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

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Date:	Mon, 28 Oct 2013 10:55:10 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Jiri Olsa <jolsa@...hat.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 7:53 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> 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
>
No, we try to poll the counter faster than it can possibly overflow.
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