linux-kernel - [PATCH 3.16 050/136] sched/fair: Scale bandwidth quota and period without losing quota/period ratio precision

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Message-ID: <lsq.1576543535.299305590@decadent.org.uk>
Date:   Tue, 17 Dec 2019 00:46:24 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Xuewei Zhang" <xueweiz@...gle.com>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        "Ingo Molnar" <mingo@...nel.org>, "Mel Gorman" <mgorman@...e.de>,
        "Phil Auld" <pauld@...hat.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Dietmar Eggemann" <dietmar.eggemann@....com>,
        "Vincent Guittot" <vincent.guittot@...aro.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ben Segall" <bsegall@...gle.com>,
        "Juri Lelli" <juri.lelli@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Anton Blanchard" <anton@...abs.org>
Subject: [PATCH 3.16 050/136] sched/fair: Scale bandwidth quota and period
 without losing quota/period ratio precision

3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Xuewei Zhang <xueweiz@...gle.com>

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@...hat.com>
Signed-off-by: Xuewei Zhang <xueweiz@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Phil Auld <pauld@...hat.com>
Cc: Anton Blanchard <anton@...abs.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vincent Guittot <vincent.guittot@...aro.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@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/sched/fair.c | 36 ++++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3753,20 +3753,28 @@ 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;