linux-kernel - Re: [PATCH 4/4] futex: Add FUTEX_LOCK with optional adaptive spinning

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Date:	Fri, 07 May 2010 09:52:33 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org,
	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>,
	Ulrich Drepper <drepper@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Avi Kivity <avi@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 4/4] futex: Add FUTEX_LOCK with optional adaptive spinning

Thomas Gleixner wrote:
> On Wed, 5 May 2010, Darren Hart wrote:
> 
>> Add a non-pi TID value based futex locking mechanism. This enables the
>> use of adaptive spinning which was problematic with the basic FUTEX_WAIT
>> operation.
> 
> You still do way too much work in that spin code with way too much
> code lines.
> 
> Can you try the following (completely uncompiled/untested) patch ?
> 
> Thanks,
> 
> 	tglx
> Subject: futex-simplify.patch
> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Fri, 07 May 2010 17:56:38 +0200
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/futex.c |   42 ++++++++++++------------------------
>  kernel/sched.c |   66 ---------------------------------------------------------
>  2 files changed, 14 insertions(+), 94 deletions(-)
> 


> +		if (to && !to->task) {
> +			ret = -ETIMEOUT;
>  			break;

I had hoped to be able to do it like this too. Unfortunately, we can't 
arm the timer until after we set TASK_INTERRUPTIBLE, and we don't do 
that until after we give up on spinning.


> -
> -		if (timeout) {
> -			now = ktime_get();
> -			if (timeout->tv64 < now.tv64)
> -				break;
>  		}

This bit clearly needs work. Either via some interpolated time 
calculation using something like jiffies, or via another timer that sets 
a bit we can check in here.

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