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

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Message-ID: <4BBD5062.4000300@us.ibm.com>
Date:	Wed, 07 Apr 2010 20:41:22 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	drepper@...il.com
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.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

drepper@...il.com wrote:
> On Tue, Apr 6, 2010 at 16:16, Thomas Gleixner <tglx@...utronix.de> wrote:
>> I know that you can do any weird stuff with the futex value, but I
>> don't see the "dramatic" limitation. Care to elaborate ?
> 
> If we have to fill in the PID we can represent only three states in a 
> futex: 0, PID, -PID.  Today we can represent 2^32 states.  Quite a 
> difference.

For general futexes sure, but not for robust or PI mutexes. Having 
adaptive futexes be based on the TID|WAITERS_FLAG policy certainly isn't 
breaking new ground, and is consistent with the other kernel-side futex 
locking implementations.

However, I agree that a FUTEX_SET_WAIT_ADAPTIVE sort of call might be 
very powerful. However, that might be just academic until I can show an 
improvement with adaptive futexes.

>> The per thread pinned page would be unconditional, right ?
> 
> Only if the process would be using these adaptive mutexes.  It could be 
> conditional.

What about the concern of this TLS approach only working for process 
private locks? I would very much like to make this work for both shared 
and private locks.

Thanks,

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