linux-kernel - Re: [PATCH] i386-pda UP optimization

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Message-ID: <20061115190606.GB9303@elte.hu>
Date:	Wed, 15 Nov 2006 20:06:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Arjan van de Ven <arjan@...radead.org>, Andi Kleen <ak@...e.de>,
	Eric Dumazet <dada1@...mosbay.com>, akpm@...l.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386-pda UP optimization


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

> Arjan van de Ven wrote:
> > segment register accesses really are not cheap. 
> > Also really it'll be better to use the register userspace is not using,
> > but we had that discussion before; could you remind me why you picked 
> > %gs in the first place?
> >   
> 
> To leave open the possibility of using the compiler's TLS support in 
> the kernel for percpu.  I also measured the cost of reloading %gs vs 
> %fs, and found no difference between reloading a null selector vs a 
> non-null selector.

what point would there be in using it? It's not like the kernel could 
make use of the thread keyword anytime soon (it would need /all/ 
architectures to support it) ... and the kernel doesnt mind how the 
current per_cpu() primitives are implemented, via assembly or via C. In 
any case, it very much matters to see the precise cost of having the pda 
selector value in %gs versus %fs.

	Ingo
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