netdev - Re: Packet gets stuck in NOLOCK pfifo

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Date:   Tue, 23 Jun 2020 09:42:59 -0400
From:   Michael Zhivich <mzhivich@...mai.com>
To:     <jonas.bonn@...rounds.com>
CC:     <davem@...emloft.net>, <john.fastabend@...il.com>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <pabeni@...hat.com>, <johunt@...mai.com>, <mzhivich@...mai.com>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc 

> From: Jonas Bonn <jonas.bonn@...rounds.com>
> To: Paolo Abeni <pabeni@...hat.com>,
> 	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
> 	LKML <linux-kernel@...r.kernel.org>,
> 	"David S . Miller" <davem@...emloft.net>,
> 	John Fastabend <john.fastabend@...il.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@...rounds.com> (raw)
> In-Reply-To: <95c5a697932e19ebd6577b5dac4d7052fe8c4255.camel@...hat.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