netdev - Re: [net-next PATCH 0/3] Symmetric queue selection using XPS for Rx queues

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Date:   Tue, 8 May 2018 08:15:08 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     amritha.nambiar@...el.com, netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [net-next PATCH 0/3] Symmetric queue selection using XPS for Rx queues

On Thu, Apr 19, 2018 at 7:41 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> On Thu, Apr 19, 2018 at 6:07 PM Amritha Nambiar <amritha.nambiar@...el.com>
> wrote:
>
>> This patch series implements support for Tx queue selection based on
>> Rx queue map. This is done by configuring Rx queue map per Tx-queue
>> using sysfs attribute. If the user configuration for Rx queues does
>> not apply, then the Tx queue selection falls back to XPS using CPUs and
>> finally to hashing.
>
>> XPS is refactored to support Tx queue selection based on either the
>> CPU map or the Rx-queue map. The config option CONFIG_XPS needs to be
>> enabled. By default no receive queues are configured for the Tx queue.
>
>> - /sys/class/net/eth0/queues/tx-*/xps_rxqs
>
>> This is to enable sending packets on the same Tx-Rx queue pair as this
>> is useful for busy polling multi-threaded workloads where it is not
>> possible to pin the threads to a CPU. This is a rework of Sridhar's
>> patch for symmetric queueing via socket option:
>> https://www.spinics.net/lists/netdev/msg453106.html
>
I suspect this is an artifact of flow director which I believe
required queue pairs to be able to work (i.e. receive queue chose
hardware is determined send queue). But that was only required because
of hardware design, I don't see the rationale for introducing queue
pairs in the software stack. There's no need to correlate the transmit
path with receive path, no need to enforce a 1-1 mapping between RX
and TX queues, and the OOO mitigations should be sufficient when TX
queue changes for a flow.

Tom

>> ---
>
>> Amritha Nambiar (3):
>>        net: Refactor XPS for CPUs and Rx queues
>>        net: Enable Tx queue selection based on Rx queues
>>        net-sysfs: Add interface for Rx queue map per Tx queue
>
>
>>   include/linux/netdevice.h |   82 +++++++++++++++
>>   include/net/sock.h        |   18 +++
>>   net/core/dev.c            |  240
> +++++++++++++++++++++++++++++++--------------
>>   net/core/net-sysfs.c      |   85 ++++++++++++++++
>>   net/core/sock.c           |    5 +
>>   net/ipv4/tcp_input.c      |    7 +
>>   net/ipv4/tcp_ipv4.c       |    1
>>   net/ipv4/tcp_minisocks.c  |    1
>>   8 files changed, 357 insertions(+), 82 deletions(-)
>
>
> Without a clear documentation (for example in
> Documentation/networking/scaling.txt)
> , I really do not understand what problem you want to solve, and why we
> need ~300 additional LOC in kernel.
>
> Referring to an old thread (
> https://www.spinics.net/lists/netdev/msg453106.html ) is not the way to go.
>
> Sorry :/