Date: Mon, 29 Oct 2012 15:05:56 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Alexander Duyck <alexander.duyck@gmail.com>
Cc: konrad@kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
        rob@landley.net, akpm@linux-foundation.org, joerg.roedel@amd.com,
        bhelgaas@google.com, shuahkhan@gmail.com,
        fujita.tomonori@lab.ntt.co.jp, linux-kernel@vger.kernel.org,
        x86@kernel.org, Alexander Duyck <alexander.h.duyck@intel.com>
Subject: Re: [PATCH v3 0/7] Improve swiotlb performance by using physical
 addresses
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On Mon, Oct 29, 2012 at 11:18:09AM -0700, Alexander Duyck wrote:
> On Mon, Oct 15, 2012 at 10:19 AM, Alexander Duyck
> <alexander.h.duyck@intel.com> wrote:
> > While working on 10Gb/s routing performance I found a significant amount of
> > time was being spent in the swiotlb DMA handler. Further digging found that a
> > significant amount of this was due to virtual to physical address translation
> > and calling the function that did it. It accounted for nearly 60% of the
> > total swiotlb overhead.
> >
> > This patch set works to resolve that by replacing the io_tlb_start and
> > io_tlb_end virtual addresses with a physical addresses. In addition it changes
> > the io_tlb_overflow_buffer from a virtual to a physical address. I followed
> > through with the cleanup to the point that the only functions that really
> > require the virtual address for the DMA buffer are the init, free, and
> > bounce functions.
> >
> > In the case of devices that are using the bounce buffers these patches should
> > result in only a slight performance gain if any. This is due to the locking
> > overhead required to map and unmap the buffers.
> >
> > In the case of devices that are not making use of bounce buffers these patches
> > can significantly reduce their overhead. In the case of an ixgbe routing test
> > for example, these changes result in 7 fewer calls to __phys_addr and
> > allow is_swiotlb_buffer to become inlined due to a reduction in the number of
> > instructions. When running a routing throughput test using small packets I
> > saw roughly a 6% increase in packets rates after applying these patches. This
> > appears to match up with the CPU overhead reduction I was tracking via perf.
> >
> > Before:
> > Results 10.0Mpps
> >
> > After:
> > Results 10.6Mpps
> >
> > Finally, I updated the parameter names for several of the core function calls
> > as there was some ambiguity in naming. Specifically virtual address pointers
> > were named dma_addr. When I changed these pointers to physical I instead used
> > the name tlb_addr as this value represented a physical address in the
> > io_tlb_start region and is less likely to be confused with a bus address.
> >
> > v2:
> > I reviewed the changes and realized that the first patch that was dropping
> > io_tlb_end and calculating the value didn't actually gain me much once I had
> > gone through and translated the rest of the addresses to physical addresses.
> > As such I have updated the patch so that it instead is converting io_tlb_end
> > from a virtual address to a physical address.  This actually helps to reduce
> > the overhead for is_swiotlb_buffer and swiotlb_dma_supported by several
> > instructions.
> >
> > v3:
> > After reviewing the patches I realized I was causing some namespace pollution
> > since a "static char *" was being replaced with "phys_addr_t" when it should
> > have been "static phys_addr_t".  As such I have updated the first 3 patches to
> > correctly replace static pointers with static physical addresses.
> >
> > ---
> >
> > Alexander Duyck (7):
> >       swiotlb:  Do not export swiotlb_bounce since there are no external consumers
> >       swiotlb: Use physical addresses instead of virtual in swiotlb_tbl_sync_single
> >       swiotlb: Use physical addresses for swiotlb_tbl_unmap_single
> >       swiotlb: Return physical addresses when calling swiotlb_tbl_map_single
> >       swiotlb: Make io_tlb_overflow_buffer a physical address
> >       swiotlb: Make io_tlb_start a physical address instead of a virtual one
> >       swiotlb: Make io_tlb_end a physical address instead of a virtual one
> >
> >
> >  drivers/xen/swiotlb-xen.c |   25 ++--
> >  include/linux/swiotlb.h   |   20 ++-
> >  lib/swiotlb.c             |  269 +++++++++++++++++++++++----------------------
> >  3 files changed, 163 insertions(+), 151 deletions(-)
> >
> 
> Is there any ETA on when this patch series might be pulled into a
> tree?  I'm just wondering if I need to rebase this patch series and
> resubmit it, and if so what tree I need to rebase it off of?

No need to rebase it. I did a test on V2 version with Xen, but I still
need to do a IA64/Calgary/AMD Vi/Intel VT-d/GART test before
pushing it out.

> 
> Thanks,
> 
> Alex
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