Re: weird loadavg on idle machine post 5.7

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 03, 2020 at 11:02:26AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 02, 2020 at 10:36:27PM +0100, Mel Gorman wrote:
> 
> > > commit c6e7bd7afaeb3af55ffac122828035f1c01d1d7b (refs/bisect/bad)
> > > Author: Peter Zijlstra <peterz@infradead.org>
> 
> > Peter, I'm not supremely confident about this but could it be because
> > "p->sched_contributes_to_load = !!task_contributes_to_load(p)" potentially
> > happens while a task is still being dequeued? In the final stages of a
> > task switch we have
> > 
> >         prev_state = prev->state;
> >         vtime_task_switch(prev);
> >         perf_event_task_sched_in(prev, current);
> >         finish_task(prev);
> > 
> > finish_task is when p->on_cpu is cleared after the state is updated.
> > With the patch, we potentially update sched_contributes_to_load while
> > p->state is transient so if the check below is true and ttwu_queue_wakelist
> > is used then sched_contributes_to_load was based on a transient value
> > and potentially wrong.
> 
> I'm not seeing it. Once a task hits schedule(), p->state doesn't change,
> except through wakeup.
> 
> And while dequeue depends on p->state, it doesn't change it.
> 
> At this point in ttwu() we know p->on_rq == 0, which implies dequeue has
> started, which means we've (at least) stopped executing the task -- we
> started or finished schedule().
> 
> Let me stare at this more...

So ARM/Power/etc.. can speculate the load such that the
task_contributes_to_load() value is from before ->on_rq.

The compiler might similar re-order things -- although I've not found it
doing so with the few builds I looked at.

So I think at the very least we should do something like this. But i've
no idea how to reproduce this problem.

Mel's patch placed it too far down, as the WF_ON_CPU path also relies on
this, and by not resetting p->sched_contributes_to_load it would skew
accounting even worse.

---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fcd56f04b706..cba8a56d0f7f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2799,9 +2799,6 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	}
 
 #ifdef CONFIG_SMP
-	p->sched_contributes_to_load = !!task_contributes_to_load(p);
-	p->state = TASK_WAKING;
-
 	/*
 	 * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be
 	 * possible to, falsely, observe p->on_cpu == 0.
@@ -2823,6 +2820,9 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	 */
 	smp_rmb();
 
+	p->sched_contributes_to_load = !!task_contributes_to_load(p);
+	p->state = TASK_WAKING;
+
 	/*
 	 * If the owning (remote) CPU is still in the middle of schedule() with
 	 * this task as prev, considering queueing p on the remote CPUs wake_list