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

[Phil Auld <pauld@redhat.com>]

On Fri, Jul 23, 2021 at 09:38:44AM -0400 Phil Auld wrote:
> On Fri, Jul 09, 2021 at 01:38:20PM +0200 Peter Zijlstra wrote:
> > 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?
> >
> 
> You're right here. (I see now that you were asking about the rmb specifically
> in the first question) The rmb is not needed. 
> 
> I was unable to reproducde it with the upstream kernel. I still think it is
> a problem though since the code in question is all the same. The recent
> changes to unbound workqueues which make it more likely to run on the
> submitting cpu may be masking the problem since it obviously requires
> multiple cpus to hit.
> 
> If I can isolate those changes I can try to revert them in upstream and
> see if I can get it there.
> 
> I suppose pulling those changes back could get us past this but
> I'm not a fan of just hiding it by making it harder to hit.
> 
> I've not gotten to the disable TTWU_QUEUE test, that's next...
>

ETOOMANYTREES

Sorry for the noise. I was using the wrong tree to compare with upstream.
The offending tree is missing f97bb5272d9e ("sched: Fix data-race in wakeup").
I thought the loadavg increase sounded familiar...


Cheers,
Phil

> 
> Cheers,
> Phil
> 
> -- 
> 

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