Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH net] udp: Allow kernel service to avoid udp socket rx queue | Date | Thu, 04 Oct 2018 10:37:43 +0100 |
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Paolo Abeni <pabeni@redhat.com> wrote:
> > There's a problem somewhere skb_recv_udp() that doesn't decrease the > > sk_rmem_alloc counter when a packet is extracted from the receive queue by > > a kernel service. > > If this is the case, it's really bad and need an explicit fix. However > it looks like sk_rmem_alloc is reclaimed by skb_recv_udp(), as it ends- > up calling udp_rmem_release() on succesfull dequeue.
It certainly *looks* like it should do that, but nonetheless, the tracepoint I put in shows it going up and up. I can try putting in a tracepoint by the subtraction, see what that shows.
> > Further, there doesn't seem any point in having the socket buffer being > > added to the UDP socket's rx queue since the rxrpc's data_ready handler > > takes it straight back out again (more or less, there seem to be occasional > > hold ups there). > > I really would really try to avoid adding another indirect call in the > data-path, unless strictly needed (to avoid more RETPOLINE overhead for > all other use-case). If skipping altogether the enqueuing makes sense > (I guess so, mostily for performance reasons), I *think* you can use > the already existing encap_rcv hook, initializing it to the rxrpc input > function, and updating such function to pull the udp header and ev. > initializing the pktinfo, if needed. Please see e.g. l2tp usage.
I looked at that, but it seems that a global conditional is required to enable it - presumably for performance reasons. I presume I would need to:
(1) Allocate a new UDP_ENCAP_* flag.
(2) Replicate at least some of the stuff that gets done between the check in udp_queue_rcv_skb() and the call of __udp_enqueue_schedule_skb() such as calling packet filtering.
I'm not sure whether I need to call things like ipv4_pktinfo_prepare(), sock_rps_save_rxhash(), sk_mark_napi_id() or sk_incoming_cpu_update() - are they of necessity to the UDP socket?
> > Putting in some tracepoints show a significant delay occurring between packets > > coming in and thence being delivered to rxrpc: > > > > <idle>-0 [001] ..s2 67.631844: net_rtl8169_napi_rx: enp3s0 skb=07db0a32 > > ... > > <idle>-0 [001] ..s4 68.292778: rxrpc_rx_packet: d5ce8d37:bdb93c60:00000002:09c7 00000006 00000000 02 20 ACK 660967981 skb=07db0a32 > > > > The "660967981" is the time difference in nanoseconds between the sk_buff > > timestamp and the current time. It seems to match the time elapsed between > > the two trace lines reasonably well. I've seen anything up to about 4s. > > Can you please provide more data?
I can give you a whole trace if you like.
> specifically can you please add: > * a perf probe in rxrpc_data_ready() just after skb_recv_udp() > reporting the sk->sk_rmem_alloc and skb->truesize > * a perf probe in __udp_enqueue_schedule_skb() just before the 'if > (rmem > sk->sk_rcvbuf)' test reporting again sk->sk_rmem_alloc, skb- > >truesize and sk->sk_rcvbuf > And then provide the perf record -g -e ... /perf script output?
Can't this be done by putting tracepoints there instead? I don't know how to do the perf stuff. What can that get that can't be obtained with a tracepoint?
Thanks, David
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