linux-kernel - Re: [patch] sched: fix broken smt/mc optimizations with CFS

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Date:	Thu, 23 Aug 2007 10:42:46 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [patch] sched: fix broken smt/mc optimizations with CFS

On Thu, 2007-08-23 at 14:13 +0200, Ingo Molnar wrote:
> * Ingo Molnar <mingo@...e.hu> wrote:
> 
> > [...] So how about the patch below instead?
> 
> the right patch attached.
> 
> -------------------------------->
> Subject: sched: fix broken SMT/MC optimizations
> From: "Siddha, Suresh B" <suresh.b.siddha@...el.com>
> 
> On a four package system with HT - HT load balancing optimizations were
> broken.  For example, if two tasks end up running on two logical threads
> of one of the packages, scheduler is not able to pull one of the tasks
> to a completely idle package.
> 
> In this scenario, for nice-0 tasks, imbalance calculated by scheduler
> will be 512 and find_busiest_queue() will return 0 (as each cpu's load
> is 1024 > imbalance and has only one task running).

Is there an upper bound on the number of tasks that can migrate during a
new idle balance? The reason that I'm asking is because I've been seeing
some large latency in the new idle path into load_balance().. I'm not
sure if lots of tasks are getting migrated or if it's just the iteration
over tasks in the rb-tree ..

Daniel

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