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Subject: Re: [patch 1/2] sched: check for prev_cpu == this_cpu in
 wake_affine()
From: Suresh Siddha <suresh.b.siddha@intel.com>
Reply-To: Suresh Siddha <suresh.b.siddha@intel.com>
To: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>,
       Arjan van de Ven <arjan@linux.jf.intel.com>,
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       Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
       Yanmin Zhang <yanmin_zhang@linux.jf.intel.com>,
       Gautham R Shenoy <ego@in.ibm.com>
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Organization: Intel Corp
Date: Mon, 08 Mar 2010 11:09:51 -0800
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hi Mike,

On Fri, 2010-03-05 at 11:36 -0800, Mike Galbraith wrote:
> Yeah, but with the 1 task + non-sync wakeup scenario, we miss the boat
> because select_idle_sibling() uses wake_affine() success as it's
> enabler.

But the wake_affine() decision is broken when this_cpu == prev_cpu. All
we need to do is to fix that, to recover that ~9% improvement.

> I have a slightly different patch for that in my tree.  There's no need
> to even call wake_affine() since the result is meaningless.

I don't think your below fix is correct because:


> -	if (affine_sd && wake_affine(affine_sd, p, sync))
> -		return cpu;
> +	if (affine_sd) {
> +		if (cpu == prev_cpu)
> +			return cpu;


by this time, we have overwritten cpu using the select_idle_sibling()
logic and cpu no longer points to this_cpu.

What we need is a comparison with this_cpu.

thanks,
suresh