Re: [RFC -v3 PATCH 2/3] sched: add yield_to function

Mike Galbraith
2011-01-13T03:26:16Z

On Wed, 2011-01-12 at 22:02 -0500, Rik van Riel wrote:

> Cgroups only makes the matter worse - libvirt places
> each KVM guest into its own cgroup, so a VCPU will
> generally always be alone on its own per-cgroup, per-cpu
> runqueue! That can lead to pulling a VCPU onto our local
> CPU because we think we are alone, when in reality we
> share the CPU with others...

How can that happen? If the task you're trying to accelerate isn't in
your task group, the whole attempt should be a noop.

> Removing the pulling code allows me to use all 4
> CPUs with a 4-VCPU KVM guest in an uncontended situation.
>
> > + /* Tell the scheduler that we'd really like pse to run next. */
> > + p_cfs_rq->next = pse;
>
> Using set_next_buddy propagates this up to the root,
> allowing the scheduler to actually know who we want to
> run next when cgroups is involved.
>
> > + /* We know whether we want to preempt or not, but are we allowed? */
> > + if (preempt&& same_thread_group(p, task_of(p_cfs_rq->curr)))
> > + resched_task(task_of(p_cfs_rq->curr));
>
> With this in place, we can get into the situation where
> we will gladly give up CPU time, but not actually give
> any to the other VCPUs in our guest.
>
> I believe we can get rid of that test, because pick_next_entity
> already makes sure it ignores ->next if picking ->next would
> lead to unfairness.

Preempting everybody who is in your way isn't playing nice neighbor, so
I think at least the same_thread_group() test needs to stay. But that's
Peter's call. Starting a zillion threads to play wakeup preempt and
lets hog the cpu isn't nice either, but it's allowed.

> Removing this test (and simplifying yield_to_task_fair) seems
> to lead to more predictable test results.

Less is more :)

-Mike