[CPU Hotplug PATCH] Restore Idle task's priority during CPU_DEAD notification

[CPU Hotplug PATCH] Restore Idle task's priority during CPU_DEAD notification

[Srivatsa Vaddagiri]

Linus,
	Patch below (against 2.6.7-rc1) fixes a CPU Hotplug problem wherein
idle task's "->prio" value is not restored to MAX_PRIO during CPU_DEAD 
handling. Without this patch, once a CPU is offlined and then later onlined, it 
becomes "more or less" useless (does not run any task other than its idle task!)
Please apply.


---

 linux-2.6.7-rc1-vatsa/kernel/sched.c |    1 +
 1 files changed, 1 insertion(+)

diff -puN kernel/sched.c~restore_idle_prio kernel/sched.c
--- linux-2.6.7-rc1/kernel/sched.c~restore_idle_prio	2004-05-25 17:04:19.000000000 +0530
+++ linux-2.6.7-rc1-vatsa/kernel/sched.c	2004-05-25 17:04:34.000000000 +0530
@@ -3569,6 +3569,7 @@ static int migration_call(struct notifie
 		rq = task_rq_lock(rq->idle, &flags);
 		deactivate_task(rq->idle, rq);
 		__setscheduler(rq->idle, SCHED_NORMAL, MAX_PRIO);
+		rq->idle->prio = MAX_PRIO;
 		task_rq_unlock(rq, &flags);
  		BUG_ON(rq->nr_running != 0);
 

_
On Wed, May 26, 2004 at 12:06:46PM +1000, Rusty Russell wrote:
> On Wed, 2004-05-26 at 02:35, Raj, Ashok wrote:
> > Thanks Vatsa...
> > 
> > It seems to work right now....
> 
> It seems obviously correct and harmless to me.  As it also fixes a
> problem, I'd say this should go to Linus & Andrew ASAP.
> 
> Thanks!
> Rusty.
> -- 
> Anyone who quotes me in their signature is an idiot -- Rusty Russell
> 

-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017

Re: [CPU Hotplug PATCH] Restore Idle task's priority during CPU_DEAD notification

[Ingo Molnar]

* Srivatsa Vaddagiri <vatsa@in.ibm.com> wrote:

> @@ -3569,6 +3569,7 @@ static int migration_call(struct notifie
>  		rq = task_rq_lock(rq->idle, &flags);
>  		deactivate_task(rq->idle, rq);
>  		__setscheduler(rq->idle, SCHED_NORMAL, MAX_PRIO);
> +		rq->idle->prio = MAX_PRIO;
>  		task_rq_unlock(rq, &flags);
>   		BUG_ON(rq->nr_running != 0);

Looks good. A small nit: while your patch creates a perfectly correct
idle thread too, i'd prefer the modified variant below. The
__setscheduler() call is (technically) incorrect because in the
SCHED_NORMAL case the prio should be zero. So it's a bit cleaner to set
up the static priority to MAX_PRIO and then revert the policy to
SCHED_NORMAL via __setscheduler(). Ok?

	Ingo

From: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

--- linux/kernel/sched.c.orig	
+++ linux/kernel/sched.c	
@@ -3566,7 +3566,8 @@ static int migration_call(struct notifie
 		/* Idle task back to normal (off runqueue, low prio) */
 		rq = task_rq_lock(rq->idle, &flags);
 		deactivate_task(rq->idle, rq);
-		__setscheduler(rq->idle, SCHED_NORMAL, MAX_PRIO);
+		rq->idle->static_prio = MAX_PRIO;
+		__setscheduler(rq->idle, SCHED_NORMAL, 0);
 		task_rq_unlock(rq, &flags);
  		BUG_ON(rq->nr_running != 0);
 

Re: [CPU Hotplug PATCH] Restore Idle task's priority during CPU_DEAD notification

[Srivatsa Vaddagiri]

On Wed, May 26, 2004 at 01:27:26PM +0200, Ingo Molnar wrote:
> Looks good. A small nit: while your patch creates a perfectly correct
> idle thread too, i'd prefer the modified variant below. The
> __setscheduler() call is (technically) incorrect because in the
> SCHED_NORMAL case the prio should be zero. So it's a bit cleaner to set
> up the static priority to MAX_PRIO and then revert the policy to
> SCHED_NORMAL via __setscheduler(). Ok?

Fine. Since we will have to setup the static priority everytime CPU_DEAD is
invoked, would it make sense if we setup this static priority (once and for
all) in init_idle instead?

Untested patch below:


---

 linux-2.6.7-rc1-vatsa/kernel/sched.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff -puN kernel/sched.c~restore_idle_prio kernel/sched.c
--- linux-2.6.7-rc1/kernel/sched.c~restore_idle_prio	2004-05-25 17:04:19.000000000 +0530
+++ linux-2.6.7-rc1-vatsa/kernel/sched.c	2004-05-26 17:17:44.000000000 +0530
@@ -3246,7 +3246,7 @@ void __devinit init_idle(task_t *idle, i
 	idle_rq->curr = idle_rq->idle = idle;
 	deactivate_task(idle, rq);
 	idle->array = NULL;
-	idle->prio = MAX_PRIO;
+	idle->prio = idle->static_prio = MAX_PRIO;
 	idle->state = TASK_RUNNING;
 	set_task_cpu(idle, cpu);
 	double_rq_unlock(idle_rq, rq);
@@ -3568,7 +3568,7 @@ static int migration_call(struct notifie
 		/* Idle task back to normal (off runqueue, low prio) */
 		rq = task_rq_lock(rq->idle, &flags);
 		deactivate_task(rq->idle, rq);
-		__setscheduler(rq->idle, SCHED_NORMAL, MAX_PRIO);
+		__setscheduler(rq->idle, SCHED_NORMAL, 0);
 		task_rq_unlock(rq, &flags);
  		BUG_ON(rq->nr_running != 0);
 

_



-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017