linux-kernel - Re: [RFC] sched/fair: hard lockup in sched_cfs_period

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Date:   Mon, 11 Mar 2019 10:44:25 -0700
From:   bsegall@...gle.com
To:     Phil Auld <pauld@...hat.com>
Cc:     mingo@...hat.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC]  sched/fair: hard lockup in sched_cfs_period_timer

Phil Auld <pauld@...hat.com> writes:

> On Wed, Mar 06, 2019 at 11:25:02AM -0800 bsegall@...gle.com wrote:
>> Phil Auld <pauld@...hat.com> writes:
>> 
>> > On Tue, Mar 05, 2019 at 12:45:34PM -0800 bsegall@...gle.com wrote:
>> >> Phil Auld <pauld@...hat.com> writes:
>> >> 
>> >> > Interestingly, if I limit the number of child cgroups to the number of 
>> >> > them I'm actually putting processes into (16 down from 2500) the problem
>> >> > does not reproduce.
>> >> 
>> >> That is indeed interesting, and definitely not something we'd want to
>> >> matter. (Particularly if it's not root->a->b->c...->throttled_cgroup or
>> >> root->throttled->a->...->thread vs root->throttled_cgroup, which is what
>> >> I was originally thinking of)
>> >> 
>> >
>> > The locking may be a red herring.
>> >
>> > The setup is root->throttled->a where a is 1-2500. There are 4 threads in
>> > each of the first 16 a groups.  The parent, throttled, is where the 
>> > cfs_period/quota_us are set. 
>> >
>> > I wonder if the problem is the walk_tg_tree_from() call in unthrottle_cfs_rq(). 
>> >
>> > The distribute_cfg_runtime looks to be O(n * m) where n is number of 
>> > throttled cfs_rqs and m is the number of child cgroups. But I'm not 
>> > completely clear on how the hierarchical cgroups play together here. 
>> >
>> > I'll pull on this thread some. 
>> >
>> > Thanks for your input.
>> >
>> >
>> > Cheers,
>> > Phil
>> 
>> Yeah, that isn't under the cfs_b lock, but is still part of distribute
>> (and under rq lock, which might also matter). I was thinking too much
>> about just the cfs_b regions. I'm not sure there's any good general
>> optimization there.
>>
>
> It's really an edge case, but the watchdog NMI is pretty painful.
>
>> I suppose cfs_rqs (tgs/cfs_bs?) could have "nearest
>> ancestor with a quota" pointer and ones with quota could have
>> "descendants with quota" list, parallel to the children/parent lists of
>> tgs. Then throttle/unthrottle would only have to visit these lists, and
>> child cgroups/cfs_rqs without their own quotas would just check
>> cfs_rq->nearest_quota_cfs_rq->throttle_count. throttled_clock_task_time
>> can also probably be tracked there.
>
> That seems like it would add a lot of complexity for this edge case. Maybe
> it would be acceptible to use the safety valve like my first example, or
> something like the below which will tune the period up until it doesn't
> overrun for ever.  The down side of this one is it does change the user's
> settings, but that could be preferable to an NMI crash.

Yeah, I'm not sure what solution is best here, but one of the solutions
should be done.

>
> Cheers,
> Phil
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 310d0637fe4b..78f9e28adc7b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4859,16 +4859,42 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
>  	return HRTIMER_NORESTART;
>  }
>  
> +extern const u64 max_cfs_quota_period;
> +s64 cfs_quota_period_autotune_thresh = 100 * NSEC_PER_MSEC;
> +int cfs_quota_period_autotune_shift  = 4; /* 100 / 16 = 6.25% */

Letting it spin for 100ms and then only increasing by 6% seems extremely
generous. If we went this route I'd probably say "after looping N
times, set the period to time taken / N + X%" where N is like 8 or
something. I think I'd probably perfer something like this to the
previous "just abort and let it happen again next interrupt" one.

> +
>  static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  {
>  	struct cfs_bandwidth *cfs_b =
>  		container_of(timer, struct cfs_bandwidth, period_timer);
> +	s64 nsprev, nsnow, new_period;
> +	ktime_t now;
>  	int overrun;
>  	int idle = 0;
>  
>  	raw_spin_lock(&cfs_b->lock);
> +	nsprev = ktime_to_ns(hrtimer_cb_get_time(timer));
>  	for (;;) {
> -		overrun = hrtimer_forward_now(timer, cfs_b->period);
> +		/* 
> +		 * Note this reverts the change to use hrtimer_forward_now, which avoids calling hrtimer_cb_get_time
> +		 * for a value we already have
> +		 */
> +		now = hrtimer_cb_get_time(timer);
> +		nsnow = ktime_to_ns(now);
> +		if (nsnow - nsprev >= cfs_quota_period_autotune_thresh) {
> +			new_period = ktime_to_ns(cfs_b->period);
> +			new_period += new_period >> cfs_quota_period_autotune_shift;
> +			if (new_period <= max_cfs_quota_period) {
> +				cfs_b->period = ns_to_ktime(new_period);
> +				cfs_b->quota += cfs_b->quota >> cfs_quota_period_autotune_shift;
> +				pr_warn_ratelimited(
> +					"cfs_period_timer [cpu%d] : Running too long, scaling up (new period %lld, new quota = %lld)\n", 
> +					smp_processor_id(), cfs_b->period/NSEC_PER_USEC, cfs_b->quota/NSEC_PER_USEC);
> +			}
> +			nsprev = nsnow;
> +		}
> +
> +		overrun = hrtimer_forward(timer, now, cfs_b->period);
>  		if (!overrun)
>  			break;