On Fri, Dec 6, 2019 at 5:04 PM Xuewei Zhang <xueweiz@xxxxxxxxxx> wrote:
Sorry. I forgot to add the upstream line:
"commit 4929a4e6faa0f13289a67cae98139e727f0d4a97 upstream."
Sending a new patch. Please ignore this one. Sorry about that.
>
> The 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.
>
> Tested-by: Phil Auld <pauld@xxxxxxxxxx>
> Signed-off-by: Xuewei Zhang <xueweiz@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Acked-by: Phil Auld <pauld@xxxxxxxxxx>
> Cc: Anton Blanchard <anton@xxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Fixes: 2e8e19226398 ("sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup")
> Link: https://lkml.kernel.org/r/20191004001243.140897-1-xueweiz@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
> 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 f77fcd37b226..f0abb8fe0ae9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4868,20 +4868,28 @@ 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 losing 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;
> --
> 2.24.0.393.g34dc348eaf-goog
>
