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

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Message-ID: <50F7D8C2.1030506@linux.vnet.ibm.com>
Date:	Thu, 17 Jan 2013 16:26:02 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Ivo Sieben <meltedpianoman@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.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

On 01/16/2013 05:32 PM, Ivo Sieben wrote:
> 2013/1/16 Preeti U Murthy <preeti@...ux.vnet.ibm.com>:
>>
>> Yes.Thank you very much for the explanation :) But I dont see how the
>> context switching goes away with your patch.With your patch, when the
>> higher priority thread comes in when the lower priority thread is
>> running in the critical section,it will see the wait queue empty and
>> "continue its execution" without now wanting to enter the critical
>> section.So this means it will preempt the lower priority thread because
>> it is not waiting on a lock anyway.There is a context switch here right?
>> I dont see any problem in scheduling due to this,but I do think your
>> patch is essential.
>>
> 
> 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.

Regards
Preeti U Murthy

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