> > @@ -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
