Re: [PATCH] sched: At sched_fork use __set_task_cpu().

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2012-01-31 at 14:18 +0530, Kamalesh Babulal wrote:
> * Rakib Mullick <rakib.mullick@gmail.com> [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 <rakib.mullick@gmail.com>
>   Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>

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)
> > 
> > 
> > --
>