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c46e665f03
On Broadwell INST_RETIRED.ALL cannot be used with any period that doesn't have the lowest 6 bits cleared. And the period should not be smaller than 128. Add a new callback to enforce this, and set it for Broadwell. This is erratum BDM57 and BDM11. How does this handle the case when an app requests a specific period with some of the bottom bits set The apps thinks it is sampling at X occurences per sample, when it is in fact at X - 63 (worst case). Short answer: Any useful instruction sampling period needs to be 4-6 orders of magnitude larger than 128, as an PMI every 128 instructions would instantly overwhelm the system and be throttled. So the +-64 error from this is really small compared to the period, much smaller than normal system jitter. Long answer: <write up by Peter:> IFF we guarantee perf_event_attr::sample_period >= 128. Suppose we start out with sample_period=192; then we'll set period_left to 192, we'll end up with left = 128 (we truncate the lower bits). We get an interrupt, find that period_left = 64 (>0 so we return 0 and don't get an overflow handler), up that to 128. Then we trigger again, at n=256. Then we find period_left = -64 (<=0 so we return 1 and do get an overflow). We increment with sample_period so we get left = 128. We fire again, at n=384, period_left = 0 (<=0 so we return 1 and get an overflow). And on and on. So while the individual interrupts are 'wrong' we get then with interval=256,128 in exactly the right ratio to average out at 192. And this works for everything >=128. So the num_samples*fixed_period thing is still entirely correct +- 127, which is good enough I'd say, as you already have that error anyhow. So no need to 'fix' the tools, al we need to do is refuse to create INST_RETIRED:ALL events with sample_period < 128. Signed-off-by: Andi Kleen <ak@linux.intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andi Kleen <ak@linux.intel.com> Cc: Arnaldo Carvalho de Melo <acme@kernel.org> Cc: Kan Liang <kan.liang@intel.com> Cc: Maria Dimakopoulou <maria.n.dimakopoulou@gmail.com> Cc: Mark Davies <junk@eslaf.co.uk> Cc: Stephane Eranian <eranian@google.com> Link: http://lkml.kernel.org/r/1409683455-29168-4-git-send-email-andi@firstfloor.org Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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