Re: [PATCH net-next v2] vhost_net: do not stall on zerocopy depletion

[David Miller <davem@davemloft.net>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Fri,  6 Oct 2017 13:22:31 -0400

> From: Willem de Bruijn <willemb@google.com>
> 
> Vhost-net has a hard limit on the number of zerocopy skbs in flight.
> When reached, transmission stalls. Stalls cause latency, as well as
> head-of-line blocking of other flows that do not use zerocopy.
> 
> Instead of stalling, revert to copy-based transmission.
> 
> Tested by sending two udp flows from guest to host, one with payload
> of VHOST_GOODCOPY_LEN, the other too small for zerocopy (1B). The
> large flow is redirected to a netem instance with 1MBps rate limit
> and deep 1000 entry queue.
> 
>   modprobe ifb
>   ip link set dev ifb0 up
>   tc qdisc add dev ifb0 root netem limit 1000 rate 1MBit
> 
>   tc qdisc add dev tap0 ingress
>   tc filter add dev tap0 parent ffff: protocol ip \
>       u32 match ip dport 8000 0xffff \
>       action mirred egress redirect dev ifb0
> 
> Before the delay, both flows process around 80K pps. With the delay,
> before this patch, both process around 400. After this patch, the
> large flow is still rate limited, while the small reverts to its
> original rate. See also discussion in the first link, below.
> 
> Without rate limiting, {1, 10, 100}x TCP_STREAM tests continued to
> send at 100% zerocopy.
> 
> The limit in vhost_exceeds_maxpend must be carefully chosen. With
> vq->num >> 1, the flows remain correlated. This value happens to
> correspond to VHOST_MAX_PENDING for vq->num == 256. Allow smaller
> fractions and ensure correctness also for much smaller values of
> vq->num, by testing the min() of both explicitly. See also the
> discussion in the second link below.
> 
> Changes
>   v1 -> v2
>     - replaced min with typed min_t
>     - avoid unnecessary whitespace change
> 
> Link:http://lkml.kernel.org/r/CAF=yD-+Wk9sc9dXMUq1+x_hh=3ThTXa6BnZkygP3tgVpjbp93g@mail.gmail.com
> Link:http://lkml.kernel.org/r/20170819064129.27272-1-den@klaipeden.com
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Applied, thanks Willem.