Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

[Josh Hunt <johunt@akamai.com>]

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.

Thanks
Josh