linux-kernel - Re: x86: clean up smpboot.c's use of udelay+schedule

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Date:	Tue, 31 Jan 2012 15:30:23 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	arjanvandeven@...il.com, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: x86: clean up smpboot.c's use of udelay+schedule


* Arjan van de Ven <arjan@...radead.org> wrote:

> On Tue, 31 Jan 2012 13:43:41 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> > > +			usleep_range(100, 200);
> > 
> > I'm wondering whether we could shorten this delay to say 10 
> > usecs and thus save 0.1 msecs (or more) from a typical SMP 
> > bootup?
> 
> doesn't matter really; [...]

It matters somewhat, especially if PeterZ's suggestion is used, 
which is far more clean as well. The magic delays are not really 
justified anymore.

> [...] bringing up a cpu is several orders more expensive (> 
> 100msec in 3.2, in 3.3 this got optimized to maybe 30 msec) 
> 0.1 msec is the least of anyone's worries at this point ;-)

It's 3% of the 30 msecs overhead.

> ( would be nice if this was a completion, but this is rather 
> fragile code in general... at least not making it spin is an 
> incremental improvement )

Completions arent hard to use and the scheduler should be up and 
running at this stage already.

Thanks,

	Ingo
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