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

Scott Wood
2020-06-17T22:50:01Z

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