[PATCH v3 1/3] sched: Fix UCLAMP_FLAG_IDLE setting

[Quentin Perret]

The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last
active task to maintain the last uclamp.max and prevent blocked util
from suddenly becoming visible.

However, there is an asymmetry in how the flag is set and cleared which
can lead to having the flag set whilst there are active tasks on the rq.
Specifically, the flag is cleared in the uclamp_rq_inc() path, which is
called at enqueue time, but set in uclamp_rq_dec_id() which is called
both when dequeueing a task _and_ in the update_uclamp_active() path. As
a result, when both uclamp_rq_{dec,ind}_id() are called from
update_uclamp_active(), the flag ends up being set but not cleared,
hence leaving the runqueue in a broken state.

Fix this by clearing the flag in uclamp_idle_reset() which is also
called in both paths to ensure things remain symmetrical.

Fixes: e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX")
Reported-by: Rick Yiu <rickyiu@xxxxxxxxxx>
Reviewed-by: Qais Yousef <qais.yousef@xxxxxxx>
Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
---
 kernel/sched/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4ca80df205ce..e514a093a0ba 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -980,6 +980,7 @@ static inline void uclamp_idle_reset(struct rq *rq, enum uclamp_id clamp_id,
 	if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE))
 		return;
 
+	rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
 	WRITE_ONCE(rq->uclamp[clamp_id].value, clamp_value);
 }
 
@@ -1252,10 +1253,6 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
 
 	for_each_clamp_id(clamp_id)
 		uclamp_rq_inc_id(rq, p, clamp_id);
-
-	/* Reset clamp idle holding when there is one RUNNABLE task */
-	if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
-		rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
 }
 
 static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
-- 
2.32.0.288.g62a8d224e6-goog