thompson@st.com>
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