> From: Jonas Bonn <jonas.b...@netrounds.com> > To: Paolo Abeni <pab...@redhat.com>, > "netdev@vger.kernel.org" <netdev@vger.kernel.org>, > LKML <linux-ker...@vger.kernel.org>, > "David S . Miller" <da...@davemloft.net>, > John Fastabend <john.fastab...@gmail.com> > Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc > Date: Fri, 11 Oct 2019 02:39:48 +0200 > Message-ID: <465a540e-5296-32e7-f6a6-79942dfe2...@netrounds.com> (raw) > In-Reply-To: <95c5a697932e19ebd6577b5dac4d7052fe8c4255.ca...@redhat.com> > > Hi Paolo, > > On 09/10/2019 21:14, Paolo Abeni wrote: > > Something alike the following code - completely untested - can possibly > > address the issue, but it's a bit rough and I would prefer not adding > > additonal complexity to the lockless qdiscs, can you please have a spin > > a it? > > We've tested a couple of variants of this patch today, but unfortunately > it doesn't fix the problem of packets getting stuck in the queue. > > A couple of comments: > > i) On 5.4, there is the BYPASS path that also needs the same treatment > as it's essentially replicating the behavour of qdisc_run, just without > the queue/dequeue steps > > ii) We are working a lot with the 4.19 kernel so I backported to the > patch to this version and tested there. Here the solution would seem to > be more robust as the BYPASS path does not exist. > > Unfortunately, in both cases we continue to see the issue of the "last > packet" getting stuck in the queue. > > /Jonas
Hello Jonas, Paolo, We have observed the same problem with pfifo_fast qdisc when sending periodic small packets on a TCP flow with multiple simultaneous connections on a 4.19.75 kernel. We've been able to catch it in action using perf probes (see trace below). For qdisc = 0xffff900d7c247c00, skb = 0xffff900b72c334f0, it takes 200270us to traverse the networking stack on a system that's not otherwise busy. qdisc only resumes processing when another enqueued packet comes in, so the packet could have been stuck indefinitely. proc-19902 19902 [032] 580644.045480: probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) qdisc=0xffff900d7c247c00 skb=0xffff900bfc294af0 band=2 atomic_qlen=0 proc-19902 19902 [032] 580644.045480: probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 skb=0xffffffff9b69d8c0 band=2 proc-19927 19927 [014] 580644.045480: probe:tcp_transmit_skb2: (ffffffff9b6dc4e5) skb=0xffff900b72c334f0 sk=0xffff900d62958040 source=0x4b4e dest=0x9abe proc-19902 19902 [032] 580644.045480: probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) qdisc=0xffff900d7c247c00 skb=0x0 band=3 atomic_qlen=0 proc-19927 19927 [014] 580644.045481: probe:ip_finish_output2: (ffffffff9b6bc650) net=0xffffffff9c107c80 sk=0xffff900d62958040 skb=0xffff900b72c334f0 __func__=0x0 proc-19902 19902 [032] 580644.045481: probe:sch_direct_xmit: (ffffffff9b69e570) skb=0xffff900bfc294af0 q=0xffff900d7c247c00 dev=0xffff900d6a140000 txq=0xffff900d6a181180 root_lock=0x0 validate=1 ret=-1 again=155 proc-19927 19927 [014] 580644.045481: net:net_dev_queue: dev=eth0 skbaddr=0xffff900b72c334f0 len=115 proc-19902 19902 [032] 580644.045482: probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 skb=0xffffffff9b69d8c0 band=1 proc-19927 19927 [014] 580644.045483: probe:pfifo_fast_enqueue: (ffffffff9b69d9f0) skb=0xffff900b72c334f0 qdisc=0xffff900d7c247c00 to_free=18446622925407304000 proc-19902 19902 [032] 580644.045483: probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) qdisc=0xffff900d7c247c00 skb=0x0 band=3 atomic_qlen=0 proc-19927 19927 [014] 580644.045483: probe:pfifo_fast_enqueue_end: (ffffffff9b69da9f) skb=0xffff900b72c334f0 qdisc=0xffff900d7c247c00 to_free=0xffff91d0f67ab940 atomic_qlen=1 proc-19902 19902 [032] 580644.045484: probe:__qdisc_run_2: (ffffffff9b69ea5a) q=0xffff900d7c247c00 packets=1 proc-19927 19927 [014] 580644.245745: probe:pfifo_fast_enqueue: (ffffffff9b69d9f0) skb=0xffff900d98fdf6f0 qdisc=0xffff900d7c247c00 to_free=18446622925407304000 proc-19927 19927 [014] 580644.245745: probe:pfifo_fast_enqueue_end: (ffffffff9b69da9f) skb=0xffff900d98fdf6f0 qdisc=0xffff900d7c247c00 to_free=0xffff91d0f67ab940 atomic_qlen=2 proc-19927 19927 [014] 580644.245746: probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 skb=0xffffffff9b69d8c0 band=0 proc-19927 19927 [014] 580644.245746: probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) qdisc=0xffff900d7c247c00 skb=0xffff900b72c334f0 band=2 atomic_qlen=1 proc-19927 19927 [014] 580644.245747: probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 skb=0xffffffff9b69d8c0 band=2 proc-19927 19927 [014] 580644.245747: probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) qdisc=0xffff900d7c247c00 skb=0xffff900d98fdf6f0 band=2 atomic_qlen=0 proc-19927 19927 [014] 580644.245748: probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 skb=0xffffffff9b69d8c0 band=2 proc-19927 19927 [014] 580644.245748: probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) qdisc=0xffff900d7c247c00 skb=0x0 band=3 atomic_qlen=0 proc-19927 19927 [014] 580644.245749: qdisc:qdisc_dequeue: dequeue ifindex=5 qdisc handle=0x0 parent=0xF txq_state=0x0 packets=2 skbaddr=0xffff900b72c334f0 proc-19927 19927 [014] 580644.245749: probe:sch_direct_xmit: (ffffffff9b69e570) skb=0xffff900b72c334f0 q=0xffff900d7c247c00 dev=0xffff900d6a140000 txq=0xffff900d6a181180 root_lock=0x0 validate=1 ret=-1 again=155 proc-19927 19927 [014] 580644.245750: net:net_dev_start_xmit: dev=eth0 queue_mapping=14 skbaddr=0xffff900b72c334f0 vlan_tagged=0 vlan_proto=0x0000 vlan_tci=0x0000 protocol=0x0800 ip_summed=3 len=115 data_len=0 network_offset=14 transport_offset_valid=1 transport_offset=34 tx_flags=0 gso_size=0 gso_segs=1 gso_type=0x1 I was wondering if you had any more luck in finding a solution or workaround for this problem (that is, aside from switching to a different qdisc)? Thanks, ~ Michael
