On Fri, Dec 6, 2019 at 5:04 PM Xuewei Zhang xueweiz@google.com 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:
- Create a parent cgroup, set quota to 1,000us and period to 10,000us.
- Create a few children cgroups.
- 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@redhat.com Signed-off-by: Xuewei Zhang xueweiz@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Phil Auld pauld@redhat.com Cc: Anton Blanchard anton@ozlabs.org Cc: Ben Segall bsegall@google.com Cc: Dietmar Eggemann dietmar.eggemann@arm.com Cc: Juri Lelli juri.lelli@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Mel Gorman mgorman@suse.de Cc: Peter Zijlstra peterz@infradead.org Cc: Steven Rostedt rostedt@goodmis.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Vincent Guittot vincent.guittot@linaro.org Fixes: 2e8e19226398 ("sched/fair: Limit sched_cfs_period_timer() loop to avoid hard lockup") Link: https://lkml.kernel.org/r/20191004001243.140897-1-xueweiz@google.com Signed-off-by: Ingo Molnar mingo@kernel.org
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