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

[Peter Zijlstra]

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.