linux-kernel - Re: [patch] change futex

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Message-ID: <20070312110204.GD2231@elte.hu>
Date:	Mon, 12 Mar 2007 12:02:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Roland McGrath <roland@...hat.com>, akpm@...ux-foundation.org,
	mm-commits@...r.kernel.org, drepper@...hat.com, oleg@...sign.ru,
	sebastien.dugue@...l.net, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] change futex_wait() to hrtimers


* Nick Piggin <npiggin@...e.de> wrote:

> > i dont think we should try to do this. We should not and cannot do 
> > anything about all of the artifacts that comes with the use of 
> > relative timeouts and schedule_timeout().
> > 
> > basically, using jiffies here (which schedule_timeout() does) is 
> > /fundamentally/ imprecise. If you get many interrupts, rounding 
> > errors sum up - and there's nothing we can do about it!
> 
> Well I did convert futex_wait to an absolute timeout based version in 
> the subsequent incremental patch. I think that is OK?

it still has the rounding artifacts: using timer_list there is no way to 
do a precise long sleep based on many small sleeps.

even if this means more work for you (i'm sorry about that!) i'm quite 
sure we should take Sebastien's hrtimers based implementation of 
futex_wait(), and use the nanosleep method to restart it. There's no 
point in further tweaking the imprecise approach: whenever some timeout 
needs to be restarted, it's a candidate for hrtimers.

until then, glibc already handles timeouts and restarts it manually.

	Ingo
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