linux-kernel - Re: [PATCH 07/13] powerpc: Preemptible mmu

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Date:	Fri, 9 Apr 2010 18:46:14 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH 07/13] powerpc: Preemptible mmu_gather

On Fri, Apr 09, 2010 at 10:14:07AM +0200, Peter Zijlstra wrote:
> On Fri, 2010-04-09 at 14:07 +1000, Nick Piggin wrote:
> > On Thu, Apr 08, 2010 at 09:17:44PM +0200, Peter Zijlstra wrote:
> > > Fix up powerpc to the new mmu_gather stuffs.
> > > 
> > > PPC has an extra batching queue to RCU free the actual pagetable
> > > allocations, use the ARCH extentions for that for now.
> > > 
> > > For the ppc64_tlb_batch, which tracks the vaddrs to unhash from the
> > > hardware hash-table, keep using per-cpu arrays but flush on context
> > > switch and use a TIF bit to track the laxy_mmu state.
> > 
> > Hm. Pity powerpc can't just use tlb flush gathering for this batching,
> > (which is what it was designed for). Then it could avoid these tricks.
> > What's preventing this? Adding a tlb gather for COW case in
> > copy_page_range?
> 
> I'm not quite sure what about that, didn't fully investigate it, just
> wanted to get something working for now.

No it's not your problem, but just perhaps a good add-on to your
patchset. Thanks for thinking about it, though.

> 
> Of of the things is that both power and sparc need more than the struct
> page we normally gather.
> 
> I did think of making the mmu_gather have something like
> 
> struct mmu_page {
>   struct page *page;
> #ifdef HAVE_ARCH_TLB_VADDR
>   unsigned long vaddr;
> #endif
> };

Well you could also have a per-arch struct for this, which they can
fill in their own info with (I think powerpc takes the pte as well)

> 
> struct mmu_gather {
>   ...
>   unsigned int nr;
>   struct mmu_page *pages;
> };
> 
> 
> and doing that vaddr collection right along with it in the same batch.
> 
> I think that that would work, Ben, Dave?
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