linux-kernel - Re: [patch 3/21] x86, bts: wait until traced task has been scheduled out

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Date:	Wed, 1 Apr 2009 14:53:14 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"roland@...hat.com" <roland@...hat.com>,
	"eranian@...glemail.com" <eranian@...glemail.com>,
	"Villacis, Juan" <juan.villacis@...el.com>,
	"ak@...ux.jf.intel.com" <ak@...ux.jf.intel.com>
Subject: Re: [patch 3/21] x86, bts: wait until traced task has been
	scheduled out


* Metzger, Markus T <markus.t.metzger@...el.com> wrote:

> >-----Original Message-----
> >From: Ingo Molnar [mailto:mingo@...e.hu]
> >Sent: Wednesday, April 01, 2009 1:42 PM
> >To: Oleg Nesterov; Peter Zijlstra
> 
> 
> >* Oleg Nesterov <oleg@...hat.com> wrote:
> >
> >> On 03/31, Markus Metzger wrote:
> >> >
> >> > +static void wait_to_unschedule(struct task_struct *task)
> >> > +{
> >> > +	unsigned long nvcsw;
> >> > +	unsigned long nivcsw;
> >> > +
> >> > +	if (!task)
> >> > +		return;
> >> > +
> >> > +	if (task == current)
> >> > +		return;
> >> > +
> >> > +	nvcsw  = task->nvcsw;
> >> > +	nivcsw = task->nivcsw;
> >> > +	for (;;) {
> >> > +		if (!task_is_running(task))
> >> > +			break;
> >> > +		/*
> >> > +		 * The switch count is incremented before the actual
> >> > +		 * context switch. We thus wait for two switches to be
> >> > +		 * sure at least one completed.
> >> > +		 */
> >> > +		if ((task->nvcsw - nvcsw) > 1)
> >> > +			break;
> >> > +		if ((task->nivcsw - nivcsw) > 1)
> >> > +			break;
> >> > +
> >> > +		schedule();
> >>
> >> schedule() is a nop here. We can wait unpredictably long...
> >>
> >> Ingo, do have have any ideas to improve this helper?
> >
> >hm, there's a similar looking existing facility:
> >wait_task_inactive(). Have i missed some subtle detail that makes it
> >inappropriate for use here?
> 
> wait_task_inactive() waits until the task is no longer 
> TASK_RUNNING.

No, that's wrong, wait_task_inactive() waits until the task 
deschedules.

	Ingo
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