[PATCH v4.19] sched/fair: Scale bandwidth quota and period without losing quota/period ratio precision - Linux-stable-mirror

7 Dec 2019

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@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


    