Messages in this thread | | | Subject | Re: [PATCH net] xsk: remove cheap_dma optimization | From | Robin Murphy <> | Date | Mon, 29 Jun 2020 16:41:16 +0100 |
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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).
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.
Robin.
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