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

[Peter Zijlstra]

On Fri, Nov 21, 2014 at 04:24:26PM +0000, Daniel Thompson wrote:
> If the overflow threshold for a counter is set above or near the
> 0xffffffff boundary then the kernel may lose track of the overflow
> causing only events that occur *after* the overflow to be recorded.
> Specifically the problem occurs when the value of the performance counter
> overtakes its original programmed value due to wrap around.
> 
> Typical solutions to this problem are either to avoid programming in
> values likely to be overtaken or to treat the overflow bit as the 33rd
> bit of the counter.
> 
> Its somewhat fiddly to refactor the code to correctly handle the 33rd bit
> during irqsave sections (context switches for example) so instead we take
> the simpler approach of avoiding values likely to be overtaken.
> 
> We set the limit to half of max_period because this matches the limit
> imposed in __hw_perf_event_init(). This causes a doubling of the interrupt
> rate for large threshold values, however even with a very fast counter
> ticking at 4GHz the interrupt rate would only be ~1Hz.
> 
> Signed-off-by: Daniel Thompson <daniel.thompson at linaro.org>
> ---
>  arch/arm/kernel/perf_event.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> 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?