Re: [PATCH] sched: __set_cpus_allowed_ptr(): Check cpus_mask, not cpus_ptr

[Scott Wood <swood@redhat.com>]

On Wed, 2020-06-17 at 15:15 +0100, Valentin Schneider wrote:
> On 17/06/20 13:17, Sebastian Andrzej Siewior wrote:
> > From: Scott Wood <swood@redhat.com>
> > 
> > This function is concerned with the long-term cpu mask, not the
> > transitory mask the task might have while migrate disabled.  Before
> > this patch, if a task was migrate disabled at the time
> > __set_cpus_allowed_ptr() was called, and the new mask happened to be
> > equal to the cpu that the task was running on, then the mask update
> > would be lost.
> > 
> > Signed-off-by: Scott Wood <swood@redhat.com>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> >  kernel/sched/core.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1637,7 +1637,7 @@ static int __set_cpus_allowed_ptr(struct
> >               goto out;
> >       }
> > 
> > -	if (cpumask_equal(p->cpus_ptr, new_mask))
> > +	if (cpumask_equal(&p->cpus_mask, new_mask))
> >               goto out;
> > 
> >       /*
> 
> Makes sense, but what about the rest of the checks? Further down there is
> 
>         /* Can the task run on the task's current CPU? If so, we're done
> */
>         if (cpumask_test_cpu(task_cpu(p), new_mask))
>                 goto out;
> 
> If the task is currently migrate disabled and for some stupid reason it
> gets affined elsewhere, we could try to move it out - which AFAICT we
> don't
> want to do because migrate disabled. So I suppose you'd want an extra
> bailout condition here when the task is migrate disabled.
> 
> ISTR in RT you do re-check the affinity and potentially move the task away
> when re-enabling migration, so that should work out all fine.

On RT the above test is:

        /* Can the task run on the task's current CPU? If so, we're done */
        if (cpumask_test_cpu(task_cpu(p), new_mask) ||
            p->cpus_ptr != &p->cpus_mask)
                goto out;

...so we do bail out if we're migrate disabled.

-Scott