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

[Peter Zijlstra]

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 :/