linux-kernel - Re: perf_events: proposed fix for broken intr throttling (repost)

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Message-ID: <CABPqkBTBczzc96zMbvYMG7xQJwxa1nc429XDTPxfc3Rah46Q1g@mail.gmail.com>
Date:	Thu, 5 Jan 2012 13:31:27 +0000
From:	Stephane Eranian <eranian@...gle.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, asharma@...com,
	vince@...ter.net, wcohen@...hat.com
Subject: Re: perf_events: proposed fix for broken intr throttling (repost)

On Thu, Jan 5, 2012 at 1:19 PM, Gleb Natapov <gleb@...hat.com> wrote:
> On Thu, Jan 05, 2012 at 01:08:41PM +0000, Stephane Eranian wrote:
>> Peter,
>>
>> I looked into this some more this morning. I don't think your proposed
>> scheme can work.
>> Unless, I misunderstood you, you were suggesting that we could perhaps
>> use a lazy
>> approach in perf_event_task_tick() and walk the event list only when
>> we have, at least,  one
>> event to unthrottle, i.e., similar to what is done with nr_freq. That
>> cannot work. The problem is
>> that you'd let all events get throttled before you'd unthrottle them
>> in the next timer tick.
>> At each overflow, hwc->interrupt would get incremented until it
>> reached MAX_INTERRUPTS.
>> Then, the event would be stopped (throttled), you'd do
>> ctx->nr_throttled = 1. At the next
>> timer tick, perf_event_task_tick() would then unthrottle the event. In
>> that scheme, the
>> event would be throttled for at most a tick. But in fact, the event
>> never generated that
>> many overflows/tick to justify throttling.
>>
>> I think there is no other way than what I suggested in my initial email:
>>   1- revert the nr_freq optimization
>>   2- reset hwc->interrupt on all events at each tick
>>
> I think my original patch did that: https://lkml.org/lkml/2011/11/15/114
>
Yes, looks like it, because it is systematically calling perf_ctx_adjust_freq()
which does reset the hwc->interrupts counter on ALL events.

But the pmu_disabled trick is not very pretty. We have to find some
middle ground
solution here.
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