From mboxrd@z Thu Jan  1 00:00:00 1970
From: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Subject: Re: [PATCH] tty: Only wakeup the line discipline idle queue when
 queue is active
Date: Thu, 17 Jan 2013 16:26:02 +0530
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To: Ivo Sieben <meltedpianoman@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, Andi Kleen <andi@firstfloor.org>, Oleg Nesterov <oleg@redhat.com>, Jiri Slaby <jslaby@suse.cz>, Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-serial@vger.kernel.org, Alan Cox <alan@linux.intel.com>, Greg KH <gregkh@linuxfoundation.org>

On 01/16/2013 05:32 PM, Ivo Sieben wrote:
> 2013/1/16 Preeti U Murthy <preeti@linux.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