On Wed, Jul 07, 2021 at 03:04:57PM -0400, Phil Auld wrote:
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().
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--;
Is this really needed ?! (this question is a big fat clue the comment is insufficient). AFAICT try_to_wake_up() has a LOAD-ACQUIRE on p->on_rq and hence the p->sched_contributed_to_load must already happen after.
@@ -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++;
That comment is terrible, look at all the other barrier comments around there for clues; in effect you're worrying about:
p->sched_contributes_to_load = X R1 = p->on_rq WMB RMB p->on_rq = Y R2 = p->sched_contributes_to_load
Right?
Bah bah bah.. I so detest having to add barriers here for silly accounting. Let me think about this a little.