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SubjectRe: [PATCH] sched/fair: Limit sched_cfs_period_timer loop to avoid hard lockup
On Fri, Mar 15, 2019 at 11:11:50AM +0100 Peter Zijlstra wrote:
> On Wed, Mar 13, 2019 at 11:08:26AM -0400, Phil Auld wrote:

...

> Computers _suck_ at /100. And since you're free to pick the constant,
> pick a power of two, computers love those.
>
> > +
> > + if (new_period > max_cfs_quota_period)
> > + new_period = max_cfs_quota_period;
> > +
> > + cfs_b->period = ns_to_ktime(new_period);
> > + cfs_b->quota += (cfs_b->quota * ((new_period - old_period) * 100)/old_period)/100;
>
> srsly!? Again, you can pick the constant to be anything, and you pick
> such a horrid number?!
>

In my defense here, all the fair.c imbalance pct code also uses 100 :)


> > + pr_warn_ratelimited(
> > + "cfs_period_timer[cpu%d]: period too short, scaling up (new cfs_period_us %lld, cfs_quota_us = %lld)\n",
> > + smp_processor_id(), cfs_b->period/NSEC_PER_USEC, cfs_b->quota/NSEC_PER_USEC);
>
> period was ktime_t, remember...

Indeed. Worked but was incorrect.

>
> Would not something simpler like the below also work?


With my version:

[ 4246.030004] cfs_period_timer[cpu16]: period too short, scaling up (new cfs_period_us 5276, cfs_quota_us = 303973)
[ 4246.346978] cfs_period_timer[cpu16]: period too short, scaling up (new cfs_period_us 17474, cfs_quota_us = 1006569)

(most of the time it's only one message. Sometimes it does a smaller increase once first like this)



with the below:

[ 117.235804] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 2492, cfs_quota_us = 143554)
[ 117.346807] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 2862, cfs_quota_us = 164863)
[ 117.470569] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 3286, cfs_quota_us = 189335)
[ 117.574883] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 3774, cfs_quota_us = 217439)
[ 117.652907] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 4335, cfs_quota_us = 249716)
[ 118.090535] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 4978, cfs_quota_us = 286783)
[ 122.098009] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 5717, cfs_quota_us = 329352)
[ 126.255209] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 6566, cfs_quota_us = 378240)
[ 126.358060] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 7540, cfs_quota_us = 434385)
[ 126.538358] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 8660, cfs_quota_us = 498865)
[ 126.614304] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 9945, cfs_quota_us = 572915)
[ 126.817085] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 11422, cfs_quota_us = 657957)
[ 127.352038] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 13117, cfs_quota_us = 755623)
[ 127.598043] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 15064, cfs_quota_us = 867785)


Plus on repeats I see an occasional

[ 152.803384] sched_cfs_period_timer: 9 callbacks suppressed



I'll rework the maths in the averaged version and post v2 if that makes sense.

It may have the extra timer fetch, although maybe I could rework it so that it used the
nsstart time the first time and did not need to do it twice in a row. I had originally
reverted the hrtimer_forward_now() to hrtimer_forward() but put that back.


Thanks for looking at it.


Also, fwiw, this was reported earlier by Anton Blanchard in https://lkml.org/lkml/2018/12/3/1047


Cheers,
Phil

>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ea74d43924b2..b71557be6b42 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4885,6 +4885,8 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
> return HRTIMER_NORESTART;
> }
>
> +extern const u64 max_cfs_quota_period;
> +
> static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
> {
> struct cfs_bandwidth *cfs_b =
> @@ -4892,6 +4894,7 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
> unsigned long flags;
> int overrun;
> int idle = 0;
> + int count = 0;
>
> raw_spin_lock_irqsave(&cfs_b->lock, flags);
> for (;;) {
> @@ -4899,6 +4902,28 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
> if (!overrun)
> break;
>
> + if (++count > 3) {
> + u64 new, old = ktime_to_ns(cfs_b->period);
> +
> + new = (old * 147) / 128; /* ~115% */
> + new = min(new, max_cfs_quota_period);
> +
> + cfs_b->period = ns_to_ktime(new);
> +
> + /* since max is 1s, this is limited to 1e9^2, which fits in u64 */
> + cfs_b->quota *= new;
> + cfs_b->quota /= old;
> +
> + pr_warn_ratelimited(
> + "cfs_period_timer[cpu%d]: period too short, scaling up (new cfs_period_us %lld, cfs_quota_us = %lld)\n",
> + smp_processor_id(),
> + new/NSEC_PER_USEC,
> + cfs_b->quota/NSEC_PER_USEC);
> +
> + /* reset count so we don't come right back in here */
> + count = 0;
> + }
> +
> idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
> }
> if (idle)

--

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