Re: [PATCH] sched: fix evaluation of skip_for_load in move_tasks()

[Peter Williams]

Siddha, Suresh B wrote:
> On Tue, Apr 25, 2006 at 01:08:00PM +1000, Peter Williams wrote:
> > 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.
> >
> > Additionally, busiest_best_prio_seen is being set when tasks are moved 
> > instead of when they are skipped which will cause problems when the 
> > current task does not have prio=busiest_best_prio.
> >
> > Solution:
> >
> > Negate the expression and apply de Marcos rule to simplify it and move 
> > the setting of busiest_best_prio_seen.
> >
> > This patch is on top of 
> > sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch
> >
> > Signed-off-by: Peter Williams <pwil3058@xxxxxxxxxxxxxx>
> >
> > --
> > Peter Williams                                   pwil3058@xxxxxxxxxxxxxx
> >
> > "Learning, n. The kind of ignorance distinguishing the studious."
> >   -- Ambrose Bierce
>
> > Index: MM-2.6.17-rc1-mm3/kernel/sched.c
> > ===================================================================
> > --- MM-2.6.17-rc1-mm3.orig/kernel/sched.c	2006-04-25 12:53:39.000000000 +1000
> > +++ MM-2.6.17-rc1-mm3/kernel/sched.c	2006-04-25 12:56:14.000000000 +1000
> > @@ -2059,7 +2059,10 @@ static int move_tasks(runqueue_t *this_r
> >  	busiest_best_prio = rq_best_prio(busiest);
> >  	/*
> >  	 * Enable handling of the case where there is more than one task
> > -	 * with the best priority.
> > +	 * with the best priority.   If the current running task is one
> > +	 * of those with prio==busiest_best_prio we know it won't be moved
> > +	 * and therefore it's safe to override the skip (based on load) of
> > +	 * any task we find with that prio.
> >  	 */
> >  	busiest_best_prio_seen = busiest_best_prio == busiest->curr->prio;
> >  
> > @@ -2108,9 +2111,10 @@ 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;
>
> I think we need to change this to
> 	if (skip_for_load && idx < this_best_prio && idx == busiest_best_prio)
> 		skip_for_load = !busiest_best_prio_seen;
>
> Otherwise we will reset skip_for_load to '0' even for the tasks whose prio is 
> less than this_best_prio but not equal to busiest_best_prio.

And why is that a problem?  The intention of this code is to make sure 
at least one busiest_best_prio task doesn't get moved as a result of the 
"skip for reasons of load weight" mechanism being overridden by the "idx 
< this_best_prio" exception.  I can't see how this intention is being 
subverted.

Peter
--
Peter Williams                                   pwil3058@xxxxxxxxxxxxxx

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