[PATCH v4 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity
will at kernel.org
Tue Dec 1 11:56:57 EST 2020
On Fri, Nov 27, 2020 at 01:19:16PM +0000, Qais Yousef wrote:
> On 11/24/20 15:50, Will Deacon wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index d2003a7d5ab5..818c8f7bdf2a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1860,24 +1860,18 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> > }
> > /*
> > - * Change a given task's CPU affinity. Migrate the thread to a
> > - * proper CPU and schedule it away if the CPU it's executing on
> > - * is removed from the allowed bitmask.
> > - *
> > - * NOTE: the caller must have a valid reference to the task, the
> > - * task must not exit() & deallocate itself prematurely. The
> > - * call is not atomic; no spinlocks may be held.
> > + * Called with both p->pi_lock and rq->lock held; drops both before returning.
> nit: wouldn't it be better for the caller to acquire and release the locks?
> Not a big deal but it's always confusing when half of the work done outside the
> function and the other half done inside.
That came up in the last version of the patches iirc, but the problem is
that __set_cpus_allowed_ptr_locked() can trigger migration, which can
drop the lock and take another one for the new runqueue.
Given that this function is internal to the scheduler, I think we can
probably live with it.
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