[tip: sched/urgent] sched/core: Fix psi_dequeue() for Proxy Execution

["tip-bot2 for John Stultz" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     c2ae8b0df2d1bb7a063f9e356e4e9a06cd4afe11
Gitweb:        https://git.kernel.org/tip/c2ae8b0df2d1bb7a063f9e356e4e9a06cd4afe11
Author:        John Stultz <jstultz@google.com>
AuthorDate:    Fri, 05 Dec 2025 01:27:09 
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 06 Dec 2025 10:13:16 +01:00

sched/core: Fix psi_dequeue() for Proxy Execution

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@amd.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Haiyue Wang <haiyuewa@163.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://patch.msgid.link/20251205012721.756394-1-jstultz@google.com
Closes: https://lore.kernel.org/lkml/20251117185550.365156-1-kprateek.nayak@amd.com/
---
 kernel/sched/stats.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
index cbf7206..c903f1a 100644
--- a/kernel/sched/stats.h
+++ b/kernel/sched/stats.h
@@ -180,8 +180,13 @@ static inline void psi_dequeue(struct task_struct *p, int flags)
 	 * avoid walking all ancestors twice, psi_task_switch() handles
 	 * TSK_RUNNING and TSK_IOWAIT for us when it moves TSK_ONCPU.
 	 * Do nothing here.
+	 *
+	 * In the SCHED_PROXY_EXECUTION case we may do sleeping
+	 * dequeues that are not followed by a task switch, so check
+	 * TSK_ONCPU is set to ensure the task switch is imminent.
+	 * Otherwise clear the flags as usual.
 	 */
-	if (flags & DEQUEUE_SLEEP)
+	if ((flags & DEQUEUE_SLEEP) && (p->psi_flags & TSK_ONCPU))
 		return;
 
 	/*