linux-kernel - RE: [PATCH 1/1] kthread: disable preemption during complete()

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Message-ID: <alpine.LFD.2.02.1208010913030.32033@ionos>
Date:	Wed, 1 Aug 2012 09:14:22 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Boonstoppel <pboonstoppel@...dia.com>
cc:	Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Henrique de Moraes Holschuh <ibm-acpi@....eng.br>,
	Andy Walls <awalls@...metrocast.net>,
	Diwakar Tundlam <dtundlam@...dia.com>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>
Subject: RE: [PATCH 1/1] kthread: disable preemption during complete()

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
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