Re: [PATCH net-next] net: dsa: add GRO support via gro_cells

Alexander Lobakin
2020-04-06T19:11:35Z

06.04.2020, 20:57, "Florian Fainelli" <f.fainelli@gmail.com>:
> On 4/6/2020 10:34 AM, Alexander Lobakin wrote:
>>  06.04.2020, 18:21, "Alexander Lobakin" <bloodyreaper@yandex.ru>:
>>>  06.04.2020, 17:48, "Andrew Lunn" <andrew@lunn.ch>:
>>>>   On Mon, Apr 06, 2020 at 01:59:10PM +0300, Alexander Lobakin wrote:
>>>>>    gro_cells lib is used by different encapsulating netdevices, such as
>>>>>    geneve, macsec, vxlan etc. to speed up decapsulated traffic processing.
>>>>>    CPU tag is a sort of "encapsulation", and we can use the same mechs to
>>>>>    greatly improve overall DSA performance.
>>>>>    skbs are passed to the GRO layer after removing CPU tags, so we don't
>>>>>    need any new packet offload types as it was firstly proposed by me in
>>>>>    the first GRO-over-DSA variant [1].
>>>>>
>>>>>    The size of struct gro_cells is sizeof(void *), so hot struct
>>>>>    dsa_slave_priv becomes only 4/8 bytes bigger, and all critical fields
>>>>>    remain in one 32-byte cacheline.
>>>>>    The other positive side effect is that drivers for network devices
>>>>>    that can be shipped as CPU ports of DSA-driven switches can now use
>>>>>    napi_gro_frags() to pass skbs to kernel. Packets built that way are
>>>>>    completely non-linear and are likely being dropped without GRO.
>>>>>
>>>>>    This was tested on to-be-mainlined-soon Ethernet driver that uses
>>>>>    napi_gro_frags(), and the overall performance was on par with the
>>>>>    variant from [1], sometimes even better due to minimal overhead.
>>>>>    net.core.gro_normal_batch tuning may help to push it to the limit
>>>>>    on particular setups and platforms.
>>>>>
>>>>>    [1] https://lore.kernel.org/netdev/20191230143028.27313-1-alobakin@dlink.ru/
>>>>
>>>>   Hi Alexander
>>>
>>>  Hi Andrew!
>>>
>>>>   net-next is closed at the moment. So you should of posted this with an
>>>>   RFC prefix.
>>>
>>>  I saw that it's closed, but didn't knew about "RFC" tags for that period,
>>>  sorry.
>>>
>>>>   The implementation looks nice and simple. But it would be nice to have
>>>>   some performance figures.
>>>
>>>  I'll do, sure. I think I'll collect the stats with various main receiving
>>>  functions in Ethernet driver (napi_gro_frags(), napi_gro_receive(),
>>>  netif_receive_skb(), netif_receive_skb_list()), and with and without this
>>>  patch to make them as complete as possible.
>>
>>  OK, so here we go.
>>
>>  My device is 1.2 GHz 4-core MIPS32 R2. Ethernet controller representing
>>  the CPU port is capable of S/G, fraglists S/G, TSO4/6 and GSO UDP L4.
>>  Tests are performed through simple IPoE VLAN NAT forwarding setup
>>  (port0 <-> port1.218) with iperf3 in TCP mode.
>>  net.core.gro_normal_batch is always set to 16 as that value seems to be
>>  the most effective for that particular hardware and drivers.
>>
>>  Packet counters on eth0 are the real numbers of ongoing frames. Counters
>>  on portX are pure-software and are updated inside networking stack.
>>
>>  ---------------------------------------------------------------------
>>
>>  netif_receive_skb() in Eth driver, no patch:
>>
>>  [ ID] Interval Transfer Bitrate Retr
>>  [ 5] 0.00-120.01 sec 9.00 GBytes 644 Mbits/sec 413 sender
>>  [ 5] 0.00-120.00 sec 8.99 GBytes 644 Mbits/sec receiver
>>
>>  eth0
>>  RX packets:7097731 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:7097702 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port0
>>  RX packets:426050 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:6671829 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1
>>  RX packets:6671681 errors:0 dropped:0 overruns:0 carrier:0
>>  TX packets:425862 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1.218
>>  RX packets:6671677 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:425851 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  ---------------------------------------------------------------------
>>
>>  netif_receive_skb_list() in Eth driver, no patch:
>>
>>  [ ID] Interval Transfer Bitrate Retr
>>  [ 5] 0.00-120.01 sec 9.48 GBytes 679 Mbits/sec 129 sender
>>  [ 5] 0.00-120.00 sec 9.48 GBytes 679 Mbits/sec receiver
>>
>>  eth0
>>  RX packets:7448098 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:7448073 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port0
>>  RX packets:416115 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:7032121 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1
>>  RX packets:7031983 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:415941 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1.218
>>  RX packets:7031978 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:415930 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  ---------------------------------------------------------------------
>>
>>  napi_gro_receive() in Eth driver, no patch:
>>
>>  [ ID] Interval Transfer Bitrate Retr
>>  [ 5] 0.00-120.01 sec 10.0 GBytes 718 Mbits/sec 107 sender
>>  [ 5] 0.00-120.00 sec 10.0 GBytes 718 Mbits/sec receiver
