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

From: "Metzger, Markus T" <markus.t.metzger@intel.com>
To: Ingo Molnar <mingo@elte.hu>, Oleg Nesterov <oleg@redhat.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"markus.t.metzger@gmail.com" <markus.t.metzger@gmail.com>,
	"roland@redhat.com" <roland@redhat.com>,
	"eranian@googlemail.com" <eranian@googlemail.com>,
	"Villacis, Juan" <juan.villacis@intel.com>,
	"ak@linux.jf.intel.com" <ak@linux.jf.intel.com>
Subject: RE: [patch 3/21] x86, bts: wait until traced task has been scheduled out
Date: Wed, 1 Apr 2009 13:43:26 +0100	[thread overview]
Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E926D537D@irsmsx504.ger.corp.intel.com> (raw)
In-Reply-To: <20090401114140.GB23678@elte.hu>

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@elte.hu]
>Sent: Wednesday, April 01, 2009 1:42 PM
>To: Oleg Nesterov; Peter Zijlstra

>* Oleg Nesterov <oleg@redhat.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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