On systems with weaker memory ordering (e.g. power) commit dbfb089d360b ("sched: Fix loadavg accounting race") causes increasing values of load average (via rq->calc_load_active and calc_load_tasks) due to the wakeup CPU not always seeing the write to task->sched_contributes_to_load in __schedule(). Missing that we fail to decrement nr_uninterruptible when waking up a task which incremented nr_uninterruptible when it slept.
The rq->lock serialization is insufficient across different rq->locks.
Add smp_wmb() to schedule and smp_rmb() before the read in ttwu_do_activate().
Fixes: dbfb089d360b ("sched: Fix loadavg accounting race") Cc: Peter Zijlstra peterz@infradead.org Cc: Ingo Molnar mingo@kernel.org Cc: Juri Lelli juri.lelli@redhat.com Cc: Vincent Guittot vincent.guittot@linaro.org Cc: Waiman Long longman@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Phil Auld pauld@redhat.com --- kernel/sched/core.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 4ca80df205ce..ced7074716eb 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2992,6 +2992,8 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
lockdep_assert_held(&rq->lock);
+ /* Pairs with smp_wmb in __schedule() */ + smp_rmb(); if (p->sched_contributes_to_load) rq->nr_uninterruptible--;
@@ -5084,6 +5086,11 @@ static void __sched notrace __schedule(bool preempt) !(prev_state & TASK_NOLOAD) && !(prev->flags & PF_FROZEN);
+ /* + * Make sure the previous write is ordered before p->on_rq etc so + * that it is visible to other cpus in the wakeup path (ttwu_do_activate()). + */ + smp_wmb(); if (prev->sched_contributes_to_load) rq->nr_uninterruptible++;