Re: [PATCH] arm: perf: Prevent wraparound during overflow

[Will Deacon]

On Wed, Nov 19, 2014 at 03:52:26PM +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@xxxxxxxxxx>
> ---
> 
> Notes:
>     There is similar code in the arm64 tree which retains the assumptions of
>     the original arm code regarding 32-bit wide performance counters. If
>     this patch doesn't get beaten up during review I'll also share a similar
>     patch for arm64.
>     
> 
>  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..b50a770f8c99 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 > (s64)(armpmu->max_period >> 1))
> +		left = armpmu->max_period >> 1;

The s64 cast looks off here, can we just drop it entirely?

Will
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