> -----Original Message-----
> From: netdev-ow...@vger.kernel.org [mailto:netdev-ow...@vger.kernel.org]
> On Behalf Of Christoph Hellwig
> Sent: Wednesday, July 8, 2020 6:50 PM
> To: Robin Murphy <robin.mur...@arm.com>
> Cc: Björn Töpel <bjorn.to...@intel.com>; Christoph Hellwig <h...@lst.de>;
> Daniel Borkmann <dan...@iogearbox.net>; maxi...@mellanox.com;
> konrad.w...@oracle.com; jonathan.le...@gmail.com;
> linux-ker...@vger.kernel.org; iommu@lists.linux-foundation.org;
> net...@vger.kernel.org; b...@vger.kernel.org; da...@davemloft.net;
> magnus.karls...@intel.com
> Subject: Re: [PATCH net] xsk: remove cheap_dma optimization
>
> On Mon, Jun 29, 2020 at 04:41:16PM +0100, Robin Murphy wrote:
> > On 2020-06-28 18:16, Björn Töpel wrote:
> >>
> >> On 2020-06-27 09:04, Christoph Hellwig wrote:
> >>> On Sat, Jun 27, 2020 at 01:00:19AM +0200, Daniel Borkmann wrote:
> >>>> Given there is roughly a ~5 weeks window at max where this removal
> could
> >>>> still be applied in the worst case, could we come up with a fix /
> >>>> proposal
> >>>> first that moves this into the DMA mapping core? If there is something
> >>>> that
> >>>> can be agreed upon by all parties, then we could avoid re-adding the 9%
> >>>> slowdown. :/
> >>>
> >>> I'd rather turn it upside down - this abuse of the internals blocks work
> >>> that has basically just missed the previous window and I'm not going
> >>> to wait weeks to sort out the API misuse. But we can add optimizations
> >>> back later if we find a sane way.
> >>>
> >>
> >> I'm not super excited about the performance loss, but I do get
> >> Christoph's frustration about gutting the DMA API making it harder for
> >> DMA people to get work done. Lets try to solve this properly using
> >> proper DMA APIs.
> >>
> >>
> >>> That being said I really can't see how this would make so much of a
> >>> difference. What architecture and what dma_ops are you using for
> >>> those measurements? What is the workload?
> >>>
> >>
> >> The 9% is for an AF_XDP (Fast raw Ethernet socket. Think AF_PACKET, but
> >> faster.) benchmark: receive the packet from the NIC, and drop it. The DMA
> >> syncs stand out in the perf top:
> >>
> >> 28.63% [kernel] [k] i40e_clean_rx_irq_zc
> >> 17.12% [kernel] [k] xp_alloc
> >> 8.80% [kernel] [k] __xsk_rcv_zc
> >> 7.69% [kernel] [k] xdp_do_redirect
> >> 5.35% bpf_prog_992d9ddc835e5629 [k]
> bpf_prog_992d9ddc835e5629
> >> 4.77% [kernel] [k] xsk_rcv.part.0
> >> 4.07% [kernel] [k] __xsk_map_redirect
> >> 3.80% [kernel] [k]
> dma_direct_sync_single_for_cpu
> >> 3.03% [kernel] [k]
> dma_direct_sync_single_for_device
> >> 2.76% [kernel] [k]
> i40e_alloc_rx_buffers_zc
> >> 1.83% [kernel] [k] xsk_flush
> >> ...
> >>
> >> For this benchmark the dma_ops are NULL (dma_is_direct() == true), and
> >> the main issue is that SWIOTLB is now unconditionally enabled [1] for
> >> x86, and for each sync we have to check that if is_swiotlb_buffer()
> >> which involves a some costly indirection.
> >>
> >> That was pretty much what my hack avoided. Instead we did all the checks
> >> upfront, since AF_XDP has long-term DMA mappings, and just set a flag
> >> for that.
> >>
> >> Avoiding the whole "is this address swiotlb" in
> >> dma_direct_sync_single_for_{cpu, device]() per-packet
> >> would help a lot.
> >
> > I'm pretty sure that's one of the things we hope to achieve with the
> > generic bypass flag :)
> >
> >> Somewhat related to the DMA API; It would have performance benefits for
> >> AF_XDP if the DMA range of the mapped memory was linear, i.e. by IOMMU
> >> utilization. I've started hacking a thing a little bit, but it would be
> >> nice if such API was part of the mapping core.
