linux-kernel - Re: priority based thread wakeup

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Date:	Fri, 18 Jan 2008 08:33:59 -0500
From:	Gregory Haskins <gregory.haskins.ml@...il.com>
To:	Mark Hansen <Mark.Hansen@...rusrtps.com.au>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	rostedt@...dmis.org
Subject: Re: priority based thread wakeup

Mark Hansen wrote:
> Hello,
> 
> Firstly, may I apologise as I am not a member of the LKML, and ask that 
> I be CC'd in any responses that may be forthcoming.
> 
> My question concerns the following patch which was incorporated into the 
> 2.6.22 kernel (quoted from that change log):
> 
>> Today, all threads waiting for a given futex are woken in FIFO 
>> order (first waiter woken first) instead of priority order.
>>
>> This patch makes use of plist (pirotity ordered lists) instead 
>> of simple list in futex_hash_bucket.
>>
>> All non-RT threads are stored with priority MAX_RT_PRIO, causing
>> them to be woken last, in FIFO order (RT-threads are woken first, 
>> in priority order).
>>
>> Signed-off-by: Sebastien Dugue <sebastien.du...@...l.net>
>> Signed-off-by: Pierre Peiffer <pierre.peif...@...l.net>
> 
> After updating to this version of the kernel, I was able to observe the 
> above fix, where multiple RT threads invoking pthread_cond_wait(), and 
> the highest priority thread will acquire the mutex first, after the 
> thread holding the mutex calls pthread_cond_signal(); 
> pthread_mutex_unlock()
> 
> However, since kernel 2.6.23, it seems that the functionality relating 
> to this "priority based wakeup" has disappeared. 
> 
> I understand there have been significant changes in this kernel 
> concerning the "Completely Fair Scheduler" replacing the "mainline" 
> scheduler; however my understanding is that the RT functionality would 
> be preserved. This does not appear to be the case based on repeating the 
> experiment described above.
> 
> I was wondering if this functionality is considered no longer 
> desirable/necessary?
> 
> If not, is it anticipated that this functionality could/would be 
> included in a later kernel?
>

Hi Mark,
   In a coffee deprived stupor ;), I responded to this mail in the wrong 
thread, here:

http://lkml.org/lkml/2008/1/18/207

Sorry for the confusion
HTH

-Greg


> Regards,
> Mark Hansen
> 
> 
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