Re: [PATCH] sched/fair: scale quota and period without losing quota/period ratio precision

Phil Auld
2019-10-07T15:14:34Z

On Thu, Oct 03, 2019 at 05:12:43PM -0700 Xuewei Zhang wrote:
> quota/period ratio is used to ensure a child task group won't get more
> bandwidth than the parent task group, and is calculated as:
> normalized_cfs_quota() = [(quota_us << 20) / period_us]
>
> If the quota/period ratio was changed during this scaling due to
> precision loss, it will cause inconsistency between parent and child
> task groups. See below example:
> A userspace container manager (kubelet) does three operations:
> 1) Create a parent cgroup, set quota to 1,000us and period to 10,000us.
> 2) Create a few children cgroups.
> 3) Set quota to 1,000us and period to 10,000us on a child cgroup.
>
> These operations are expected to succeed. However, if the scaling of
> 147/128 happens before step 3), quota and period of the parent cgroup
> will be changed:
> new_quota: 1148437ns, 1148us
> new_period: 11484375ns, 11484us
>
> And when step 3) comes in, the ratio of the child cgroup will be 104857,
> which will be larger than the parent cgroup ratio (104821), and will
> fail.
>
> Scaling them by a factor of 2 will fix the problem.
>
> Fixes: 2e8e19226398 ("sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup")
> Signed-off-by: Xuewei Zhang <xueweiz@google.com>

I managed to get it to trigger the second case. It took 50,000 children (20x my initial tests).

[ 1367.850630] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 4340, cfs_quota_us = 250000)
[ 1370.390832] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 8680, cfs_quota_us = 500000)
[ 1372.914689] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 17360, cfs_quota_us = 1000000)
[ 1375.447431] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 34720, cfs_quota_us = 2000000)
[ 1377.982785] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 69440, cfs_quota_us = 4000000)
[ 1380.481702] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 138880, cfs_quota_us = 8000000)
[ 1382.894692] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 277760, cfs_quota_us = 16000000)
[ 1385.264872] cfs_period_timer[cpu11]: period too short, scaling up (new cfs_period_us = 555520, cfs_quota_us = 32000000)
[ 1393.965140] cfs_period_timer[cpu11]: period too short, but cannot scale up without losing precision (cfs_period_us = 555520, cfs_quota_us = 32000000)

I suspect going higher could cause the original lockup, but that'd be the case with the old code as well.
And this also gets us out of it faster.

Tested-by: Phil Auld <pauld@redhat.com>

Cheers,
Phil

> ---
> kernel/sched/fair.c | 36 ++++++++++++++++++++++--------------
> 1 file changed, 22 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 83ab35e2374f..b3d3d0a231cd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4926,20 +4926,28 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
> 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 = div64_u64(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(),
> - div_u64(new, NSEC_PER_USEC),
> - div_u64(cfs_b->quota, NSEC_PER_USEC));
> + /*
> + * Grow period by a factor of 2 to avoid lossing precision.
> + * Precision loss in the quota/period ratio can cause __cfs_schedulable
> + * to fail.
> + */
> + new = old * 2;
> + if (new < max_cfs_quota_period) {
> + cfs_b->period = ns_to_ktime(new);
> + cfs_b->quota *= 2;
> +
> + 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(),
> + div_u64(new, NSEC_PER_USEC),
> + div_u64(cfs_b->quota, NSEC_PER_USEC));
> + } else {
> + pr_warn_ratelimited(
> + "cfs_period_timer[cpu%d]: period too short, but cannot scale up without losing precision (cfs_period_us = %lld, cfs_quota_us = %lld)\n",
> + smp_processor_id(),
> + div_u64(old, NSEC_PER_USEC),
> + div_u64(cfs_b->quota, NSEC_PER_USEC));
> + }
>
> /* reset count so we don't come right back in here */
> count = 0;
> --
> 2.23.0.581.g78d2f28ef7-goog
>

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