linux-kernel - [PATCH] sched/fair: Fix the nohz.next

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Date:   Sun,  5 Feb 2017 01:57:35 -0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Wanpeng Li <wanpeng.li@...mail.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH] sched/fair: Fix the nohz.next_balance update mess 

From: Wanpeng Li <wanpeng.li@...mail.com>

The commit: 
  c5afb6a87f2 ("sched/fair: Fix nohz.next_balance update")

intends to update nohz.next_balance in two steps.

1) The ILB CPU utilizes next_balance variable in nohz_idle_balance() 
   to gather the shortest next balance of other idle CPUs before 
   updating nohz.next_balance. 
2) The ILB CPU updates the nohz.next_balance according to its own 
   next_balance after load balance on behalf of other idle CPUs.

However, there is a mess which breaks the original intention of the 
first step, every idle CPUs update nohz.next_balance during ILB CPU 
on behalf of them to do load balance, and then the ILB CPU utilizes 
next_balance variable in nohz_idle_balance() to gather the shortest 
next balance of other idle CPUs before updating nohz.next_balance.

This patch fixes it by don't update nohz.next_balance for other idle 
CPUs when ILB CPU on behalf of them to do load balance. 

Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
CC: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 274c747..83948a4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8750,7 +8750,8 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
 		 * balance for itself and we need to update the
 		 * nohz.next_balance accordingly.
 		 */
-		if ((idle == CPU_IDLE) && time_after(nohz.next_balance, rq->next_balance))
+		if ((idle == CPU_IDLE) && time_after(nohz.next_balance, rq->next_balance) &&
+			!test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_rq()->cpu)))
 			nohz.next_balance = rq->next_balance;
 #endif
 	}
-- 
2.7.4