linux-kernel - Re: [RFC] perf_events: ctx_flexible_sched

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Date:	Thu, 6 May 2010 16:41:46 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, mingo@...e.hu,
	Paul Mackerras <paulus@...ba.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] perf_events: ctx_flexible_sched_in() not maximizing PMU 
	utilization

On Thu, May 6, 2010 at 4:20 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2010-05-06 at 16:03 +0200, Stephane Eranian wrote:
>> Hi,
>>
>> Looking at ctx_flexible_sched_in(), the logic is that if  group_sched_in()
>> fails for a HW group, then no other HW group in the list is even tried.
>> I don't understand this restriction. Groups are independent of each other.
>> The failure of one group should not block others from being scheduled,
>> otherwise you under-utilize the PMU.
>>
>> What is the reason for this restriction? Can we lift it somehow?
>
> Sure, but it will make scheduling much more expensive. The current
> scheme will only ever check the first N events because it stops at the
> first that fails, and since you can max fix N events on the PMU its
> constant time.
>
You may fail not because the PMU is full but because an event is incompatible
with the others, i.e., there may still be room for more evens. By relying on the
RR to get coverage for all events, you also increase blind spots for
events which
have been skipped. Longer blind spots implies less accuracy when you scale.

> To fix this issue you'd have to basically always iterate all events and
> only stop once the PMU is fully booked, which reduces to an O(n) worst
> case algorithm.
>

Yes, but if you have X events and you don't know if you have at least N
that are compatible with each other, then you have to scan the whole list.

> But yeah, I did think of making the thing an RB-tree and basically
> schedule on service received, that should fix the lop-sided RR we get
> with constrained events.
>
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