sched/fair: Scale bandwidth quota and period without losing quota/period ratio precision

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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b765a58cf20f1b9c5ce10e68d42a651d88e91913..5e65c7eea872965ed508fe7d279eea277092479f 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4391,20 +4391,28 @@ 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;