* Peter Zijlstra <pet...@infradead.org> [2012-09-10 18:03:55]:
> On Mon, 2012-09-10 at 08:16 -0500, Andrew Theurer wrote:
> > > > @@ -4856,8 +4859,6 @@ again:
> > > > if (curr->sched_class != p->sched_class)
> > > > goto out;
> > > >
> > > > - if (task_running(p_rq, p) || p->state)
> > > > - goto out;
> > >
> > > Is it possible that by this time the current thread takes double rq
> > > lock, thread p could actually be running? i.e is there merit to keep
> > > this check around even with your similar check above?
> >
> > I think that's a good idea. I'll add that back in.
>
> Right, it needs to still be there, the test before acquiring p_rq is an
> optimistic test to avoid work, but you have to still test it once you
> acquire p_rq since the rest of the code relies on this not being so.
>
> How about something like this instead.. ?
>
> ---
> kernel/sched/core.c | 35 ++++++++++++++++++++++++++---------
> 1 file changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c46a011..c9ecab2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4300,6 +4300,23 @@ void __sched yield(void)
> }
> EXPORT_SYMBOL(yield);
>
> +/*
> + * Tests preconditions required for sched_class::yield_to().
> + */
> +static bool __yield_to_candidate(struct task_struct *curr, struct
> task_struct *p)
> +{
> + if (!curr->sched_class->yield_to_task)
> + return false;
> +
> + if (curr->sched_class != p->sched_class)
> + return false;
Peter,
Should we also add a check if the runq has a skip buddy (as pointed out
by Raghu) and return if the skip buddy is already set. Something akin
to
if (p_rq->cfs_rq->skip)
return false;
So if somebody has already acquired a double run queue lock and almost
set the next buddy, we dont need to take run queue lock and also avoid
overwriting the already set skip buddy.
> +
> + if (task_running(p_rq, p) || p->state)
> + return false;
> +
> + return true;
> +}
> +
> /**
> * yield_to - yield the current processor to another thread in
> * your thread group, or accelerate that thread toward the
> @@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p, bool
> preempt)
> rq = this_rq();
>
> again:
> + /* optimistic test to avoid taking locks */
> + if (!__yield_to_candidate(curr, p))
> + goto out_irq;
> +
> p_rq = task_rq(p);
> double_rq_lock(rq, p_rq);
> while (task_rq(p) != p_rq) {
> @@ -4330,14 +4351,9 @@ bool __sched yield_to(struct task_struct *p, bool
> preempt)
> goto again;
> }
>
> - if (!curr->sched_class->yield_to_task)
> - goto out;
> -
> - if (curr->sched_class != p->sched_class)
> - goto out;
> -
> - if (task_running(p_rq, p) || p->state)
> - goto out;
> + /* validate state, holding p_rq ensures p's state cannot change */
> + if (!__yield_to_candidate(curr, p))
> + goto out_unlock;
>
> yielded = curr->sched_class->yield_to_task(rq, p, preempt);
> if (yielded) {
> @@ -4350,8 +4366,9 @@ bool __sched yield_to(struct task_struct *p, bool
> preempt)
> resched_task(p_rq->curr);
> }
>
> -out:
> +out_unlock:
> double_rq_unlock(rq, p_rq);
> +out_irq:
> local_irq_restore(flags);
>
> if (yielded)
>
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