linux-kernel - Re: [bisected] Clocksource tsc unstable git

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Date:	Tue, 09 Nov 2010 14:45:33 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	john stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...64.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andreas Herrmann <andreas.herrmann3@....com>, avi@...hat.com,
	mtosatti@...hat.com
Subject: Re: [bisected] Clocksource tsc unstable git

On Tue, 2010-11-09 at 14:21 +0100, Markus Trippelsdorf wrote:
> 
> You've missed the fact that Peter already has a patch that fixes the
> problem, but he never bothered to post it in this thread.
> 
Right, forgot about that, sorry.

Still Heiko has a good point, I've queued the below after I realized
that PREEMPT_ACTIVE would still require two tests and the on_rq bit is
actually sufficient.

---
Subject: sched: Fix runnable condition for stoptask
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Date: Tue Nov 09 14:36:52 CET 2010

Heiko reported that the TASK_RUNNING check is not sufficient for
CONFIG_PREEMPT=y since we can get preempted with !TASK_RUNNING.

He suggested adding a ->se.on_rq test to the existing TASK_RUNNING
one, however TASK_RUNNING will always have ->se.on_rq, so we might as
well reduce that to a single test.

Reported-by: Heiko Carstens <heiko.carstens@...ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
LKML-Reference: <new-submission>
---
 kernel/sched_stoptask.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/kernel/sched_stoptask.c
===================================================================
--- linux-2.6.orig/kernel/sched_stoptask.c
+++ linux-2.6/kernel/sched_stoptask.c
@@ -26,7 +26,7 @@ static struct task_struct *pick_next_tas
 {
 	struct task_struct *stop = rq->stop;
 
-	if (stop && stop->state == TASK_RUNNING)
+	if (stop && stop->se.on_rq)
 		return stop;
 
 	return NULL;

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