[Cake] Fwd: [net-next PATCH 00/14] lockless qdisc series

Dave Taht dave.taht at gmail.com
Thu Dec 7 13:53:32 EST 2017

---------- Forwarded message ----------
From: John Fastabend <john.fastabend at gmail.com>
Date: Thu, Dec 7, 2017 at 9:53 AM
Subject: [net-next PATCH 00/14] lockless qdisc series
To: willemdebruijn.kernel at gmail.com, daniel at iogearbox.net,
eric.dumazet at gmail.com, davem at davemloft.net
Cc: netdev at vger.kernel.org, jiri at resnulli.us, xiyou.wangcong at gmail.com

This series adds support for building lockless qdiscs. This is
the result of noticing the qdisc lock is a common hot-spot in
perf analysis of the Linux network stack, especially when testing
with high packet per second rates. However, nothing is free and
most qdiscs rely on the qdisc lock for their data structures so
each qdisc must be converted on a case by case basis. In this
series, to kick things off, we make pfifo_fast, mq, and mqprio
lockless. Follow up series can address additional qdiscs as needed.
For example sch_tbf might be useful. To allow this the lockless
design is an opt-in flag. In some future utopia we convert all
qdiscs and we get to drop this case analysis, but in order to
make progress we live in the real-world.

There are also a handful of optimizations I have behind this
series and a few code cleanups that I couldn't figure out how
to fit neatly into this series with out increasing the patch
count. Once this is in additional patches can address this. The
most notable is in skb_dequeue we can push the consumer lock
out a bit and consume multiple skbs off the skb_array in pfifo
fast per iteration. Ideally we could push arrays of packets at
drivers as well but we would need the infrastructure for this.
The other notable improvement is to do less locking in the
overrun cases where bad tx queue list and gso_skb are being
hit. Although, nice in theory in practice this is the error
case and I haven't found a benchmark where this matters yet.

For testing...

My first test case uses multiple containers (via cilium) where
multiple client containers use 'wrk' to benchmark connections with
a server container running lighttpd. Where lighttpd is configured
to use multiple threads, one per core. Additionally this test has
a proxy agent running so all traffic takes an extra hop through a
proxy container. In these cases each TCP packet traverses the egress
qdisc layer at least four times and the ingress qdisc layer an
additional four times. This makes for a good stress test IMO, perf
details below.

The other micro-benchmark I run is injecting packets directly into
qdisc layer using pktgen. This uses the benchmark script,


Benchmarks taken in two cases, "base" running latest net-next no
changes to qdisc layer and "qdisc" tests run with qdisc lockless
updates. Numbers reported in req/sec. All virtual 'veth' devices
run with pfifo_fast in the qdisc test case.

`wrk -t16 -c $conns -d30 "http://[$SERVER_IP4]:80"`

conns    16      32     64   1024
base:   18831  20201  21393  29151
qdisc:  19309  21063  23899  29265

notice in all cases we see performance improvement when running
with qdisc case.

Microbenchmarks using pktgen are as follows,

`pktgen_bench_xmit_mode_queue_xmit.sh -t 1 -i eth2 -c 20000000

base(mq):          2.1Mpps
base(pfifo_fast):  2.1Mpps
qdisc(mq):         2.6Mpps
qdisc(pfifo_fast): 2.6Mpps

notice numbers are the same for mq and pfifo_fast because only
testing a single thread here. In both tests we see a nice bump
in performance gain. The key with 'mq' is it is already per
txq ring so contention is minimal in the above cases. Qdiscs
such as tbf or htb which have more contention will likely show
larger gains when/if lockless versions are implemented.

Thanks to everyone who helped with this work especially Daniel
Borkmann, Eric Dumazet and Willem de Bruijn for discussing the
design and reviewing versions of the code.

Changes from the RFC: dropped a couple patches off the end,
fixed a bug with skb_queue_walk_safe not unlinking skb in all
cases, fixed a lockdep splat with pfifo_fast_destroy not calling
*_bh lock variant, addressed _most_ of Willem's comments, there
was a bug in the bulk locking (final patch) of the RFC series.

@Willem, I left out lockdep annotation for a follow on series
to add lockdep more completely, rather than just in code I

Comments and feedback welcome.



John Fastabend (14):
      net: sched: cleanup qdisc_run and __qdisc_run semantics
      net: sched: allow qdiscs to handle locking
      net: sched: remove remaining uses for qdisc_qlen in xmit path
      net: sched: provide per cpu qstat helpers
      net: sched: a dflt qdisc may be used with per cpu stats
      net: sched: explicit locking in gso_cpu fallback
      net: sched: drop qdisc_reset from dev_graft_qdisc
      net: sched: use skb list for skb_bad_tx
      net: sched: check for frozen queue before skb_bad_txq check
      net: sched: helpers to sum qlen and qlen for per cpu logic
      net: sched: add support for TCQ_F_NOLOCK subqueues to sch_mq
      net: sched: add support for TCQ_F_NOLOCK subqueues to sch_mqprio
      net: skb_array: expose peek API
      net: sched: pfifo_fast use skb_array

 include/linux/skb_array.h |    5 +
 include/net/gen_stats.h   |    3
 include/net/pkt_sched.h   |   10 +
 include/net/sch_generic.h |   79 +++++++-
 net/core/dev.c            |   31 +++
 net/core/gen_stats.c      |    9 +
 net/sched/sch_api.c       |    8 +
 net/sched/sch_generic.c   |  440 ++++++++++++++++++++++++++++++++-------------
 net/sched/sch_mq.c        |   34 +++
 net/sched/sch_mqprio.c    |   69 +++++--
 10 files changed, 512 insertions(+), 176 deletions(-)



Dave Täht
CEO, TekLibre, LLC
Tel: 1-669-226-2619

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