linux-kernel - Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

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Date:	Mon, 5 Apr 2010 16:29:12 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Peter W. Morreale" <pmorreale@...ell.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Chris Mason <chris.mason@...cle.com>,
	John Cooper <john.cooper@...rd-harmonic.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
	spinning

* Darren Hart (dvhltc@...ibm.com) wrote:
> 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.

You also have a notification scheme (preempt_notifiers will fire on
schedule out).  However, you'd have to register the notifiers from a
non-current context (i.e. on slow path acquire reaching out to lock owner
and registering notifier on lock owner's behalf, which would kind of
defeat the point AFAICT).

thanks,
-chris
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