linux-kernel - Re: [PATCH 4/8] sched/core: Skip wakeup when task is already running.

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Message-ID: <534E59FC.2090001@cn.fujitsu.com>
Date:	Wed, 16 Apr 2014 19:22:52 +0900
From:	Dongsheng Yang <yangds.fnst@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<rostedt@...dmis.org>, <fweisbec@...il.com>, <mingo@...hat.com>,
	<acme@...stprotocols.net>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] sched/core: Skip wakeup when task is already running.

On 04/15/2014 10:53 PM, Peter Zijlstra wrote:
> On Tue, Apr 15, 2014 at 09:32:53PM +0900, Dongsheng Yang wrote:
>
> How can you get there with ->state == RUNNING? try_to_wake_up*() bail
> when !(->state & state).
Yes, try_to_wake_up() did this check. But other callers would miss it.

With the following code ,I can get the actual message of waking up
a running task

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9f63275..1369cae 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1418,8 +1418,10 @@ static void ttwu_activate(struct rq *rq, struct 
task_stru
  static void
  ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
  {
-       if (p->state == TASK_RUNNING)
+       if (p->state == TASK_RUNNING) {
+               printk("Wakeup a running task.");
                 return;
+       }

         check_preempt_curr(rq, p, wake_flags);
         trace_sched_wakeup(p, true);


# grep "Wakeup" /var/log/messages
Apr 15 20:16:21 localhost kernel: [    5.436505] Wakeup a running task.
Apr 15 20:16:21 localhost kernel: [    7.776042] Wakeup a running task.
Apr 15 20:16:21 localhost kernel: [    9.324274] Wakeup a running task.

So, I think there are some caller of ttwu_do_wakeup() is attempt to wake
up a running task.
> .
>

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