Merge branch 'socket-sendmsg-zerocopy'
Willem de Bruijn says: ==================== socket sendmsg MSG_ZEROCOPY Introduce zerocopy socket send flag MSG_ZEROCOPY. This extends the shared page support (SKBTX_SHARED_FRAG) from sendpage to sendmsg. Implement the feature for TCP initially, as large writes benefit most. On a send call with MSG_ZEROCOPY, the kernel pins user pages and links these directly into the skbuff frags[] array. Each send call with MSG_ZEROCOPY that transmits data will eventually queue a completion notification on the error queue: a per-socket u32 incremented on each such call. A request may have to revert to copy to succeed, for instance when a device cannot support scatter-gather IO. In that case a flag is passed along to notify that the operation succeeded without zerocopy optimization. The implementation extends the existing zerocopy infra for tuntap, vhost and xen with features needed for TCP, notably reference counting to handle cloning on retransmit and GSO. For more details, see also the netdev 2.1 paper and presentation at https://netdevconf.org/2.1/session.html?debruijn Changelog: v3 -> v4: - dropped UDP, RAW and PF_PACKET for now Without loopback support, datagrams are usually smaller than the ~8KB size threshold needed to benefit from zerocopy. - style: a few reverse chrismas tree - minor: SO_ZEROCOPY returns ENOTSUPP on unsupported protocols - minor: squashed SO_EE_CODE_ZEROCOPY_COPIED patch - minor: rebased on top of net-next with kmap_atomic fix v2 -> v3: - fix rebase conflict: SO_ZEROCOPY 59 -> 60 v1 -> v2: - fix (kbuild-bot): do not remove uarg until patch 5 - fix (kbuild-bot): move zerocopy_sg_from_iter doc with function - fix: remove unused extern in header file RFCv2 -> v1: - patch 2 - review comment: in skb_copy_ubufs, always allocate order-0 page, also when replacing compound source pages. - patch 3 - fix: always queue completion notification on MSG_ZEROCOPY, also if revert to copy. - fix: on syscall abort, correctly revert notification state - minor: skip queue notification on SOCK_DEAD - minor: replace BUG_ON with WARN_ON in recoverable error - patch 4 - new: add socket option SOCK_ZEROCOPY. only honor MSG_ZEROCOPY if set, ignore for legacy apps. - patch 5 - fix: clear zerocopy state on skb_linearize - patch 6 - fix: only coalesce if prev errqueue elem is zerocopy - minor: try coalescing with list tail instead of head - minor: merge bytelen limit patch - patch 7 - new: signal when data had to be copied - patch 8 (tcp) - optimize: avoid setting PSH bit when exceeding max frags. that limits GRO on the client. do not goto new_segment. - fix: fail on MSG_ZEROCOPY | MSG_FASTOPEN - minor: do not wait for memory: does not work for optmem - minor: simplify alloc - patch 9 (udp) - new: add PF_INET6 - fix: attach zerocopy notification even if revert to copy - minor: simplify alloc size arithmetic - patch 10 (raw hdrinc) - new: add PF_INET6 - patch 11 (pf_packet) - minor: simplify slightly - patch 12 - new msg_zerocopy regression test: use veth pair to test all protocols: ipv4/ipv6/packet, tcp/udp/raw, cork all relevant ethtool settings: rx off, sg off all relevant packet lengths: 0, <MAX_HEADER, max size RFC -> RFCv2: - review comment: do not loop skb with zerocopy frags onto rx: add skb_orphan_frags_rx to orphan even refcounted frags call this in __netif_receive_skb_core, deliver_skb and tun: same as commit 1080e512 ("net: orphan frags on receive") - fix: hold an explicit sk reference on each notification skb. previously relied on the reference (or wmem) held by the data skb that would trigger notification, but this breaks on skb_orphan. - fix: when aborting a send, do not inc the zerocopy counter this caused gaps in the notification chain - fix: in packet with SOCK_DGRAM, pull ll headers before calling zerocopy_sg_from_iter - fix: if sock_zerocopy_realloc does not allow coalescing, do not fail, just allocate a new ubuf - fix: in tcp, check return value of second allocation attempt - chg: allocate notification skbs from optmem to avoid affecting tcp write queue accounting (TSQ) - chg: limit #locked pages (ulimit) per user instead of per process - chg: grow notification ids from 16 to 32 bit - pass range [lo, hi] through 32 bit fields ee_info and ee_data - chg: rebased to davem-net-next on top of v4.10-rc7 - add: limit notification coalescing sharing ubufs limits overhead, but delays notification until the last packet is released, possibly unbounded. Add a cap. - tests: add snd_zerocopy_lo pf_packet test - tests: two bugfixes (add do_flush_tcp, ++sent not only in debug) Limitations / Known Issues: - TCP may build slightly smaller than max TSO packets due to exceeding MAX_SKB_FRAGS frags when zerocopy pages are unaligned. - All SKBTX_SHARED_FRAG may require additional __skb_linearize or skb_copy_ubufs calls in u32, skb_find_text, similar to skb_checksum_help. Notification skbuffs are allocated from optmem. For sockets that cannot effectively coalesce notifications, the optmem max may need to be increased to avoid hitting -ENOBUFS: sysctl -w net.core.optmem_max=1048576 In application load, copy avoidance shows a roughly 5% systemwide reduction in cycles when streaming large flows and a 4-8% reduction in wall clock time on early tensorflow test workloads. For the single-machine veth tests to succeed, loopback support has to be temporarily enabled by making skb_orphan_frags_rx map to skb_orphan_frags. * Performance The below table shows cycles reported by perf for a netperf process sending a single 10 Gbps TCP_STREAM. The first three columns show Mcycles spent in the netperf process context. The second three columns show time spent systemwide (-a -C A,B) on the two cpus that run the process and interrupt handler. Reported is the median of at least 3 runs. std is a standard netperf, zc uses zerocopy and % is the ratio. Netperf is pinned to cpu 2, network interrupts to cpu3, rps and rfs are disabled and the kernel is booted with idle=halt. NETPERF=./netperf -t TCP_STREAM -H $host -T 2 -l 30 -- -m $size perf stat -e cycles $NETPERF perf stat -C 2,3 -a -e cycles $NETPERF --process cycles-- ----cpu cycles---- std zc % std zc % 4K 27,609 11,217 41 49,217 39,175 79 16K 21,370 3,823 18 43,540 29,213 67 64K 20,557 2,312 11 42,189 26,910 64 256K 21,110 2,134 10 43,006 27,104 63 1M 20,987 1,610 8 42,759 25,931 61 Perf record indicates the main source of these differences. Process cycles only at 1M writes (perf record; perf report -n): std: Samples: 42K of event 'cycles', Event count (approx.): 21258597313 79.41% 33884 netperf [kernel.kallsyms] [k] copy_user_generic_string 3.27% 1396 netperf [kernel.kallsyms] [k] tcp_sendmsg 1.66% 694 netperf [kernel.kallsyms] [k] get_page_from_freelist 0.79% 325 netperf [kernel.kallsyms] [k] tcp_ack 0.43% 188 netperf [kernel.kallsyms] [k] __alloc_skb zc: Samples: 1K of event 'cycles', Event count (approx.): 1439509124 30.36% 584 netperf.zerocop [kernel.kallsyms] [k] gup_pte_range 14.63% 284 netperf.zerocop [kernel.kallsyms] [k] __zerocopy_sg_from_iter 8.03% 159 netperf.zerocop [kernel.kallsyms] [k] skb_zerocopy_add_frags_iter 4.84% 96 netperf.zerocop [kernel.kallsyms] [k] __alloc_skb 3.10% 60 netperf.zerocop [kernel.kallsyms] [k] kmem_cache_alloc_node * Safety The number of pages that can be pinned on behalf of a user with MSG_ZEROCOPY is bound by the locked memory ulimit. While the kernel holds process memory pinned, a process cannot safely reuse those pages for other purposes. Packets looped onto the receive stack and queued to a socket can be held indefinitely. Avoid unbounded notification latency by restricting user pages to egress paths only. skb_orphan_frags_rx() will create a private copy of pages even for refcounted packets when these are looped, as did skb_orphan_frags for the original tun zerocopy implementation. Pages are not remapped read-only. Processes can modify packet contents while packets are in flight in the kernel path. Bytes on which kernel control flow depends (headers) are copied to avoid TOCTTOU attacks. Datapath integrity does not otherwise depend on payload, with three exceptions: checksums, optional sk_filter/tc u32/.. and device + driver logic. The effect of wrong checksums is limited to the misbehaving process. TC filters that access contents may have to be excluded by adding an skb_orphan_frags_rx. Processes can also safely avoid OOM conditions by bounding the number of bytes passed with MSG_ZEROCOPY and by removing shared pages after transmission from their own memory map. ==================== Signed-off-by:David S. Miller <davem@davemloft.net>
Showing
Please register or sign in to comment