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Date: Fri, 22 Feb 2013 10:54:16 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Michael Wang <wangyun@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, Paul Turner <pjt@google.com>,
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        Ram Pai <linuxram@us.ibm.com>,
        "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
        Namhyung Kim <namhyung@kernel.org>
Subject: Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()
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* Mike Galbraith <efault@gmx.de> wrote:

> On Fri, 2013-02-22 at 09:36 +0100, Peter Zijlstra wrote: 
> > On Fri, 2013-02-22 at 10:37 +0800, Michael Wang wrote:
> > > But that's really some benefit hardly to be estimate, especially when
> > > the workload is heavy, the cost of wake_affine() is very high to
> > > calculated se one by one, is that worth for some benefit we could not
> > > promise?
> > 
> > Look at something like pipe-test.. wake_affine() used to 
> > ensure both client/server ran on the same cpu, but then I 
> > think we added select_idle_sibling() and wrecked it again :/
> 
> Yeah, that's the absolute worst case for 
> select_idle_sibling(), 100% synchronous, absolutely nothing to 
> be gained by cross cpu scheduling. Fortunately, most tasks do 
> more than that, but nonetheless, select_idle_sibling() 
> definitely is a two faced little b*tch.  I'd like to see the 
> evil b*tch die, but something needs to replace it's pretty 
> face.  One thing that you can do is simply don't call it when 
> the context switch rate is incredible.. its job is to recover 
> overlap, if you're scheduling near your max, there's no win 
> worth the cost.

Couldn't we make the cutoff dependent on sched_migration_cost? 
If the wakeup comes in faster than that then don't spread.

Thanks,

	Ingo