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

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Message-ID: <alpine.LFD.2.02.1207261242240.32033@ionos>
Date:	Thu, 26 Jul 2012 12:47:48 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Tejun Heo <tj@...nel.org>,
	Peter Boonstoppel <pboonstoppel@...dia.com>,
	"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 Zijlstra wrote:
> On Wed, 2012-07-25 at 15:40 -0700, Tejun Heo wrote:
> > > This patch disables preemption during complete(), since we call
> > > schedule() directly afterwards, so it will correctly enter
> > > TASK_UNINTERRUPTIBLE. This speeds up kthread creation/binding during
> > > cpu hotplug significantly.
> 
> 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.

> > > +
> > > +	/*
> > > +	 * Disable preemption so we enter TASK_UNINTERRUPTIBLE after
> > > +	 * complete() instead of possibly being preempted. This speeds
> > > +	 * up clients that do a kthread_bind() directly after
> > > +	 * creation.
> > > +	 */
> > > +	preempt_disable();
> > 
> > Shouldn't this happen before setting current state to UNINTERRUPTIBLE?
> > What prevents preemption happening right above preempt_disable()?
> 
> Nothing, it also doesn't matter that much, you could get preempted right
> before preempt_disable() and end up in the same place.
> 
> The main thing is avoiding the wakeup preemption from the complete()
> because we're going to sleep right after anyway.
> 
> 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.
 
> > >  	complete(&create->done);
> > > +	preempt_enable_no_resched();
> > > +
> > >  	schedule();
> 
> Other than that it seems fine, although I know tglx just loves new
> preempt_enable_no_resched() sites ;-)

The ones which are immediately followed by a call to schedule() are at
least not causing any headache for RT :)

Thanks,

	tglx
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