linux-kernel - [PATCH v2] sched/deadline: Fix the intention to re-evalute tick dependency for offline cpu

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Date:	Fri, 12 Aug 2016 16:19:00 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@....com>,
	Luca Abeni <luca.abeni@...tn.it>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [PATCH v2] sched/deadline: Fix the intention to re-evalute tick dependency for offline cpu

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

The dl task will be replenished after dl task timer fire and start a 
new period. It will be enqueued and to re-evaluate its dependency on 
the tick in order to restart it. However, if cpu is hot-unplug, 
irq_work_queue will splash since the target cpu is offline.

As a result:

    WARNING: CPU: 2 PID: 0 at kernel/irq_work.c:69 irq_work_queue_on+0xad/0xe0
    Call Trace:
     dump_stack+0x99/0xd0
     __warn+0xd1/0xf0
     warn_slowpath_null+0x1d/0x20
     irq_work_queue_on+0xad/0xe0
     tick_nohz_full_kick_cpu+0x44/0x50
     tick_nohz_dep_set_cpu+0x74/0xb0
     enqueue_task_dl+0x226/0x480
     activate_task+0x5c/0xa0
     dl_task_timer+0x19b/0x2c0
     ? push_dl_task.part.31+0x190/0x190
  
This can be triggered by hot-unplug the full dynticks cpu which dl 
task is running on. 

We enqueue the dl task on the offline CPU, because we need to do 
replenish for start_dl_timer(). So, as Juri pointed out, we would 
need to do is calling replenish_dl_entity() directly, instead of 
enqueue_task_dl(). pi_se shouldn't be a problem as the task shouldn't 
be boosted if it was throttled.

This patch fix it by just replenish dl entity to avoid the intention 
to re-evaluate tick dependency if the cpu is offline.

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@....com>
Cc: Luca Abeni <luca.abeni@...tn.it>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
v1 -> v2:
 * replenish dl entity

 kernel/sched/deadline.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index d091f4a..b141bd2 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -641,11 +641,15 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 		goto unlock;
 	}
 
-	enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
-	if (dl_task(rq->curr))
-		check_preempt_curr_dl(rq, p, 0);
-	else
-		resched_curr(rq);
+	if (unlikely(!rq->online))
+		replenish_dl_entity(dl_se, dl_se);
+	else {
+		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
+		if (dl_task(rq->curr))
+			check_preempt_curr_dl(rq, p, 0);
+		else
+			resched_curr(rq);
+	}
 
 #ifdef CONFIG_SMP
 	/*
-- 
1.9.1