Messages in this thread | | | Subject | Re: Packet gets stuck in NOLOCK pfifo_fast qdisc | From | Jonas Bonn <> | Date | Wed, 1 Jul 2020 09:53:08 +0200 |
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On 30/06/2020 21:14, Josh Hunt wrote: > On 6/23/20 6:42 AM, Michael Zhivich wrote: >>> From: Jonas Bonn <jonas.bonn@netrounds.com> >>> To: Paolo Abeni <pabeni@redhat.com>, >>> "netdev@vger.kernel.org" <netdev@vger.kernel.org>, >>> LKML <linux-kernel@vger.kernel.org>, >>> "David S . Miller" <davem@davemloft.net>, >>> John Fastabend <john.fastabend@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-f6a6-79942dfe2618@netrounds.com> (raw) >>> In-Reply-To: <95c5a697932e19ebd6577b5dac4d7052fe8c4255.camel@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 >> > > Jonas/Paolo > > Do either of you know if there's been any development on a fix for this > issue? If not we can propose something.
Hi Josh,
No, I haven't been able to do any more work on this and the affected user switched qdisc (to avoid this problem) so I lost the reliable reproducer that I had...
/Jonas
> > Thanks > Josh
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