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

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E926D537D@irsmsx504.ger.corp.intel.com>
Date:	Wed, 1 Apr 2009 13:43:26 +0100
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	"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

>-----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.

I need to wait until the task has been scheduled out at least once.


regards,
markus.

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