netdev - Re: [Intel-wired-lan] [RFC PATCH 2/2] ixgbe: setup XPS via netif_set

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Date:   Thu, 15 Mar 2018 10:22:45 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [Intel-wired-lan] [RFC PATCH 2/2] ixgbe: setup XPS via netif_set_xps()

On Thu, Mar 15, 2018 at 10:05 AM, Paolo Abeni <pabeni@...hat.com> wrote:
> Hi,
>
> On Thu, 2018-03-15 at 09:43 -0700, Alexander Duyck wrote:
>> On Thu, Mar 15, 2018 at 8:08 AM, Paolo Abeni <pabeni@...hat.com> wrote:
>> > Before this commit, ixgbe with the default setting lacks XPS mapping
>> > for CPUs id greater than the number of tx queues.
>> >
>> > As a consequence the xmit path for such CPUs experience a relevant cost
>> > in __netdev_pick_tx, mainly due to skb_tx_hash(), as reported by the perf
>> > tool:
>> >
>> > 7.55%--netdev_pick_tx
>> >         |
>> >         --6.92%--__netdev_pick_tx
>> >                   |
>> >                   --6.35%--__skb_tx_hash
>> >                             |
>> >                             --5.94%--__skb_get_hash
>> >                                       |
>> >                                       --3.22%--__skb_flow_dissect
>> >
>> > in the following  scenario:
>> >
>> > ethtool -L em1 combined 1
>> > taskset 2 netperf -H 192.168.1.1 -t UDP_STREAM -- -m 1
>> > MIGRATED UDP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.101.1 () port 0 AF_INET
>> > Socket  Message  Elapsed      Messages
>> > Size    Size     Time         Okay Errors   Throughput
>> > bytes   bytes    secs            #      #   10^6bits/sec
>> >
>> > 212992       1   10.00     11497225      0       9.20
>> >
>> > After this commit the perf tool reports:
>> >
>> > 0.85%--__netdev_pick_tx
>> >
>> > and netperf reports:
>> >
>> > MIGRATED UDP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.101.1 () port 0 AF_INET
>> > Socket  Message  Elapsed      Messages
>> > Size    Size     Time         Okay Errors   Throughput
>> > bytes   bytes    secs            #      #   10^6bits/sec
>> >
>> > 212992       1   10.00     12736058      0      10.19
>> >
>> > roughly +10% in xmit tput.
>> >
>> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
>>
>> I think we shouldn't be configuring XPS if number of Tx or Rx queues
>> is less than the number of CPUs, or ATR is not enabled.
>
> Thank you for the feedback!
>
> Please note the currently the ixgbe driver is enabling XPS regardless
> of the above considerations.
>
>> Really the XPS bits are only really supposed to be used with the ATR
>> functionality enabled. If we don't have enough queues for a 1:1
>> mapping we should probably not be programming XPS since ATR isn't
>> going to function right anyway.
>
> uhm... I don't know the details of ATR, but apparently it is for TCP
> only, while the use-case I'm referring to is plain (no tunnel)
> unconnected UDP traffic. Am I missing something?

No. Basically the ATR/XPS bits is an overreach. The original code had
the driver just using the incoming CPU to select a Tx queue via the
ndo_select_queue. I pushed it out to XPS in order to try to get the
drivers to avoid using ndo_select_queue and provide the user with a
way to at least manually disable the Tx side of it.

For now I would say we should not have the driver configuring the XPS
map if it cannot assume a 1:1 mapping. As-is there are a number of
features where having this functionality enabled doesn't make sense.
In those cases we leave cpu as -1 in ixgbe_alloc_q_vector, and leave
the affinity mask as all 0s. It might make sense to just update the
code there in the case of ixgbe so that we don't update the XPS map or
the q_vector->affinity_mask if we cannot achieve a 1:1 mapping. As is
I would say the code is probably in need of updates since
ixgbe_alloc_q_vector doesn't handle the case where we might have a
non-linear CPU layout.

ATR is a feature that has been on my list of things to fix sometime in
the near future, but I haven't had the time as I have been pulled into
too many other efforts. Ideally we should be moving away from ATR and
instead looking at doing something like supporting ndo_rx_flow_steer.

Thanks.

- Alex