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    SubjectRe: [PATCH] sched: Fix nr_uninterruptible race causing increasing load average
    On Thu, Jul 08, 2021 at 09:48:03AM +0200, Peter Zijlstra wrote:
    > On Thu, Jul 08, 2021 at 09:26:26AM +0200, Peter Zijlstra wrote:
    > > 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.
    >
    > I got the below:
    >
    > __schedule() ttwu()
    >
    > rq_lock() raw_spin_lock(&p->pi_lock)
    > smp_mb__after_spinlock(); smp_mb__after_spinlock();
    >
    > p->sched_contributes_to_load = X; if (READ_ONCE(p->on_rq) && ...)
    > goto unlock;
    > smp_acquire__after_ctrl_dep();
    >
    > smp_cond_load_acquire(&p->on_cpu, !VAL)
    >
    > deactivate_task()
    > p->on_rq = 0;
    >
    > context_switch()
    > finish_task_switch()
    > finish_task()
    > smp_store_release(p->on_cpu, 0);
    >
    > ttwu_queue()
    > rq_lock()
    > ttwu_do_activate()
    > if (p->sched_contributes_to_load)
    > ...
    > rq_unlock()
    > raw_spin_unlock(&p->pi_lock);
    > finish_lock_switch()
    > rq_unlock();
    >
    >
    >
    > The only way for ttwu() to end up in an enqueue, is if it did a
    > LOAD-ACQUIRE on ->on_cpu,

    That's not completely true; there's the WF_ON_CPU case, but in that
    scenario we IPI the CPU doing __schedule and it becomes simple UP/PO and
    everything must trivially work.

    > but that orders with the STORE-RELEASE on the
    > same, which ensures the p->sched_contributes_to_load LOAD must happen
    > after the STORE.
    >
    > What am I missing? Your Changelog/comments provide insufficient clues..

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