> >>
> >> Input: array of pages Output: array of dma addrs (and obviously dev,
> >> flags and such)
> >>
> >> For non-IOMMU len(array of pages) == len(array of dma addrs)
> >> For best-case IOMMU len(array of dma addrs) == 1 (large linear space)
> >>
> >> But that's for later. :-)
> >
> > FWIW you will typically get that behaviour from IOMMU-based
> implementations
> > of dma_map_sg() right now, although it's not strictly guaranteed. If you
> > can weather some additional setup cost of calling
> > sg_alloc_table_from_pages() plus walking the list after mapping to test
> > whether you did get a contiguous result, you could start taking advantage
> > of it as some of the dma-buf code in DRM and v4l2 does already (although
> > those cases actually treat it as a strict dependency rather than an
> > optimisation).
>
> Yikes.
>
> > I'm inclined to agree that if we're going to see more of these cases, a new
> > API call that did formally guarantee a DMA-contiguous mapping (either via
> > IOMMU or bounce buffering) or failure might indeed be handy.
>
> I was planning on adding a dma-level API to add more pages to an
> IOMMU batch, but was waiting for at least the intel IOMMU driver to be
> converted to the dma-iommu code (and preferably arm32 and s390 as well).
>
> Here is my old pseudo-code sketch for what I was aiming for from the
> block/nvme perspective. I haven't even implemented it yet, so there might
> be some holes in the design:
>
>
> /*
> * Returns 0 if batching is possible, postitive number of segments required
> * if batching is not possible, or negatie values on error.
> */
> int dma_map_batch_start(struct device *dev, size_t rounded_len,
> enum dma_data_direction dir, unsigned long attrs, dma_addr_t *addr);
> int dma_map_batch_add(struct device *dev, dma_addr_t *addr, struct page
> *page,
> unsigned long offset, size_t size);
> int dma_map_batch_end(struct device *dev, int ret, dma_addr_t start_addr);
>
Hello Christoph,
What is the different between dma_map_batch_add() and adding the buffer to sg
of dma_map_sg()?
> int blk_dma_map_rq(struct device *dev, struct request *rq,
> enum dma_data_direction dir, unsigned long attrs,
> dma_addr_t *start_addr, size_t *len)
> {
> struct req_iterator iter;
> struct bio_vec bvec;
> dma_addr_t next_addr;
> int ret;
>
> if (number_of_segments(req) == 1) {
> // plain old dma_map_page();
> return 0;
> }
>
> // XXX: block helper for rounded_len?
> *len = length_of_request(req);
> ret = dma_map_batch_start(dev, *len, dir, attrs, start_addr);
> if (ret)
> return ret;
>
> next_addr = *start_addr;
> rq_for_each_segment(bvec, rq, iter) {
> ret = dma_map_batch_add(dev, &next_addr, bvec.bv_page,
> bvec.bv_offset, bvev.bv_len);
> if (ret)
> break;
> }
>
> return dma_map_batch_end(dev, ret, *start_addr);
> }
>
> dma_addr_t blk_dma_map_bvec(struct device *dev, struct bio_vec *bvec,
> enum dma_data_direction dir, unsigned long attrs)
> {
> return dma_map_page_attrs(dev, bv_page, bvec.bv_offset, bvev.bv_len,
> dir, attrs);
> }
>
> int queue_rq()
> {
> dma_addr_t addr;
> int ret;
>
> ret = blk_dma_map_rq(dev, rq, dir, attrs. &addr, &len);
> if (ret < 0)
> return ret;
>
> if (ret == 0) {
> if (use_sgl()) {
> nvme_pci_sgl_set_data(&cmd->dptr.sgl, addr, len);
> } else {
> set_prps();
> }
> return;
> }
>
> if (use_sgl()) {
> alloc_one_sgl_per_segment();
>
> rq_for_each_segment(bvec, rq, iter) {
> addr = blk_dma_map_bvec(dev, &bdev, dir, 0);
> set_one_sgl();
> }
> } else {
> alloc_one_prp_per_page();
>
> rq_for_each_segment(bvec, rq, iter) {
> ret = blk_dma_map_bvec(dev, &bdev, dir, 0);
> if (ret)
> break;
> set_prps();
> }
> }
Thanks
Barry
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