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SubjectRe: [PATCH] sched/fair: Limit sched_cfs_period_timer loop to avoid hard lockup
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Phil Auld <pauld@redhat.com> writes:

> On Fri, Mar 15, 2019 at 05:03:47PM +0100 Peter Zijlstra wrote:
>> On Fri, Mar 15, 2019 at 11:30:42AM -0400, Phil Auld wrote:
>>
>> >> 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.
>>
>> Sure; but remember, simpler is often better, esp. for code that
>> typically 'never' runs.
>
> I reworked it to the below. This settles a bit faster. The average is sort of squishy because
> it's 3 samples divided by 4. And if we stay in a single call after updating the period the "average"
> will be even less accurate.
>
> It settles at a larger value faster so produces fewer messages and none of the callback supressed ones.
> The added complexity may not be worth it, though.
>
> I think this or your version, either one, would work.
>
> What needs to happen now to get one of them to land somewhere? Should I just repost one with my
> signed-off and let you add whatever other tags? And if so do you have a preference for which one?
>
> Also, Ben, thoughts?

It would probably make sense to have it just be ++count > 4 then I
think? But otherwise yeah, I'm fine with either.

>
> Cheers,
> Phil
>
> --
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ea74d43924b2..297fd228fdb0 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 =
> @@ -4892,14 +4894,46 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
> unsigned long flags;
> int overrun;
> int idle = 0;
> + int count = 0;
> + u64 start, now;
>
> raw_spin_lock_irqsave(&cfs_b->lock, flags);
> + now = start = ktime_to_ns(hrtimer_cb_get_time(timer));
> for (;;) {
> - overrun = hrtimer_forward_now(timer, cfs_b->period);
> + overrun = hrtimer_forward(timer, now, cfs_b->period);
> if (!overrun)
> break;
>
> + if (++count > 3) {
> + u64 new, old = ktime_to_ns(cfs_b->period);
> +
> + /* rough average of the time each loop is taking
> + * really should be (n-s)/3 but this is easier for the machine
> + */
> + new = (now - start) >> 2;
> + if (new < old)
> + new = old;
> + new = (new * 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);
> + now = ktime_to_ns(hrtimer_cb_get_time(timer));
> }
> if (idle)
> cfs_b->period_active = 0;

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