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Date: Fri, 9 Apr 2010 18:46:14 +1000
From: Nick Piggin <npiggin@suse.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Avi Kivity <avi@redhat.com>,
       Thomas Gleixner <tglx@linutronix.de>, Rik van Riel <riel@redhat.com>,
       Ingo Molnar <mingo@elte.hu>, akpm@linux-foundation.org,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       David Miller <davem@davemloft.net>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>, Mel Gorman <mel@csn.ul.ie>
Subject: Re: [PATCH 07/13] powerpc: Preemptible mmu_gather
Message-ID: <20100409084614.GN5683@laptop>
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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?