Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

[Jonas Bonn]

On 30/06/2020 21:14, Josh Hunt wrote:
> On 6/23/20 6:42 AM, Michael Zhivich wrote:
> > > From: Jonas Bonn <jonas.bonn@xxxxxxxxxxxxx>
> > > To: Paolo Abeni <pabeni@xxxxxxxxxx>,
> > > ÂÂÂÂ"netdev@xxxxxxxxxxxxxxx" <netdev@xxxxxxxxxxxxxxx>,
> > > ÂÂÂÂLKML <linux-kernel@xxxxxxxxxxxxxxx>,
> > > ÂÂÂÂ"David S . Miller" <davem@xxxxxxxxxxxxx>,
> > > ÂÂÂÂJohn Fastabend <john.fastabend@xxxxxxxxx>
> > > 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@xxxxxxxxxxxxx> (raw)
> > > In-Reply-To: <95c5a697932e19ebd6577b5dac4d7052fe8c4255.camel@xxxxxxxxxx>
> > >
> > > 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)Â 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.
> >
> > ÂÂÂ proc-19902 19902 [032] 580644.045480: 
> > probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) 
> > qdisc=0xffff900d7c247c00 skb=0xffff900bfc294af0 band=2 atomic_qlen=0
> > ÂÂÂ proc-19902 19902 [032] 580644.045480:     
> > probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 
> > skb=0xffffffff9b69d8c0 band=2
> > ÂÂÂ proc-19927 19927 [014] 580644.045480:      
> > probe:tcp_transmit_skb2: (ffffffff9b6dc4e5) skb=0xffff900b72c334f0 
> > sk=0xffff900d62958040 source=0x4b4e dest=0x9abe
> > ÂÂÂ proc-19902 19902 [032] 580644.045480: 
> > probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) 
> > qdisc=0xffff900d7c247c00 skb=0x0 band=3 atomic_qlen=0
> > ÂÂÂ proc-19927 19927 [014] 580644.045481:      
> > probe:ip_finish_output2: (ffffffff9b6bc650) net=0xffffffff9c107c80 
> > sk=0xffff900d62958040 skb=0xffff900b72c334f0 __func__=0x0
> > ÂÂÂ 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
> > ÂÂÂ proc-19927 19927 [014] 580644.045481:            
> > net:net_dev_queue: dev=eth0 skbaddr=0xffff900b72c334f0 len=115
> > ÂÂÂ proc-19902 19902 [032] 580644.045482:     
> > probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 
> > skb=0xffffffff9b69d8c0 band=1
> > ÂÂÂ proc-19927 19927 [014] 580644.045483:     
> > probe:pfifo_fast_enqueue: (ffffffff9b69d9f0) skb=0xffff900b72c334f0 
> > qdisc=0xffff900d7c247c00 to_free=18446622925407304000
> > ÂÂÂ proc-19902 19902 [032] 580644.045483: 
> > probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) 
> > qdisc=0xffff900d7c247c00 skb=0x0 band=3 atomic_qlen=0
> > ÂÂÂ proc-19927 19927 [014] 580644.045483: 
> > probe:pfifo_fast_enqueue_end: (ffffffff9b69da9f) 
> > skb=0xffff900b72c334f0 qdisc=0xffff900d7c247c00 
> > to_free=0xffff91d0f67ab940 atomic_qlen=1
> > ÂÂÂ proc-19902 19902 [032] 580644.045484:          
> > probe:__qdisc_run_2: (ffffffff9b69ea5a) q=0xffff900d7c247c00 packets=1
> > ÂÂÂ proc-19927 19927 [014] 580644.245745:     
> > probe:pfifo_fast_enqueue: (ffffffff9b69d9f0) skb=0xffff900d98fdf6f0 
> > qdisc=0xffff900d7c247c00 to_free=18446622925407304000
> > ÂÂÂ proc-19927 19927 [014] 580644.245745: 
> > probe:pfifo_fast_enqueue_end: (ffffffff9b69da9f) 
> > skb=0xffff900d98fdf6f0 qdisc=0xffff900d7c247c00 
> > to_free=0xffff91d0f67ab940 atomic_qlen=2
> > ÂÂÂ proc-19927 19927 [014] 580644.245746:     
> > probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 
> > skb=0xffffffff9b69d8c0 band=0
> > ÂÂÂ proc-19927 19927 [014] 580644.245746: 
> > probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) 
> > qdisc=0xffff900d7c247c00 skb=0xffff900b72c334f0 band=2 atomic_qlen=1
> > ÂÂÂ proc-19927 19927 [014] 580644.245747:     
> > probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 
> > skb=0xffffffff9b69d8c0 band=2
> > ÂÂÂ proc-19927 19927 [014] 580644.245747: 
> > probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) 
> > qdisc=0xffff900d7c247c00 skb=0xffff900d98fdf6f0 band=2 atomic_qlen=0
> > ÂÂÂ proc-19927 19927 [014] 580644.245748:     
> > probe:pfifo_fast_dequeue: (ffffffff9b69d8c0) qdisc=0xffff900d7c247c00 
> > skb=0xffffffff9b69d8c0 band=2
> > ÂÂÂ proc-19927 19927 [014] 580644.245748: 
> > probe:pfifo_fast_dequeue_end: (ffffffff9b69d99d) 
> > qdisc=0xffff900d7c247c00 skb=0x0 band=3 atomic_qlen=0
> > ÂÂÂ proc-19927 19927 [014] 580644.245749:          
> > qdisc:qdisc_dequeue: dequeue ifindex=5 qdisc handle=0x0 parent=0xF 
> > txq_state=0x0 packets=2 skbaddr=0xffff900b72c334f0
> > ÂÂÂ 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
> > ÂÂÂ 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