>>
>>  eth0
>>  RX packets:7868281 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:7868267 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port0
>>  RX packets:429082 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:7439343 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1
>>  RX packets:7439199 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:428913 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1.218
>>  RX packets:7439195 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:428902 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  =====================================================================
>>
>>  netif_receive_skb() in Eth driver + patch:
>>
>>  [ ID] Interval Transfer Bitrate Retr
>>  [ 5] 0.00-120.01 sec 12.2 GBytes 870 Mbits/sec 2267 sender
>>  [ 5] 0.00-120.00 sec 12.2 GBytes 870 Mbits/sec receiver
>>
>>  eth0
>>  RX packets:9474792 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:9474777 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port0
>>  RX packets:455200 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:353288 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1
>>  RX packets:9019592 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:455035 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1.218
>>  RX packets:353144 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:455024 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  ---------------------------------------------------------------------
>>
>>  netif_receive_skb_list() in Eth driver + patch:
>>
>>  [ ID] Interval Transfer Bitrate Retr
>>  [ 5] 0.00-120.01 sec 11.6 GBytes 827 Mbits/sec 2224 sender
>>  [ 5] 0.00-120.00 sec 11.5 GBytes 827 Mbits/sec receiver
>>
>>  eth0
>>  RX packets:8981651 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:898187 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port0
>>  RX packets:436159 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:335665 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1
>>  RX packets:8545492 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:436071 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1.218
>>  RX packets:335593 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:436065 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  -----------------------------------------------------------
>>
>>  napi_gro_receive() in Eth driver + patch:
>>
>>  [ ID] Interval Transfer Bitrate Retr
>>  [ 5] 0.00-120.01 sec 11.8 GBytes 855 Mbits/sec 122 sender
>>  [ 5] 0.00-120.00 sec 11.8 GBytes 855 Mbits/sec receiver
>>
>>  eth0
>>  RX packets:9292214 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:9292190 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port0
>>  RX packets:438516 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:347236 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1
>>  RX packets:8853698 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:438331 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  port1.218
>>  RX packets:347082 errors:0 dropped:0 overruns:0 frame:0
>>  TX packets:438320 errors:0 dropped:0 overruns:0 carrier:0
>>
>>  -----------------------------------------------------------
>>
>>  The main goal is achieved: we have about 100-200 Mbps of performance
>>  boost while in-stack skbs are greatly reduced from ~8-9 millions to
>>  ~350000 (compare port0 TX and port1 RX without patch and with it).
>
> And the number of TCP retries is also lower, which likely means that we
> are making better use of the flow control built into the hardware/driver
> here?
>
> BTW do you know why you have so many retries though? It sounds like your
> flow control is missing a few edge cases, or that you have an incorrect
> configuration of your TX admission queue.

Well, I have the same question TBH. All these ~1.5 years that I'm
working on these switches I have pretty chaotic number of TCP
retransmissions each time I change something in the code. They are
less likely to happen when the average CPU load is lower, but ~100
is the best result I ever got.
Seems like I should stop trying to push software throughput to
the max for a while and pay more attention to this and to hardware
configuration instead and check if I miss something :)

>>  The main bottleneck in gro_cells setup is that GRO layer starts to
>>  work only after skb are being processed by DSA stack, so they are
>>  going frame-by-frame until that moment (RX counter on port1).
>>
>>  If one day we change the way of handling incoming packets (not
>>  through fake packet_type), we could avoid that by unblocking GRO
>>  processing in between Eth driver and DSA core.
>>  With my custom packet_offload for ETH_P_XDSA that works only for
>>  my CPU tag format I have about ~910-920 Mbps on the same platform.
>>  This way doesn't fit mainline code of course, so I'm working on
>>  alternative Rx paths for DSA, e.g. through net_device::rx_handler()
>>  etc.
>>
>>  Until then, gro_cells really improve things a lot while the actual
>>  patch is tiny.
> --
> Florian