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From: Vincent Guittot <vincent.guittot@linaro.org>
To: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
	dietmar.eggemann@arm.com, rostedt@goodmis.org,
	bsegall@google.com, mgorman@suse.de,
	linux-kernel@vger.kernel.org, iwtbavbm@gmail.com
Cc: valentin.schneider@arm.com, Vincent Guittot <vincent.guittot@linaro.org>
Subject: [PATCH v2] sched/fair: fix nohz next idle balance
Date: Tue,  9 Jun 2020 14:37:48 +0200
Message-ID: <20200609123748.18636-1-vincent.guittot@linaro.org> (raw)

With commit:
  'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")'
rebalance_domains of the local cfs_rq happens before others idle cpus have
updated nohz.next_balance and its value is overwritten.

Move the update of nohz.next_balance for other idles cpus before balancing
and updating the next_balance of local cfs_rq.

Also, the nohz.next_balance is now updated only if all idle cpus got a
chance to rebalance their domains and the idle balance has not been aborted
because of new activities on the CPU. In case of need_resched, the idle
load balance will be kick the next jiffie in order to address remaining
ilb.

Reported-by: Peng Liu <iwtbavbm@gmail.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---

- Changes v2:
  - use jiffies instead of incrementing nohz.next_balance to be sure of the
    value. The change slipped out of the previous version.

 kernel/sched/fair.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0ed04d2a8959..832164d441dd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10017,7 +10017,12 @@ static void kick_ilb(unsigned int flags)
 {
 	int ilb_cpu;
 
-	nohz.next_balance++;
+	/*
+	 * Increase nohz.next_balance only when if full ilb is triggered but
+	 * not if we only update stats.
+	 */
+	if (flags & NOHZ_BALANCE_KICK)
+		nohz.next_balance = jiffies+1;
 
 	ilb_cpu = find_new_ilb();
 
@@ -10338,6 +10343,14 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
 		}
 	}
 
+	/*
+	 * next_balance will be updated only when there is a need.
+	 * When the CPU is attached to null domain for ex, it will not be
+	 * updated.
+	 */
+	if (likely(update_next_balance))
+		nohz.next_balance = next_balance;
+
 	/* Newly idle CPU doesn't need an update */
 	if (idle != CPU_NEWLY_IDLE) {
 		update_blocked_averages(this_cpu);
@@ -10358,14 +10371,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
 	if (has_blocked_load)
 		WRITE_ONCE(nohz.has_blocked, 1);
 
-	/*
-	 * next_balance will be updated only when there is a need.
-	 * When the CPU is attached to null domain for ex, it will not be
-	 * updated.
-	 */
-	if (likely(update_next_balance))
-		nohz.next_balance = next_balance;
-
 	return ret;
 }
 
-- 
2.17.1


             reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-09 12:37 Vincent Guittot [this message]
2020-06-10 12:49 ` Peter Zijlstra
2020-06-11 14:12 ` Valentin Schneider
2020-06-11 14:28   ` Peter Zijlstra
2020-06-11 14:43 ` Mel Gorman
2020-06-16 12:21 ` [tip: sched/core] sched/fair: Fix NOHZ " tip-bot2 for Vincent Guittot

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