Messages in this thread | | | Date | Thu, 30 Apr 2015 10:51:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] x86, perf: Add an aperfmperf driver |
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On Wed, Apr 29, 2015 at 06:17:05PM -0700, Andy Lutomirski wrote: > >> > + /* no sampling */ > >> > + if (event->hw.sample_period) > >> > + return -EINVAL; > >> > >> You could have set pmu::capabilities = > >> PERF_PMU_CAP_NO_INTERRUPT which would also have killed that dead. > > > > > > That checks attr.sample_period. I'm a bit confused about the > > relationship between event->hw and event->attr. Do I not need to > > check hw.sample_period?
event->attr is the perf_event_attr used to instantiate the event. event->hw is the hardware/working state of the event.
You'll notice that attr::sample_period is part of a union and when !attr::freq will be used as the actual hw::sample_period. However when attr::freq we'll compute hw::sample_period based on actual event rates such that we'll approx attr::sample_freq.
Setting pmu::capabilities = PERF_PMU_CAP_NO_INTERRUPT would be the best solution here.
> > Before I submit v2, do you think this is actually worth doing? I can > > see it being useful for answering questions like "did this workload > > end up running at full speed". > > > > To clarify, this is partially redundant with "cpu-cycles" and > "ref-cycles". That being said, these are simpler, actually documented > as being appropriate for measuring cpu performance states, and don't > have any scheduling constraints.
On the whole useful question; I dunno. It seems like something worth providing for the reasons you state. But I don't really get around to doing much userspace these days so I might not be the best to answer this.
Also, you could extend this with IA32_PPERF (Skylake and later, see SDM-201501 book 3 section 14.4.5.1).
> Also, is perf stat able to count while idle? perf stat -a -e > cpu-cycles sleep 1 reports very small numbers.
Yes, perf stat -a (iow cpu events) should count while idle, note however that not all events count during halt, so its very much event dependent.
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