Re: [PATCH] sched: Avoid unnecessarily moving highest priority task move_tasks()

[Peter Williams <pwil3058@bigpond.net.au>]

Siddha, Suresh B wrote:
> On Tue, Apr 25, 2006 at 09:04:41AM +1000, Peter Williams wrote:
>> Looks like there's a NOT missing doesn't it.  So there is an error but I 
>> don't think that your patch is the right way to fix it.  We just need to 
>> negate the above assignment.  E.g.
>>
>> skip_for_load = !(busiest_best_prio_seen || idx != busiest_best_prio);
>>
>> or
>>
>> skip_for_load = !busiest_best_prio_seen && idx == busiest_best_prio;
>>
>> whichever is more efficient.
> 
> That just will not be enough. busiest_best_prio needs to be set to '1'
> whenever we skip the first best priority task(not whenever we move it)

Yes, I just realized that.  It needs to be done after skip_for_load has 
been finalized.

> 
> And we also need to initialize busiest_best_prio_seen inside this check.
> (like in my patch)
> 	if (busiest->expired->nr_active) {

Why?  It's already initialized.  If the current running task has 
prio==busiest_best_prio then we know that can_migrate_task() will 
prevent it from being moved so it's safe to move any other tasks we meet 
with that priority.

> 
> And we need to reset busiest_best_prio_seen to '0' whenever we finished
> the checking of expired list (and move onto active list) and there are
> no best prio tasks on expired list..

No we don't.  Once we've skipped one it's OK to move any others that we 
find.  We'll never move more than one as a result of overriding the skip 
anyhow .

> 
>>> @@ -2072,6 +2067,13 @@ static int move_tasks(runqueue_t *this_r
>>>  	if (busiest->expired->nr_active) {
>>>  		array = busiest->expired;
>>>  		dst_array = this_rq->expired;
>>> +		/*
>>> +		 * We already have one or more busiest best prio tasks on
>>> +		 * active list.
>> This is a pretty bold assertion.  How do we know that this is true.
> 
> That comment refers to when 'busiest_best_prio_seen' is initialized to '1'.
> Comment needs to be fixed.

But you initialized it to zero.

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

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