Messages in this thread Patch in this message | | | Date | Mon, 5 Dec 2011 15:17:09 -0800 | From | Arun Sharma <> | Subject | Re: [RFC][PATCH 0/6] perf: x86 RDPMC and RDTSC support |
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On 12/5/11 12:16 PM, Arun Sharma wrote: > On 12/2/11 2:22 PM, Stephane Eranian wrote: >> Have you tried with inheritance disabled? >> I don't remember if perf stat has it enabled buy default. > > Disabling inheritance using the -i switch as in: > > (for i in `seq 1 10`; do numactl --cpunodebind 1 perf stat -i -e > instructions ./lat_ctx -P1 -s32k 4; done) > > shows numbers closer to baseline, since the threads used for > benchmarking are not counting events.
I spent some more time looking into this. Running:
perf stat -e instructions ./lat_ctx -P1 -s32k 4 -N 1000000 & perf record -ag -- sleep 3
didn't show high cycles counts on any perf_events related functions in the context switch path. The only PMU related stuff that showed up had to do with x86_pmu_enable called from an IPI.
So just as an experiment I disabled perf_rotate_context() via:
--- a/kernel/sched.c +++ b/kernel/sched.c @@ -4252,8 +4252,6 @@ void scheduler_tick(void) curr->sched_class->task_tick(rq, curr, 0); raw_spin_unlock(&rq->lock);
- perf_event_task_tick(); -
That seems to bring the lat_ctx numbers very close to baseline.
I suspect that some optimizations are possible in perf_rotate_context that don't involve enabling/disabling the PMU via an IPI for simple cases that involve one or two hardware events (eg: fixed counters).
-Arun Sample stack trace:
lat_ctx 29874 [012] 350729.824173: cycles: ffffffff8101149c intel_pmu_enable_all ([kernel.kallsyms]) ffffffff810115f7 intel_pmu_nhm_enable_all ([kernel.kallsyms]) ffffffff8100e877 x86_pmu_enable ([kernel.kallsyms]) ffffffff810a99de perf_pmu_enable ([kernel.kallsyms]) ffffffff8100d682 x86_pmu_commit_txn ([kernel.kallsyms]) ffffffff810aa4f9 group_sched_in ([kernel.kallsyms]) ffffffff810ab06d __perf_event_enable ([kernel.kallsyms]) ffffffff810a8c93 remote_function ([kernel.kallsyms]) ffffffff81065eec generic_smp_call_function_single_interrupt ([kernel.kallsyms]) ffffffff81018064 smp_call_function_single_interrupt ([kernel.kallsyms]) ffffffff81461fcb call_function_single_interrupt ([kernel.kallsyms]) 401ec4 bread (/root/lat_ctx)
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