Re: [PATCH 07/13] powerpc: Preemptible mmu_gather

Nick Piggin
2010-04-09T08:46:27Z

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?