On Tue, 2012-01-31 at 14:18 +0530, Kamalesh Babulal wrote:
> * Rakib Mullick <[email protected]> [2012-01-29 22:34:37]:
>
> > We don't use select_task_rq() from sched_fork() anymore and no chance of task gets migrated at
> > this point. Therefore, we can avoid task migration related checking/accounting, so use
> > __set_task_cpu() instead of set_task_cpu().
> >
> > Signed-off-by: Rakib Mullick <[email protected]>
> Reviewed-by: Kamalesh Babulal <[email protected]>
Since we call sched_fork() with preemption enabled _long_ after the
child is copied from the parent who is to say we (parent) didn't migrate
away and are now setting a different cpu?
One could argue that that might not be a real migration from the child's
POV, maybe, but nobody seems to be making that argument.
I really don't see the point of this..
> > ---
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index df00cb0..a38026e 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1755,7 +1755,7 @@ void sched_fork(struct task_struct *p)
> > * Silence PROVE_RCU.
> > */
> > raw_spin_lock_irqsave(&p->pi_lock, flags);
> > - set_task_cpu(p, cpu);
> > + __set_task_cpu(p, cpu);
> > raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> >
> > #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
> >
> >
> > --
>
On Wed, Feb 1, 2012 at 7:55 PM, Peter Zijlstra <[email protected]> wrote:
> On Tue, 2012-01-31 at 14:18 +0530, Kamalesh Babulal wrote:
>> * Rakib Mullick <[email protected]> [2012-01-29 22:34:37]:
>>
>> > ?We don't use select_task_rq() from sched_fork() anymore and no chance of task gets migrated at
>> > this point. Therefore, we can avoid task migration related checking/accounting, so use
>> > __set_task_cpu() instead of set_task_cpu().
>> >
>> > Signed-off-by: Rakib Mullick <[email protected]>
>> ? Reviewed-by: Kamalesh Babulal <[email protected]>
>
> Since we call sched_fork() with preemption enabled _long_ after the
> child is copied from the parent who is to say we (parent) didn't migrate
> away and are now setting a different cpu?
>
If parent gets migrated that should be accounted as parents migration
count not for child offcourse. And if we're counting child's
nr_migration count for parent's getting migrated, we're simply
screwing childs migration count. Isn't it?
> One could argue that that might not be a real migration from the child's
> POV, maybe, but nobody seems to be making that argument.
>
But I'm not seeing it from child's or parent's POV. I'm simply
addressing the point of a task's migration counter
(p->se.nr_migrations), simply this task wasn't moved.
> I really don't see the point of this..
>
I'm hoping, you'll rethink...
Thanks,
Rakib