From: Xuewei Zhang <xueweiz@xxxxxxxxxx> commit 4929a4e6faa0f13289a67cae98139e727f0d4a97 upstream. 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> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/sched/fair.c | 34 +++++++++++++++++++++------------- 1 file changed, 21 insertions(+), 13 deletions(-) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4655,20 +4655,28 @@ static enum hrtimer_restart sched_cfs_pe if (++count > 3) { u64 new, old = ktime_to_ns(cfs_b->period); - new = (old * 147) / 128; /* ~115% */ - new = min(new, max_cfs_quota_period); + /* + * 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; - 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)); + 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;