Re: [PATCH v3] sched: Fix psi_dequeue for Proxy Execution

[Peter Zijlstra]

On Fri, Dec 05, 2025 at 01:27:09AM +0000, John Stultz wrote:
> Currently, if the sleep flag is set, psi_dequeue() doesn't
> change any of the psi_flags.
> 
> This is because psi_task_switch() will clear TSK_ONCPU as well
> as other potential flags (TSK_RUNNING), and the assumption is
> that a voluntary sleep always consists of a task being dequeued
> followed shortly there after with a psi_sched_switch() call.
> 
> Proxy Execution changes this expectation, as mutex-blocked tasks
> that would normally sleep stay on the runqueue. But in the case
> where the mutex-owning task goes to sleep, or the owner is on a
> remote cpu, we will then deactivate the blocked task shortly
> after.
> 
> In that situation, the mutex-blocked task will have had its
> TSK_ONCPU cleared when it was switched off the cpu, but it will
> stay TSK_RUNNING. Then if we later dequeue it (as currently done
> if we hit a case find_proxy_task() can't yet handle, such as the
> case of the owner being on another rq or a sleeping owner)
> psi_dequeue() won't change any state (leaving it TSK_RUNNING),
> as it incorrectly expects a psi_task_switch() call to
> immediately follow.
> 
> Later on when the task get woken/re-enqueued, and psi_flags are
> set for TSK_RUNNING, we hit an error as the task is already
> TSK_RUNNING:
>   psi: inconsistent task state! task=188:kworker/28:0 cpu=28 psi_flags=4 clear=0 set=4
> 
> To resolve this, extend the logic in psi_dequeue() so that
> if the sleep flag is set, we also check if psi_flags have
> TSK_ONCPU set (meaning the psi_task_switch is imminent) before
> we do the shortcut return.
> 
> If TSK_ONCPU is not set, that means we've already switched away,
> and this psi_dequeue call needs to clear the flags.
> 
> Fixes: be41bde4c3a8 ("sched: Add an initial sketch of the find_proxy_task() function")
> Reported-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Closes: https://lore.kernel.org/lkml/20251117185550.365156-1-kprateek.nayak@xxxxxxx/
> Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
> Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Tested-by: Haiyue Wang <haiyuewa@xxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Stuck this on my post rc1 pile to look at.