Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

Sean Christopherson
2024-10-10T18:23:55Z

On Thu, Oct 10, 2024, Peter Zijlstra wrote:
> On Thu, Oct 10, 2024 at 10:19:40AM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 09, 2024 at 07:49:54PM -0700, Sean Christopherson wrote:
> >
> > > TL;DR: Code that checks task_struct.on_rq may be broken by this commit.
> >
> > Correct, and while I did look at quite a few, I did miss KVM used it,
> > damn.
> >
> > > Peter,
> > >
> > > Any thoughts on how best to handle this? The below hack-a-fix resolves the issue,
> > > but it's obviously not appropriate. KVM uses vcpu->preempted for more than just
> > > posted interrupts, so KVM needs equivalent functionality to current->on-rq as it
> > > was before this commit.
> > >
> > > @@ -6387,7 +6390,7 @@ static void kvm_sched_out(struct preempt_notifier *pn,
> > >
> > > WRITE_ONCE(vcpu->scheduled_out, true);
> > >
> > > - if (current->on_rq && vcpu->wants_to_run) {
> > > + if (se_runnable(&current->se) && vcpu->wants_to_run) {
> > > WRITE_ONCE(vcpu->preempted, true);
> > > WRITE_ONCE(vcpu->ready, true);
> > > }
> >
> > se_runnable() isn't quite right, but yes, a helper along those lines is
> > probably best. Let me try and grep more to see if there's others I
> > missed as well :/
>
> How's the below? I remember looking at the freezer thing before and
> deciding it isn't a correctness thing, but given I added the helper, I
> changed it anyway. I've added a bunch of comments and the perf thing is
> similar to KVM, it wants to know about preemptions so that had to change
> too.

Fixes KVM's woes! Thanks!