linux-kernel - Re: [PATCH] sched: Distinguish sched_wakeup event when wake up a task which did schedule out or not.

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Message-ID: <20140511145224.00412e59@gandalf.local.home>
Date:	Sun, 11 May 2014 14:52:24 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dongsheng Yang <dongsheng081251@...il.com>,
	"yangds.fnst" <yangds.fnst@...fujitsu.com>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, bsegall@...gle.com
Subject: Re: [PATCH] sched: Distinguish sched_wakeup event when wake up a
 task which did schedule out or not.

On Sun, 11 May 2014 18:35:31 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> So if the wait side has already observed cond==false, then without the
> wakeup, which still potentially has ->on_rq == true, it would block.
> Therefore the wakeup is a _real_ wakeup.
> 
> We fundamentally cannot know, on the wake side, if the wait side has or
> has not observed cond, and therefore the distinction you're trying to
> make is a false one.

I believe you may be misunderstanding Dongsheng. It has nothing to do
with the wake condition. But the "success" is basically saying, "did I
move the task on to the run queue?". That's a relevant piece of
information that the wake up event isn't currently showing.

Let me ask you this; with Donsheng's patch, will there ever be a
sched_switch event when the wakeup event sees 'false' and the
sched_switch event see the task with a state other than "R"? And if so,
how did the task doing the wakeup event, wake up that task?

-- Steve
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