linux-kernel - Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

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Date:	Fri, 20 Oct 2006 19:37:24 -0700 (PDT)
From:	Linus Torvalds <torvalds@...l.org>
To:	David Miller <davem@...emloft.net>
cc:	ralf@...ux-mips.org, nickpiggin@...oo.com.au, akpm@...l.org,
	linux-kernel@...r.kernel.org, anemo@....ocn.ne.jp,
	linux-arch@...r.kernel.org, schwidefsky@...ibm.com,
	James.Bottomley@...elEye.com
Subject: Re: [PATCH 1/3] Fix COW D-cache aliasing on fork



On Fri, 20 Oct 2006, David Miller wrote:
>
> From: Linus Torvalds <torvalds@...l.org>
> Date: Fri, 20 Oct 2006 17:38:32 -0700 (PDT)
> 
> > I think (but may be mistaken) that ARM _does_ have pure virtual caches 
> > with a process ID, but people have always ended up flushing them at 
> > context switch simply because it just causes too much trouble.
> > 
> > Sparc? VIPT too? Davem?
> 
> sun4c is VIVT, but has no SMP variants.

You don't need SMP - we have sleeping sections here, so even threads on UP 
can trigger it. 

Now, to trigger it you need to have
 - virtual indexing not just by  address, but by some "address space 
   identifier" thing too
 - (in practice) a big enough cache that switching tasks wouldn't flush it 
   anyway.

> sun4m has both VIPT and PIPT.
> 
> > But it would be good to have something for the early -rc1 sequence for 
> > 2.6.20, and maybe the MIPS COW D$ patches are it, if it has performance 
> > advantages on MIPS that can also be translated to other virtual cache 
> > users..
> 
> I think it could help for sun4m highmem configs.

Well, if you can re-create the performance numbers (Ralf - can you send 
the full series with the final "remove the now unnecessary flush" to 
Davem?), that will make deciding things easier, I think.

I suspect sparc, mips and arm are the main architectures where virtually 
indexed caching really matters enough for this to be an issue at all.

		Linus
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