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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:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 310d0637fe4b..90cc67bbf592 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4859,19 +4859,51 @@ static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
> > return HRTIMER_NORESTART;
> > }
> >
> > +extern const u64 max_cfs_quota_period;
> > +int cfs_period_autotune_loop_limit = 8;
> > +int cfs_period_autotune_cushion_pct = 15; /* percentage added to period recalculation */
>
> static const ?
>
> > +
> > 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 nsstart, nsnow, new_period;
>
> u64
>
> > int overrun;
> > int idle = 0;
> > + int count = 0;
> >
> > raw_spin_lock(&cfs_b->lock);
> > + nsstart = ktime_to_ns(hrtimer_cb_get_time(timer));
>
> we really should kill ktime :/ Anyway, you now do two indirect timer
> calls back to back :/
>
> And this is unconditional overhead.
>
> > for (;;) {
> > overrun = hrtimer_forward_now(timer, cfs_b->period);
> > if (!overrun)
> > break;
> >
> > + if (++count > cfs_period_autotune_loop_limit) {
> > + ktime_t old_period = ktime_to_ns(cfs_b->period);
> > +
> > + nsnow = ktime_to_ns(hrtimer_cb_get_time(timer));
> > + new_period = (nsnow - nsstart)/cfs_period_autotune_loop_limit;
> > +
> > + /* Make sure new period will be larger than old. */
> > + if (new_period < old_period) {
> > + new_period = old_period;
> > + }
> > + new_period += (new_period * cfs_period_autotune_cushion_pct) / 100;
>
> Computers _suck_ at /100. And since you're free to pick the constant,
> pick a power of two, computers love those.
>

Fair enough, I was thinking percents. And also that once we get in here it's not a really hot path.

> > +
> > + 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?!
>

Same as above :)


> > + 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...
>
> > +
>
> And these here lines all all waaay too long.
>
> Also, is that complexity really needed?
>
> > + /* reset count so we don't come right back in here */
> > + count = 0;
> > + }
> > +
> > idle = do_sched_cfs_period_timer(cfs_b, overrun);
> > }
> > if (idle)
>
>
> Would not something simpler like the below also work?
>

I expect it would.... but the original concept increased the period to
a bit more (15%) than the average time it was taking to go around the loop.
This version will run the loop a lot longer as it's only increasing by
15% each time.

In my setup it would go from ~2000 to ~11000 and be done. This one
will go off, raise 2000 to 2300, then fire again, raise to 2645 , etc.

Unless it's getting reset this would still be a one time thing so that
may not matter.

The math calculations do look better though. I knew there had to be a
better way to scale up the quota. It just wasn't jumping out at me :)


Thanks,

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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