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

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Date:	Mon, 11 Sep 2006 01:13:25 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
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

Ingo Molnar wrote:
> 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 ...
>   

I agree.

> the TLS ABI is just a gcc stupidity. Why did they pick the _second_ 
> extra selector, instead of the first one ...?

Well, it doesn't matter which they chose if its the same for user and 
kernel space. Why does x86-64 have swapgs rather than swapfs?

>  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 ...

If there's gcc hacking to be done, it would be trying to get it make the 
TLS offsets from the PDA/TCB positive rather than negative...

    J
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