linux-kernel - Re: [patch 3/3] sched: move sched_avg_update() to update_cpu

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Message-ID: <1281945634.1926.968.camel@laptop>
Date:	Mon, 16 Aug 2010 10:00:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org, chris@...stnet.net,
	debian00@...ceadsl.fr, hpa@...or.com, jonathan.protzenko@...il.com,
	mans@...sr.com, psastudio@...l.ru, rjw@...k.pl,
	stephan.eicher@....de, sxxe@....de, thomas@...hlinux.org,
	venki@...gle.com, wonghow@...il.com
Subject: Re: [patch 3/3] sched: move sched_avg_update() to update_cpu_load()

On Fri, 2010-08-13 at 12:45 -0700, Suresh Siddha wrote:
> plain text document attachment (move_sched_avg_update.patch)
> Currently sched_avg_update() (which updates rt_avg stats in the rq) is getting
> called from scale_rt_power() (in the load balance context) which doesn't take
> rq->lock.

Right, but I think it doesn't need to, as its never accessed cross CPU
(except maybe for /proc/sched_debug, and who cares about that).

The (other) odd case is NO_HZ idle balancing, but there the actual CPU
won't be updating the fields, so the remote CPU doing NO_HZ idle
balancing should be good to touch it.

> Fix it by moving the sched_avg_update() to more appropriate update_cpu_load()
> where the CFS load gets updated aswell.

Right, except it breaks things a bit, at the very least you really need
that update right before reading it, otherwise you can end up with >100%
fractions, which are odd indeed ;-)


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