linux-kernel - Re: [tip:perf/core] perf: Ignore non-sampling overflows

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Date:	Tue, 28 Jun 2011 13:56:03 +0200
From:	Francis Moreau <francis.moro@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Robert Richter <robert.richter@....com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [tip:perf/core] perf: Ignore non-sampling overflows

On Tue, Jun 28, 2011 at 1:05 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2011-06-28 at 12:53 +0200, Robert Richter wrote:
>> > --- a/kernel/perf_event.c
>> > +++ b/kernel/perf_event.c
>> > @@ -4240,6 +4240,13 @@ static int __perf_event_overflow(struct perf_event *event, int nmi,
>> >     struct hw_perf_event *hwc = &event->hw;
>> >     int ret = 0;
>> >
>> > +   /*
>> > +    * Non-sampling counters might still use the PMI to fold short
>> > +    * hardware counters, ignore those.
>> > +    */
>> > +   if (unlikely(!is_sampling_event(event)))
>> > +           return 0;
>> > +
>
>> do you remember the background of this change. This check silently
>> drops data of non-sampling events. I want to use perf_event_overflow()
>> to write to the buffer and want to modify the check, but don't see
>> which 'accidentally' interrupts may occur that must be ignored.
>
> IIRC this is because we always program the interrupt bit, such that when
> the counter overflows we can account and reprogram the thing. This is
> needed because no hardware counter is in fact 64 bits wide. Therefore we
> have to program the counter to its max width and properly account the
> state and reprogram on overflow.
>
> Imagine a 32bit cycle counter (@1GHz), if we were not to program that as
> taking interrupts and nobody would read that counter for about 4.2
> seconds, we'd have overflowed and lost the actual count value for the
> thing.
>
> So what we do is program is at 31bits (so that the msb can toggle and
> trigger the interrupt), and on interrupt add to event->count, and reset
> the hardware to start counting again.
>
> Now some arch/*/perf_event.c implementations unconditionally called
> perf_event_overflow() from their IRQ handler, even for such non-sampling
> counters.

Yes that's what I recall too.

-- 
Francis
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