linux-kernel - [PATCH 1/2] sched/fair: Fix bandwidth timer clock drift condition

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Date:   Mon, 18 Jun 2018 17:16:17 +0800
From:   Xunlei Pang <xlpang@...ux.alibaba.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Ben Segall <bsegall@...gle.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] sched/fair: Fix bandwidth timer clock drift condition

The current condition to judge clock drift in expire_cfs_rq_runtime()
is wrong, the two runtime_expires are actually the same when clock
drift happens, so this condtion can never hit. The orginal design was
correctly done by commit a9cf55b28610 ("sched: Expire invalid runtime"),
but was changed to be the current one due to its locking issue.

This patch introduces another way, it adds a new field in both structure
cfs_rq and cfs_bandwidth to record the expiration update sequence, and
use them to figure out if clock drift happens(true if they equal).

This fix is also needed by the following patch.

Fixes: 51f2176d74ac ("sched/fair: Fix unlocked reads of some cfs_b->quota/period")
Cc: Ben Segall <bsegall@...gle.com>
Signed-off-by: Xunlei Pang <xlpang@...ux.alibaba.com>
---
 kernel/sched/fair.c  | 14 ++++++++------
 kernel/sched/sched.h |  6 ++++--
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e497c05aab7f..9f384264e832 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4590,6 +4590,7 @@ void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)
 	now = sched_clock_cpu(smp_processor_id());
 	cfs_b->runtime = cfs_b->quota;
 	cfs_b->runtime_expires = now + ktime_to_ns(cfs_b->period);
+	cfs_b->expires_seq++;
 }
 
 static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
@@ -4612,6 +4613,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 	struct task_group *tg = cfs_rq->tg;
 	struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(tg);
 	u64 amount = 0, min_amount, expires;
+	int expires_seq;
 
 	/* note: this is a positive sum as runtime_remaining <= 0 */
 	min_amount = sched_cfs_bandwidth_slice() - cfs_rq->runtime_remaining;
@@ -4629,6 +4631,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 		}
 	}
 	expires = cfs_b->runtime_expires;
+	expires_seq = cfs_b->expires_seq;
 	raw_spin_unlock(&cfs_b->lock);
 
 	cfs_rq->runtime_remaining += amount;
@@ -4637,8 +4640,10 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 	 * spread between our sched_clock and the one on which runtime was
 	 * issued.
 	 */
-	if ((s64)(expires - cfs_rq->runtime_expires) > 0)
+	if ((s64)(expires - cfs_rq->runtime_expires) > 0) {
 		cfs_rq->runtime_expires = expires;
+		cfs_rq->expires_seq = expires_seq;
+	}
 
 	return cfs_rq->runtime_remaining > 0;
 }
@@ -4664,12 +4669,9 @@ static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 	 * has not truly expired.
 	 *
 	 * Fortunately we can check determine whether this the case by checking
-	 * whether the global deadline has advanced. It is valid to compare
-	 * cfs_b->runtime_expires without any locks since we only care about
-	 * exact equality, so a partial write will still work.
+	 * whether the global deadline(cfs_b->expires_seq) has advanced.
 	 */
-
-	if (cfs_rq->runtime_expires != cfs_b->runtime_expires) {
+	if (cfs_rq->expires_seq == cfs_b->expires_seq) {
 		/* extend local deadline, drift is bounded above by 2 ticks */
 		cfs_rq->runtime_expires += TICK_NSEC;
 	} else {
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6601baf2361c..e977e04f8daf 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -334,9 +334,10 @@ struct cfs_bandwidth {
 	u64			runtime;
 	s64			hierarchical_quota;
 	u64			runtime_expires;
+	int			expires_seq;
 
-	int			idle;
-	int			period_active;
+	short			idle;
+	short			period_active;
 	struct hrtimer		period_timer;
 	struct hrtimer		slack_timer;
 	struct list_head	throttled_cfs_rq;
@@ -551,6 +552,7 @@ struct cfs_rq {
 
 #ifdef CONFIG_CFS_BANDWIDTH
 	int			runtime_enabled;
+	int			expires_seq;
 	u64			runtime_expires;
 	s64			runtime_remaining;
 
-- 
2.14.1.40.g8e62ba1