Re: [PATCH net-next v3 0/5] virtio-net tx napi

[David Miller]

From: Willem de Bruijn 
Date: Mon, 24 Apr 2017 13:49:25 -0400

> Add napi for virtio-net transmit completion processing.

Series applied, thanks.

Re: [PATCH net-next v3 0/5] virtio-net tx napi

[Michael S. Tsirkin]

On Mon, Apr 24, 2017 at 01:49:25PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn 
> 
> Add napi for virtio-net transmit completion processing.


Acked-by: Michael S. Tsirkin 

> 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 b0c39dbdc204 ("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  2781639805
>   Gcycles 274  285
> 
> 10x:
>   Mbps  4294742531
>   Gcycles 300  296
> 
> 100x:
>   Mbps  3183028042
>   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 

[PATCH net-next v3 0/5] virtio-net tx napi

[Willem de Bruijn]

From: Willem de Bruijn 

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 b0c39dbdc204 ("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  2781639805
  Gcycles 274  285

10x:
  Mbps  4294742531
  Gcycles 300  296

100x:
  Mbps  3183028042
  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