linux-kernel - Re: [PATCH 3/3] sched: newidle balance set idle

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Date:	Wed, 09 Feb 2011 04:37:21 +0100
From:	Mike Galbraith <efault@....de>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>, Nick Piggin <npiggin@...il.com>
Subject: Re: [PATCH 3/3] sched: newidle balance set idle_timestamp only on
 successful pull

On Tue, 2011-02-08 at 10:13 -0800, Venkatesh Pallipadi wrote:
> load_balance() could return a negative value in the case of
> SMT sibling CPU being busy. Code in idle_balance() though, uses this
> return value as an indicator of successful task pull, ignoring the
> -1 return value.

Yup, garden variety bug.

> This has two problems:
> 1) Resets idle_stamp even when this return value is -1.
> Specific case is on SMT capable system, CPU A is idle and its sibling
> CPU B is busy. In this case, CPU A avg_idle will not depend on
> a task sleeping/waking up on it. Instead it will continue to hold stale
> avg_idle value for extended period of time.

Not good.

Acked-by: Mike Galbraith <efault@....de>

	-Mike

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