linux-kernel - Re: x86: unify genapic code, unify subarchitectures, remove old subarchitecture code

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Message-ID: <498F0ED9.1080906@goop.org>
Date:	Sun, 08 Feb 2009 08:56:57 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, hpa <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: x86: unify genapic code, unify subarchitectures, remove old	subarchitecture
 code

James Bottomley wrote:
> The other big problem is mm/tlb.c.  This directly uses genapic with 8
> vectors which is impossible for voyager: the QIC only has 8 separate IPI
> vectors for everything.  The two alternatives which spring to mind are
> either to rebase mm/tlb.c on top of smp_call_function.  This would add a
> small amount to the critical path, but would also allow vector scaling
> beyond the current 8 IPI vectors to a per processor number (i.e. might
> scale better beyond 8 cores).

I floated an experimental patch to do just that last year.  There were 
concerns because it had a pretty significantly hit on the performance of 
tlb-heavy benchmarks; and since then a multicast smp_call_function ends 
up kmallocing, which probably won't help matters.  I got called away to 
other things before really exploring all the options here, so it may 
well be worth reviving that work.

>   Or to keep voyager separate and move
> pieces of paravirt ops (or rather a separated piece of pv_ops) into
> smp_ops to effect the separation. 

This should be easy since you can can hook all the tlb operations via 
pv_mmu_ops.  And to avoid duplicating a lot of similar-looking code, you 
could just do a generic smp_call_function-based version.

    J
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