[PATCH] sched: Fix boolean expression in move_tasks()

[PATCH] sched: Fix boolean expression in move_tasks()

[Peter Williams]

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Problem:

In the patch 
sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch
I got a the sense of a boolean expression wrong when assigning a value 
to skip_for_load.  The expression should have been negated before being 
assigned.

Solution:

Negate the expression and apply de Marcos rule to simplify it.  This 
patch is on top of 
sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch

Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>

-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce

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Index: MM-2.6.17-rc1-mm3/kernel/sched.c
===================================================================
--- MM-2.6.17-rc1-mm3.orig/kernel/sched.c	2006-04-21 12:26:54.000000000 +1000
+++ MM-2.6.17-rc1-mm3/kernel/sched.c	2006-04-25 09:09:54.000000000 +1000
@@ -2108,7 +2108,7 @@ skip_queue:
 	 */
 	skip_for_load = tmp->load_weight > rem_load_move;
 	if (skip_for_load && idx < this_best_prio)
-		skip_for_load = busiest_best_prio_seen || idx != busiest_best_prio;
+		skip_for_load = !busiest_best_prio_seen && idx == busiest_best_prio;
 	if (skip_for_load ||
 	    !can_migrate_task(tmp, busiest, this_cpu, sd, idle, &pinned)) {
 		if (curr != head)

Re: [PATCH] sched: Fix boolean expression in move_tasks()

[Andrew Morton]

Peter Williams <pwil3058@bigpond.net.au> wrote:
>
> Negate the expression and apply de Marcos rule to simplify it.  This 
>  patch is on top of 
>  sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch
> 
>  Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
> 
>  -- 
>  Peter Williams                                   pwil3058@bigpond.net.au
> 
>  "Learning, n. The kind of ignorance distinguishing the studious."
>    -- Ambrose Bierce
> 
> 
> [smpnice-fix-boolean-expression  text/plain (824 bytes)]
>  Index: MM-2.6.17-rc1-mm3/kernel/sched.c
>  ===================================================================
>  --- MM-2.6.17-rc1-mm3.orig/kernel/sched.c	2006-04-21 12:26:54.000000000 +1000
>  +++ MM-2.6.17-rc1-mm3/kernel/sched.c	2006-04-25 09:09:54.000000000 +1000
>  @@ -2108,7 +2108,7 @@ skip_queue:
>   	 */
>   	skip_for_load = tmp->load_weight > rem_load_move;
>   	if (skip_for_load && idx < this_best_prio)
>  -		skip_for_load = busiest_best_prio_seen || idx != busiest_best_prio;
>  +		skip_for_load = !busiest_best_prio_seen && idx == busiest_best_prio;

But Suresh's
sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks-fix.patch
changed all this code:

	/*
	 * To help distribute high priority tasks accross CPUs we don't
	 * skip a task if it will be the highest priority task (i.e. smallest
	 * prio value) on its new queue regardless of its load weight
	 */
	skip_for_load = tmp->load_weight > rem_load_move;
	if (skip_for_load && idx < this_best_prio && idx == busiest_best_prio)
		skip_for_load = !busiest_best_prio_seen &&
				head->next == head->prev;
	if (skip_for_load ||
	    !can_migrate_task(tmp, busiest, this_cpu, sd, idle, &pinned)) {
		if (curr != head)
			goto skip_queue;
		idx++;
		goto skip_bitmap;
	}


What to do?

Re: [PATCH] sched: Fix boolean expression in move_tasks()

[Peter Williams]

Andrew Morton wrote:
> Peter Williams <pwil3058@bigpond.net.au> wrote:
>> Negate the expression and apply de Marcos rule to simplify it.  This 
>>  patch is on top of 
>>  sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch
>>
>>  Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
>>
>>  -- 
>>  Peter Williams                                   pwil3058@bigpond.net.au
>>
>>  "Learning, n. The kind of ignorance distinguishing the studious."
>>    -- Ambrose Bierce
>>
>>
>> [smpnice-fix-boolean-expression  text/plain (824 bytes)]
>>  Index: MM-2.6.17-rc1-mm3/kernel/sched.c
>>  ===================================================================
>>  --- MM-2.6.17-rc1-mm3.orig/kernel/sched.c	2006-04-21 12:26:54.000000000 +1000
>>  +++ MM-2.6.17-rc1-mm3/kernel/sched.c	2006-04-25 09:09:54.000000000 +1000
>>  @@ -2108,7 +2108,7 @@ skip_queue:
>>   	 */
>>   	skip_for_load = tmp->load_weight > rem_load_move;
>>   	if (skip_for_load && idx < this_best_prio)
>>  -		skip_for_load = busiest_best_prio_seen || idx != busiest_best_prio;
>>  +		skip_for_load = !busiest_best_prio_seen && idx == busiest_best_prio;
> 
> But Suresh's
> sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks-fix.patch
> changed all this code:
> 
> 	/*
> 	 * To help distribute high priority tasks accross CPUs we don't
> 	 * skip a task if it will be the highest priority task (i.e. smallest
> 	 * prio value) on its new queue regardless of its load weight
> 	 */
> 	skip_for_load = tmp->load_weight > rem_load_move;
> 	if (skip_for_load && idx < this_best_prio && idx == busiest_best_prio)
> 		skip_for_load = !busiest_best_prio_seen &&
> 				head->next == head->prev;
> 	if (skip_for_load ||
> 	    !can_migrate_task(tmp, busiest, this_cpu, sd, idle, &pinned)) {
> 		if (curr != head)
> 			goto skip_queue;
> 		idx++;
> 		goto skip_bitmap;
> 	}
> 
> 
> What to do?

Suresh's patch was wrong and this was intended as an alternative. 
Unfortunately, it is also in adequate and the setting of 
busiest_best_prio_seen needs to be moved to just after skip_for_load is 
set.  I have another patch that does that (and adds to the comments). 
Should I send that separately or roll the two patches together?

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce

Re: [PATCH] sched: Fix boolean expression in move_tasks()

[Andrew Morton]

Peter Williams <pwil3058@bigpond.net.au> wrote:
>
> > What to do?
> 
>  Suresh's patch was wrong and this was intended as an alternative. 

OIC.

>  Unfortunately, it is also in adequate and the setting of 
>  busiest_best_prio_seen needs to be moved to just after skip_for_load is 
>  set.  I have another patch that does that (and adds to the comments). 
>  Should I send that separately or roll the two patches together?

I don't mind, really - rolled together, I guess.