linux-kernel - Re: x86: disable preemption in delay

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Message-Id: <20071115232635.d190acd7.akpm@linux-foundation.org>
Date:	Thu, 15 Nov 2007 23:26:35 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86: disable preemption in delay_tsc()

On Fri, 16 Nov 2007 08:17:08 +0100 Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > It sounds like it would work OK.  What is the setup cost for a usleep?  
> > I'd have thought that code which does something like
> > 
> > 	while (i++ < 1000) {
> > 		foo();
> > 		udelay(1);
> > 	}
> > 
> > would take qiute a bit longer with such a change?
> 
> full roundtrip cost ought to be below 10 usecs, depending on the system. 

Ow.  So the above timeout would take 10x longer.  That probably won't break
anything, but quite a few drivers do udelay(1) for post-IO settling times
and they might not like it.

> There's no problem doing a non-preemptible udelay up to 10 usecs and we
> could use usleep above that.

Yup, with a few smarts in there we could work out which is the best to use,
and also compensate for the setup costs.

It doesn't sound very 2.6.24ish though.

As a quicky things perhaps we could only do the preempt_disable()/preempt_enable()
if the TSCs are unsynced?  Do we reliably know that?  I guess not..
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