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

[Daniel Thompson]

On 04/12/14 10:26, Will Deacon wrote:
> 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@xxxxxxxxxx>
> 
>   Acked-by: Will Deacon <will.deacon@xxxxxxx>
> 
> You'll probably need to refresh this at -rc1 as there are a bunch of
> changes queued for this file already. Then you can stick it into rmk's
> patch system.

I'll do that. Thanks.


> 
> Cheers,
> 
> Will
> 
>> ---
>>  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;
>>  
>>  	local64_set(&hwc->prev_count, (u64)-left);
>>  
>> -- 
>> 1.9.3
>>
>>

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