From: Quentin Perret qperret@google.com
[ Upstream commit ca4984a7dd863f3e1c0df775ae3e744bff24c303 ]
The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last uclamp active task (that is, when buckets.tasks reaches 0 for all buckets) 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 update_uclamp_active() as well.
Fixes: e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX") Reported-by: Rick Yiu rickyiu@google.com Signed-off-by: Quentin Perret qperret@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Qais Yousef qais.yousef@arm.com Tested-by: Dietmar Eggemann dietmar.eggemann@arm.com Link: https://lore.kernel.org/r/20210805102154.590709-2-qperret@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/core.c | 25 +++++++++++++++++++------ 1 file changed, 19 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 15b4d2fb6be3..1e9672d609f7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1281,6 +1281,23 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p) uclamp_rq_dec_id(rq, p, clamp_id); }
+static inline void uclamp_rq_reinc_id(struct rq *rq, struct task_struct *p, + enum uclamp_id clamp_id) +{ + if (!p->uclamp[clamp_id].active) + return; + + uclamp_rq_dec_id(rq, p, clamp_id); + uclamp_rq_inc_id(rq, p, clamp_id); + + /* + * Make sure to clear the idle flag if we've transiently reached 0 + * active tasks on rq. + */ + if (clamp_id == UCLAMP_MAX && (rq->uclamp_flags & UCLAMP_FLAG_IDLE)) + rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE; +} + static inline void uclamp_update_active(struct task_struct *p) { @@ -1304,12 +1321,8 @@ uclamp_update_active(struct task_struct *p) * affecting a valid clamp bucket, the next time it's enqueued, * it will already see the updated clamp bucket value. */ - for_each_clamp_id(clamp_id) { - if (p->uclamp[clamp_id].active) { - uclamp_rq_dec_id(rq, p, clamp_id); - uclamp_rq_inc_id(rq, p, clamp_id); - } - } + for_each_clamp_id(clamp_id) + uclamp_rq_reinc_id(rq, p, clamp_id);
task_rq_unlock(rq, p, &rf); }