On Mon, Jun 11, 2018 at 03:38:50PM +0800, Wanpeng Li wrote: > From: Wanpeng Li <[email protected]> > > Consider the task afffinity constrain when yield to a task. > > Cc: Paolo Bonzini <[email protected]> > Cc: Radim Krčmář <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Ingo Molnar <[email protected]> > Signed-off-by: Wanpeng Li <[email protected]> > --- > kernel/sched/core.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 092f7c4..11001ff 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5102,6 +5102,9 @@ int __sched yield_to(struct task_struct *p, bool > preempt) > if (task_running(p_rq, p) || p->state) > goto out_unlock; > > + if (!cpumask_test_cpu(smp_processor_id(), &p->cpus_allowed)) > + goto out_unlock; > + > yielded = curr->sched_class->yield_to_task(rq, p, preempt); > if (yielded) { > schedstat_inc(rq->yld_count);
I'm confused... why? So yield_to(@p) is documented as yielding @curr and getting @p to run 'sooner'. If they're on the same rq, yay, that means we'll likely switch from @curr to @p, however if they're not on the same rq, it should still work, except we'll reschedule 2 CPUs. Look at the code, yield_to() will lock 2 rqs: rq and p_rq. First if verifies p_rq == task_rq(p) (p could've been migrated while we were waiting to acquire the lock) if this is not so, we unlock and try again. Then we check trivial things like actually having a ->yield_to and @curr and @p being of the same class. Then we check if @p is in fact already running or not runnable at all, if either, obviously nothing to do. So now, we have rq and p_rq locked, they need not be the same and we'll call ->yield_to(), on success we'll force reschedule p_rq, unlock the rqs and reschedule ourself. So far, nothing requires @p to be runnable on the current CPU. So let us look at yield_to_task_fair() the only yield_to implementation: That again checks if @p is in fact runnable, if not, nothing to do. Then it calls set_next_buddy(&p->se), which will mark @p more likely to get picked on its rq (p_rq) _not_ our rq. Note how set_next_buddy() uses cfs_rq_of(). Then it yields @curr on the current cpu. Again, nothing cares if @curr and @p are on the same CPU and if @curr is allowed to run on the current CPU -- there are no migrations. So.. why?!
