Messages in this thread | | | From | "Liang, Kan" <> | Subject | RE: [PATCH 02/14] perf/x86: output NMI overhead | Date | Thu, 24 Nov 2016 19:40:21 +0000 |
| |
> > @@ -1492,8 +1507,10 @@ perf_event_nmi_handler(unsigned int cmd, > struct pt_regs *regs) > > start_clock = sched_clock(); > > ret = x86_pmu.handle_irq(regs); > > finish_clock = sched_clock(); > > + clock = finish_clock - start_clock; > > > > - perf_sample_event_took(finish_clock - start_clock); > > + perf_caculate_nmi_overhead(clock); > > + perf_sample_event_took(clock); > > Ah, so it's the *sampling* overhead, not the NMI overhead. > > This doesn't take into account the cost of entering/exiting the handler, > which could be larger than the sampling overhead (e.g. if the PMU is > connected through chained interrupt controllers). > > > enum perf_record_overhead_type { > > + PERF_NMI_OVERHEAD = 0, > > As above, it may be worth calling this PERF_SAMPLE_OVERHEAD; this
I think PERF_NMI stands for the NMI overhead in perf part.
PERF_SAMPLE_OVERHEAD looks too generic I think. It heard like the sum of all overheads in sampling. After all we collect the overhead in different stage of sampling. NMI handler, multiplexing, side-band events...
> doesn't count the entire cost of the NMI, and other architectures may want > to implement this, yet don't have NMI. >
I think I can change it to PERF_X86_NMI_OVERHEAD, if you think it's more clear. For other architectures, they can implement their own type of overhead, just ignore the NMI one.
> > @@ -1872,7 +1873,7 @@ __perf_remove_from_context(struct perf_event > > *event, { > > unsigned long flags = (unsigned long)info; > > > > - event_sched_out(event, cpuctx, ctx); > > + event_sched_out(event, cpuctx, ctx, false); > > if (flags & DETACH_GROUP) > > perf_group_detach(event); > > list_del_event(event, ctx); > > @@ -1918,9 +1919,9 @@ static void __perf_event_disable(struct > perf_event *event, > > update_cgrp_time_from_event(event); > > update_group_times(event); > > if (event == event->group_leader) > > - group_sched_out(event, cpuctx, ctx); > > + group_sched_out(event, cpuctx, ctx, true); > > else > > - event_sched_out(event, cpuctx, ctx); > > + event_sched_out(event, cpuctx, ctx, true); > > Why does this differ from __perf_remove_from_context()? >
Both of them are called on removing event. So I think we only need to log overhead in one place.
I just did some tests. It looks __perf_remove_from_context is called after __perf_event_disable. I think I will log overhead in __perf_remove_from_context for next version.
> What's the policy for when we do or do not measure overhead?
Currently, it's enabled all the time. Jirka suggested me to make it configurable. I will do it in next version. For next version, I still prefer to make it enable by default, since it doesn't bring additional overhead based on my test.
Thanks, Kan
| |