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Date: Mon, 5 Jan 2015 15:57:39 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Daniel Thompson <daniel.thompson@...aro.org> Cc: Russell King <linux@....linux.org.uk>, Will Deacon <will.deacon@....com>, Catalin Marinas <catalin.marinas@....com>, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, patches@...aro.org, linaro-kernel@...ts.linaro.org, John Stultz <john.stultz@...aro.org>, Sumit Semwal <sumit.semwal@...aro.org> Subject: Re: [PATCH v2 1/2] arm: perf: Prevent wraparound during overflow 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@...aro.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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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