linux-kernel - Re: [BUG 2.6.25-rc3] scheduler/hotplug: some processes are dealocked when cpu is set to offline

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Message-Id: <20080307121822.54b8c2fb.akpm@linux-foundation.org>
Date:	Fri, 7 Mar 2008 12:18:22 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
Cc:	ego@...ibm.com, mingo@...e.hu, oleg@...sign.ru,
	yi.y.yang@...el.com, linux-kernel@...r.kernel.org, rjw@...k.pl,
	tglx@...utronix.de
Subject: Re: [BUG 2.6.25-rc3] scheduler/hotplug: some processes are
 dealocked when cpu is set to offline

On Fri, 7 Mar 2008 14:02:20 +0100
"Dmitry Adamushko" <dmitry.adamushko@...il.com> wrote:

> Hi,
> 
> 'watchdog' is of SCHED_FIFO class. The standard load-balancer doesn't
> move RT tasks between cpus anymore and there is a special mechanism in
> scher_rt.c instead (I think, it's .25 material).
> 
> So I wonder, whether __migrate_task() is still capable of properly
> moving a RT task to another CPU (e.g. for the case when it's in
> TASK_RUNNING state) without breaking something in the rt migration
> mechanism (or whatever else) that would leave us with a runqueue in
> the 'inconsistent' state...
> (I've taken a quick look at the relevant code so can't confirm it yet)
> 
> maybe it'd be faster if somebody could do a quick test now with the
> following line commented out in kernel/softlockup.c :: watchdog()
> 
> -         sched_setscheduler(current, SCHED_FIFO, &param);
> 

Yup, thanks.  This:


 kernel/softirq.c      |    2 +-
 kernel/softlockup.c   |    2 +-
 kernel/stop_machine.c |    2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff -puN kernel/softlockup.c~a kernel/softlockup.c
--- a/kernel/softlockup.c~a
+++ a/kernel/softlockup.c
@@ -211,7 +211,7 @@ static int watchdog(void *__bind_cpu)
 	struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
 	int this_cpu = (long)__bind_cpu;
 
-	sched_setscheduler(current, SCHED_FIFO, &param);
+//	sched_setscheduler(current, SCHED_FIFO, &param);
 
 	/* initialize timestamp */
 	touch_softlockup_watchdog();
diff -puN kernel/stop_machine.c~a kernel/stop_machine.c
--- a/kernel/stop_machine.c~a
+++ a/kernel/stop_machine.c
@@ -188,7 +188,7 @@ struct task_struct *__stop_machine_run(i
 		struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
 
 		/* One high-prio thread per cpu.  We'll do this one. */
-		sched_setscheduler(p, SCHED_FIFO, &param);
+//		sched_setscheduler(p, SCHED_FIFO, &param);
 		kthread_bind(p, cpu);
 		wake_up_process(p);
 		wait_for_completion(&smdata.done);
diff -puN kernel/softirq.c~a kernel/softirq.c
--- a/kernel/softirq.c~a
+++ a/kernel/softirq.c
@@ -622,7 +622,7 @@ static int __cpuinit cpu_callback(struct
 
 		p = per_cpu(ksoftirqd, hotcpu);
 		per_cpu(ksoftirqd, hotcpu) = NULL;
-		sched_setscheduler(p, SCHED_FIFO, &param);
+//		sched_setscheduler(p, SCHED_FIFO, &param);
 		kthread_stop(p);
 		takeover_tasklets(hotcpu);
 		break;
_

fixes the wont-power-off regression.

But 2.6.24 runs the watchdog threads SCHED_FIFO too.  Are you saying that
it's the migration code which changed? 
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