linux-kernel - RE: [PATCH 02/14] perf/x86: output NMI overhead

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Date:   Thu, 24 Nov 2016 19:40:21 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Mark Rutland <mark.rutland@....com>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "wangnan0@...wei.com" <wangnan0@...wei.com>,
        "andi@...stfloor.org" <andi@...stfloor.org>
Subject: RE: [PATCH 02/14] perf/x86: output NMI overhead



> > @@ -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