Re: [PATCH] sched/fair: Avoid divide by zero when rebalancing domains

[Matt Fleming]

On Thu, 05 Jul, at 11:10:42AM, Valentin Schneider wrote:
> Hi,
> 
> On 04/07/18 15:24, Matt Fleming wrote:
> > It's possible that the CPU doing nohz idle balance hasn't had its own
> > load updated for many seconds. This can lead to huge deltas between
> > rq->avg_stamp and rq->clock when rebalancing, and has been seen to
> > cause the following crash:
> > 
> >  divide error: 0000 [#1] SMP
> >  Call Trace:
> >   [<ffffffff810bcba8>] update_sd_lb_stats+0xe8/0x560
> >   [<ffffffff810bd04d>] find_busiest_group+0x2d/0x4b0
> >   [<ffffffff810bd640>] load_balance+0x170/0x950
> >   [<ffffffff810be3ff>] rebalance_domains+0x13f/0x290
> >   [<ffffffff810852bc>] __do_softirq+0xec/0x300
> >   [<ffffffff8108578a>] irq_exit+0xfa/0x110
> >   [<ffffffff816167d9>] reschedule_interrupt+0xc9/0xd0
> > 
> 
> Do you have some sort of reproducer for that crash? If not I guess I can cook
> something up with a quiet userspace & rt-app, though I've never seen that one
> on arm64.
 
Unfortunately no, I don't have a reproduction case. Would love to have
one to verify the patch though.

> > Make sure we update the rq clock and load before balancing.
> > 
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  kernel/sched/fair.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 2f0a0be4d344..2c81662c858a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9597,6 +9597,16 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
> >  	 */
> >  	smp_mb();
> >  
> > +	/*
> > +	 * Ensure this_rq's clock and load are up-to-date before we
> > +	 * rebalance since it's possible that they haven't been
> > +	 * updated for multiple schedule periods, i.e. many seconds.
> > +	 */
> > +	raw_spin_lock_irq(&this_rq->lock);
> > +	update_rq_clock(this_rq);
> > +	cpu_load_update_idle(this_rq);
> > +	raw_spin_unlock_irq(&this_rq->lock);
> > +
> 
> I'm failing to understand why the updates further down below are seemingly
> not enough. After we've potentially done 
> 
>     update_rq_clock(rq);
>     cpu_load_update_idle(rq);
> 
> for all nohz cpus != this_cpu, we still end up doing:
> 
>     if (idle != CPU_NEWLY_IDLE) {
> 	    update_blocked_averages(this_cpu);
> 	    has_blocked_load |= this_rq->has_blocked_load;
>     }
> 
> which should properly update this_rq's clock and load before we attempt to do
> any balancing on it.
 
But cpu_load_update_idle() and update_blocked_averages() are not the same
thing.