linux-kernel - Re: [PATCH v3 4/4] perf,x86: add RAPL hrtimer support

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Message-ID: <20131026175305.GE1069@krava.brq.redhat.com>
Date:	Sat, 26 Oct 2013 19:53:05 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v3 4/4] perf,x86: add RAPL hrtimer support

On Sat, Oct 26, 2013 at 07:07:06PM +0200, Stephane Eranian wrote:
> On Fri, Oct 25, 2013 at 7:44 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> > On Wed, Oct 23, 2013 at 02:58:05PM +0200, Stephane Eranian wrote:

SNIP

> >> +     list_for_each_entry(event, &pmu->active_list, active_entry) {
> >> +             rapl_event_update(event);
> >> +     }
> >
> > hi,
> > I dont fully understand the reason for the timer,
> > I'm probably missing something..
> >
> The reason is rather simple and is similar to what happens with uncore.
> The counter are narrow, 32-bit and there is no interrupt capability. We
> need to poll the counters and accumulate in the sw counter to avoid missing
> an overflow.
> 
> > - the timer calls rapl_event_update for all defined events
> 
> No, only for the defined RAPL events which is what we want.

ok, that's what I meant

> 
> > - but rapl_pmu_event_read calls rapl_event_update any time the
> >   event is read (sys_read)
> >
> Yes, but we want to prevent missing a counter overflow. It may happen
> if the counter counts in a unit which increments fast.
> 
> > The rapl_event_update only read msr and updates
> > event->count|hw,prev_count.
> No, it does update the count:
>         local64_add(sdelta, &event->count);

ah, there's the shift that takes care of the
overflowed msr value.. ok

thanks,
jirka
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