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Date:	Thu, 3 Jan 2013 10:49:44 +0100
From:	Ivo Sieben <meltedpianoman@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>
Cc:	Jiri Slaby <jslaby@...e.cz>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-serial@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] tty: Only wakeup the line discipline idle queue when
 queue is active

Oleg, Peter, Ingo, Andi & Preeti,

2013/1/2 Jiri Slaby <jslaby@...e.cz>:
> On 01/02/2013 04:21 PM, Ivo Sieben wrote:
>> I don't understand your responses: do you suggest to implement this
>> "if active" behavior in:
>> * A new wake_up function called wake_up_if_active() that is part of
>> the waitqueue layer?
>
> Sounds good.
>
> --
> js
> suse labs

I want to ask you 'scheduler' people for your opinion:

Maybe you remember my previous patch where I suggested an extra
'waitqueue empty' check before entering the critical section of the
wakeup() function (If you do not remember see
https://lkml.org/lkml/2012/10/25/159)

Finally Oleg responded that a lot of callers do

	if (waitqueue_active(q))
		wake_up(...);

what made my patch pointless and adds a memory barrier. I then decided
to also implement the 'waitqueue_active' approach for my problem.

But now I get a review comment by Jiri that he would like to hide this
'if active behavior' in a wake_up_if_active() kind of function. I
think he is right that implementing this check in the wakeup function
would clean things up, right?

I would like to have your opinion on the following two suggestions:
- We still can do the original patch on the wake_up() that I
suggested. I then can do an additional code cleanup patch that removes
the double 'waitqueue_active' call (a quick grep found about 150 of
these waitqueue active calls) on several places in the code.
- Or - as an alternative - I could add extra _if_active() versions of
all wake_up() functions, that implement this extra test.

Regards,
Ivo
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