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

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Message-ID: <5FBF8E85CA34454794F0F7ECBA79798F379D364860@HQMAIL04.nvidia.com>
Date:	Thu, 26 Jul 2012 14:16:51 -0700
From:	Peter Boonstoppel <pboonstoppel@...dia.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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()

> > 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.


PeterB--
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