linux-kernel - Re: 2.6.18-rc6-mm1: GPF loop on early boot

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Date:	Mon, 11 Sep 2006 10:01:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>,
	Laurent Riffard <laurent.riffard@...e.fr>,
	Arjan van de Ven <arjan@...radead.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...source.com>
Subject: Re: 2.6.18-rc6-mm1: GPF loop on early boot


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Ingo Molnar wrote:
> >yes - but loading a null selector is a special-case: you dont have to 
> >invalidate/reload the shadow, you just have to turn access off. This 
> >might or might not make a difference on modern CPUs (it makes a 
> >difference with older CPUs) - but it's worth a try nevertheless. You 
> >measured a 9 cycles degradation with the %gs method, we could recover 
> >some of that.
> >  
> 
> It's a worthwhile experiment.  The gain would be the NULL selector 
> load, but the loss would be an additional segment reload on context 
> switch and TLS ABI incompatibility (which is more difficult to 
> quantify).

the ratio between the number of syscalls vs. the number of context 
switches is 1-2 orders of magnitude. So a loss of 9 cycles in the 
syscall path is roughly equal to a loss of 90-900 cycles in switch_to() 
costs ...

the TLS ABI is just a gcc stupidity. Why did they pick the _second_ 
extra selector, instead of the first one ...? Anyway, perhaps this could 
be solved by extending gcc with a switch to also generate __thread code 
off %fs. Probably not worth the pain though ...

	Ingo
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