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

[Valentin Schneider <valentin.schneider@arm.com>]

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.

> Make sure we update the rq clock and load before balancing.
> 
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Matt Fleming <matt@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.

>  	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
>  		if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
>  			continue;
>