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 <mi...@kernel.org>
> > Cc: Mike Galbraith <umgwanakikb...@gmail.com>
> > Cc: Peter Zijlstra <pet...@infradead.org>
> > Signed-off-by: Matt Fleming <m...@codeblueprint.co.uk>
> > ---
> > 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.
