Message-ID: <4BBA6F0B.2060008@us.ibm.com>
Date: Mon, 05 Apr 2010 16:15:23 -0700
From: Darren Hart <dvhltc@us.ibm.com>
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To: Avi Kivity <avi@redhat.com>
CC: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       Eric Dumazet <eric.dumazet@gmail.com>,
       "Peter W. Morreale" <pmorreale@novell.com>,
       Rik van Riel <riel@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
       Gregory Haskins <ghaskins@novell.com>,
       Sven-Thorsten Dietrich <sdietrich@novell.com>,
       Chris Mason <chris.mason@oracle.com>,
       John Cooper <john.cooper@third-harmonic.com>,
       Chris Wright <chrisw@sous-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
 spinning
References: <1270499039-23728-1-git-send-email-dvhltc@us.ibm.com> <4BBA5305.7010002@redhat.com> <4BBA5C00.4090703@us.ibm.com> <4BBA6279.20802@redhat.com>
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Avi Kivity wrote:

>>> An interesting (but perhaps difficult to achieve) optimization would 
>>> be to spin in userspace.
>>
>> I couldn't think of a lightweight way to determine when the owner has 
>> been scheduled out in userspace. Kernel assistance is required. You 
>> could do this on the schedule() side of things, but I figured I'd get 
>> some strong pushback if I tried to add a hook into descheduling that 
>> flipped a bit in the futex value stating the owner was about to 
>> deschedule(). Still, that might be something to explore.
> 
> In the futex value it's hopeless (since a thread can hold many locks), 

It can, but there is a futex value per lock. If the task_struct had a 
list of held futex locks (as it does for pi futex locks) the 
deschedule() path could walk that and mark the FUTEX_OWNER_SLEEPING bit.


> but I don't think it's unreasonable to set a bit in the thread local 
> storage area.  The futex format would then need to be extended to 
> contain a pointer to this bit.

This appears to be 1 bit per task instead of 1 bit per lock. Also, the 
value is thread-specific... so how would a potential waiter be able to 
determine if the owner of a particular lock was running or not with this 
method?  ... maybe I'm missing some core bit about TLS... are you 
talking about pthread_key_create() and pthread_getspecific() ?

Thanks,

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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