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

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Date:	Mon, 21 Jan 2013 08:26:03 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Ivo Sieben <meltedpianoman@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>, 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

On 01/18/2013 09:15 PM, Oleg Nesterov wrote:
> On 01/17, Preeti U Murthy wrote:
>>
>> On 01/16/2013 05:32 PM, Ivo Sieben wrote:
>>>
>>> I don't have a problem that there is a context switch to the high
>>> priority process: it has a higher priority, so it probably is more
>>> important.
>>> My problem is that even when the waitqueue is empty, the high priority
>>> thread has a risk to block on the spinlock needlessly (causing context
>>> switches to low priority task and back to the high priority task)
>>>
>> Fair enough Ivo.I think you should go ahead with merging the
>> waitqueue_active()
>>   wake_up()
>> logic into the wake_up() variants.
> 
> This is not easy. We can't simply change wake_up*() helpers or modify
> __wake_up().

Hmm.I need to confess that I don't really know what goes into a change
such as this.Since there are a lot of waitqueue_active()+wake_up()
calls,I was wondering why at all have a separate logic as
waitqueue_active(),if we could do what it does in wake_up*(). But you
guys can decide this best.
> 
> I can't understand why do you dislike Ivo's simple patch. There are
> a lot of "if (waitqueue_active) wake_up" examples. Even if we add the
> new helpers (personally I don't think this makes sense) , we can do
> this later. Why should we delay this fix?

Personally i was concerned about how this could cause a scheduler
overhead.There does not seem to be much of a problem here.Ivo's patch
for adding a waitqueue_active() for his specific problem would also do
well,unless there is a dire requirement for a clean up,which I am unable
to evaluate.

> 
> Oleg.
> 

Thank you

Regards
Preeti U Murthy

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