Merge branch 'virtio-net-tx-napi'
Willem de Bruijn says: ==================== virtio-net tx napi Add napi for virtio-net transmit completion processing. Changes: v2 -> v3: - convert __netif_tx_trylock to __netif_tx_lock on tx napi poll ensure that the handler always cleans, to avoid deadlock - unconditionally clean in start_xmit avoid adding an unnecessary "if (use_napi)" branch - remove virtqueue_disable_cb in patch 5/5 a noop in the common event_idx based loop - document affinity_hint_set constraint v1 -> v2: - disable by default - disable unless affinity_hint_set because cache misses add up to a third higher cycle cost, e.g., in TCP_RR tests. This is not limited to the patch that enables tx completion cleaning in rx napi. - use trylock to avoid contention between tx and rx napi - keep interrupts masked during xmit_more (new patch 5/5) this improves cycles especially for multi UDP_STREAM, which does not benefit from cleaning tx completions on rx napi. - move free_old_xmit_skbs (new patch 3/5) to avoid forward declaration not changed: - deduplicate virnet_poll_tx and virtnet_poll_txclean they look similar, but have differ too much to make it worthwhile. - delay netif_wake_subqueue for more than 2 + MAX_SKB_FRAGS evaluated, but made no difference - patch 1/5 RFC -> v1: - dropped vhost interrupt moderation patch: not needed and likely expensive at light load - remove tx napi weight - always clean all tx completions - use boolean to toggle tx-napi, instead - only clean tx in rx if tx-napi is enabled - then clean tx before rx - fix: add missing braces in virtnet_freeze_down - testing: add 4KB TCP_RR + UDP test results Based on previous patchsets by Jason Wang: [RFC V7 PATCH 0/7] enable tx interrupts for virtio-net http://lkml.iu.edu/hypermail/linux/kernel/1505.3/00245.html Before commit b0c39dbd ("virtio_net: don't free buffers in xmit ring") the virtio-net driver would free transmitted packets on transmission of new packets in ndo_start_xmit and, to catch the edge case when no new packet is sent, also in a timer at 10HZ. A timer can cause long stalls. VIRTIO_F_NOTIFY_ON_EMPTY avoids stalls due to low free descriptor count. It does not address a stalls due to low socket SO_SNDBUF. Increasing timer frequency decreases that stall time, but increases interrupt rate and, thus, cycle count. Currently, with no timer, packets are freed only at ndo_start_xmit. Latency of consume_skb is now unbounded. To avoid a deadlock if a sock reaches SO_SNDBUF, packets are orphaned on tx. This breaks TCP small queues. Reenable TCP small queues by removing the orphan. Instead of using a timer, convert the driver to regular tx napi. This does not have the unresolved stall issue and does not have any frequency to tune. By keeping interrupts enabled by default, napi increases tx interrupt rate. VIRTIO_F_EVENT_IDX avoids sending an interrupt if one is already unacknowledged, so makes this more feasible today. Combine that with an optimization that brings interrupt rate back in line with the existing version for most workloads: Tx completion cleaning on rx interrupts elides most explicit tx interrupts by relying on the fact that many rx interrupts fire. Tested by running {1, 10, 100} {TCP, UDP} STREAM, RR, 4K_RR benchmarks from a guest to a server on the host, on an x86_64 Haswell. The guest runs 4 vCPUs pinned to 4 cores. vhost and the test server are pinned to a core each. All results are the median of 5 runs, with variance well < 10%. Used neper (github.com/google/neper) as test process. Napi increases single stream throughput, but increases cycle cost. The optimizations bring this down. The previous patchset saw a regression with UDP_STREAM, which does not benefit from cleaning tx interrupts in rx napi. This regression is now gone for 10x, 100x. Remaining difference is higher 1x TCP_STREAM, lower 1x UDP_STREAM. The latest results are with process, rx napi and tx napi affine to the same core. All numbers are lower than the previous patchset. upstream napi TCP_STREAM: 1x: Mbps 27816 39805 Gcycles 274 285 10x: Mbps 42947 42531 Gcycles 300 296 100x: Mbps 31830 28042 Gcycles 279 269 TCP_RR Latency (us): 1x: p50 21 21 p99 27 27 Gcycles 180 167 10x: p50 40 39 p99 52 52 Gcycles 214 211 100x: p50 281 241 p99 411 337 Gcycles 218 226 TCP_RR 4K: 1x: p50 28 29 p99 34 36 Gcycles 177 167 10x: p50 70 71 p99 85 134 Gcycles 213 214 100x: p50 442 611 p99 802 785 Gcycles 237 216 UDP_STREAM: 1x: Mbps 29468 26800 Gcycles 284 293 10x: Mbps 29891 29978 Gcycles 285 312 100x: Mbps 30269 30304 Gcycles 318 316 UDP_RR: 1x: p50 19 19 p99 23 23 Gcycles 180 173 10x: p50 35 40 p99 54 64 Gcycles 245 237 100x: p50 234 286 p99 484 473 Gcycles 224 214 Note that GSO is enabled, so 4K RR still translates to one packet per request. Lower throughput at 100x vs 10x can be (at least in part) explained by looking at bytes per packet sent (nstat). It likely also explains the lower throughput of 1x for some variants. upstream: N=1 bytes/pkt=16581 N=10 bytes/pkt=61513 N=100 bytes/pkt=51558 at_rx: N=1 bytes/pkt=65204 N=10 bytes/pkt=65148 N=100 bytes/pkt=56840 ==================== Acked-by:Michael S. Tsirkin <mst@redhat.com>
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