Re: [PATCH] sched: Fix nr_uninterruptible race causing increasing load average

[Peter Zijlstra]

On Thu, Jul 08, 2021 at 09:25:45AM -0400, Phil Auld wrote:
> Hi Peter,
> 
> 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.
> >
> 
> Yes, it is needed.  We've got idle power systems with load average of 530.21.
> Calc_load_tasks is 530, and the sum of both nr_uninterruptible and
> calc_load_active across all the runqueues is 530. Basically monotonically
> non-decreasing load average. With the patch this no longer happens.

Have you tried without the rmb here? Do you really need both barriers?