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Date: Wed, 1 Aug 2012 09:14:22 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Boonstoppel <pboonstoppel@nvidia.com>
cc: Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>,
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        Diwakar Tundlam <dtundlam@nvidia.com>, Oleg Nesterov <oleg@redhat.com>,
        Ingo Molnar <mingo@elte.hu>
Subject: RE: [PATCH 1/1] kthread: disable preemption during complete()
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On Thu, 26 Jul 2012, Peter Boonstoppel wrote:

> > > tglx has patches that make the kthread create/destroy stuff from hotplug
> > > go away.. that seems like the better approach.
> 
> > Right. That cpu hotplug setup/teardown stuff is ugly.
> 
> If that stuff gets removed complete that's great. The only change I'm aware of right now is the workqueue one: http://thread.gmane.org/gmane.linux.kernel/1329164
> 
> > > The main thing is avoiding the wakeup preemption from the complete()
> > > because we're going to sleep right after anyway.
> 
> You are very likely to be preempted by the complete(), since the newly created thread has a relatively high vruntime.
> 
> > > The comment doesn't really make that clear.
> 
> > Right, the comment is crap. It has nothing to do with kthread_bind()
> > and stuff. The whole purpose is to avoid the pointless preemption
> > after wakeup.
> 
> The only case I want to solve is the kthread_bind()->wait_task_inactive() scenario. On our platforms this patch reduces average cpu_up() time from about 9ms to 8ms, but max time goes down from 37ms to 8.5ms. cpu_up() latency becomes much more predictable.
> 

There is a bunch of patches in the queue, which kills the full
setup/teardown of per cpu threads and puts those threads into a "park"
position instead.

Thanks,

	tglx