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

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Message-ID: <20120105131908.GH31206@redhat.com>
Date:	Thu, 5 Jan 2012 15:19:08 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Stephane Eranian <eranian@...gle.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 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

--
			Gleb.
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