linux-kernel - Re: [PATCH] tty: Only wakeup the line discipline idle queue when queue is active

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Date:	Tue, 15 Jan 2013 10:16:17 +0100
From:	Ivo Sieben <meltedpianoman@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	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>,
	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

Hi

2013/1/3 Oleg Nesterov <oleg@...hat.com>:
>> 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.
>
> Plus this change doesn't look 100% correct, at least in theory.
>
>> I then decided
>> to also implement the 'waitqueue_active' approach for my problem.
>
> Well, if you ask me I think this is the best solution ;)
>
> But I won't insist.
>
>> 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.
>
> In this case we should also fix some users of add_wait_queue().
>
>> - Or - as an alternative - I could add extra _if_active() versions of
>> all wake_up() functions, that implement this extra test.
>
> Not sure this will actually help to make the code cleaner. The last
> patch you sent looks simple and clean. IMHO it doesn't make sense
> to create _if_active helper for each wake_up*.
>
> Oleg.
>

The comments by Oleg point out to me that the 'if waitqueueu_active'
is a common practice for checking a waitqueue to be non empty. It is
applied this way on several places in the kernel code.

@Jiri, Alan, Greg:
Don't you agree my patch makes sense?
It solves an issue for me, and I really would like this patch to be approved.

Regards,

Ivo Sieben
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