Date:   Tue, 12 Sep 2017 15:53:11 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH] net: Introduce a socket option to enable picking tx
 queue based on rx queue.

On Tue, Sep 12, 2017 at 3:31 PM, Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
>
>
> On 9/12/2017 8:47 AM, Eric Dumazet wrote:
>>
>> On Mon, 2017-09-11 at 23:27 -0700, Samudrala, Sridhar wrote:
>>>
>>> On 9/11/2017 8:53 PM, Eric Dumazet wrote:
>>>>
>>>> On Mon, 2017-09-11 at 20:12 -0700, Tom Herbert wrote:
>>>>
>>>>> Two ints in sock_common for this purpose is quite expensive and the
>>>>> use case for this is limited-- even if a RX->TX queue mapping were
>>>>> introduced to eliminate the queue pair assumption this still won't
>>>>> help if the receive and transmit interfaces are different for the
>>>>> connection. I think we really need to see some very compelling results
>>>>> to be able to justify this.
>>>
>>> Will try to collect and post some perf data with symmetric queue
>>> configuration.
>>>
>>>> Yes, this is unreasonable cost.
>>>>
>>>> XPS should really cover the case already.
>>>>
>>>
>>> Eric,
>>>
>>> Can you clarify how XPS covers the RX-> TX queue mapping case?
>>> Is it possible to configure XPS to select TX queue based on the RX queue
>>> of a flow?
>>> IIUC, it is based on the CPU of the thread doing the transmit OR based
>>> on skb->priority to TC mapping?
>>> It may be possible to get this effect if the the threads are pinned to a
>>> core, but if the app threads are
>>> freely moving, i am not sure how XPS can be configured to select the TX
>>> queue based on the RX queue of a flow.
>>
>> If application is freely moving, how NIC can properly select the RX
>> queue so that packets are coming to the appropriate queue ?
>
> The RX queue is selected via RSS and we don't want to move the flow based on
> where the thread is running.

Unless flow director is enabled on the Intel device... This was, I
believe, one of the first attempts to introduce a queue pair notion to
general purpose NICs. The idea was that the device records the TX
queue for a flow and then uses that to determine receive queue in a
symmetric fashion. aRFS is similar, but was under SW control how the
mapping is done. As Eric mentioned there are scalability issues with
these mechanisms, but we also found that flow director can easily
reorder packets whenever the thread moves.

>>
>>
>> This is called aRFS, and it does not scale to millions of flows.
>> We tried in the past, and this went nowhere really, since the setup cost
>> is prohibitive and DDOS vulnerable.
>>
>> XPS will follow the thread, since selection is done on current cpu.
>>
>> The problem is RX side. If application is free to migrate, then special
>> support (aRFS) is needed from the hardware.
>
> This may be true if most of the rx processing is happening in the interrupt
> context.
> But with busy polling,  i think we don't need aRFS as a thread should be
> able to poll
> any queue irrespective of where it is running.

It's not just a problem with interrupt processing, in general we like
to have all receive processing an subsequent transmit of a reply to be
done on one CPU. Silo'ing is good for performance and parallelism.
This can sometimes be relaxed in situations where CPUs share a cache
so crossing CPUs is not not costly.

>>
>>
>> At least for passive connections, we already have all the support in the
>> kernel so that you can have one thread per NIC queue, dealing with
>> sockets that have incoming packets all received on one NIC RX queue.
>> (And of course all TX packets will use the symmetric TX queue)
>>
>> SO_REUSEPORT plus appropriate BPF filter can achieve that.
>>
>> Say you have 32 queues, 32 cpus.
>>
>> Simply use 32 listeners, 32 threads (or 32 pools of threads)
>
> Yes. This will work if each thread is pinned to a core associated with the
> RX interrupt.
> It may not be possible to pin the threads to a core.
> Instead we want to associate a thread to a queue and do all the RX and TX
> completion
> of a queue in the same thread context via busy polling.
>
When that happens it's possible for RX to be done on the completely
wrong CPU which we know is suboptimal. However, this shouldn't
negatively affect TX side since XPS will just use the queue
appropriate for running CPU. Like Eric said, this is really a receive
problem more than a transmit problem. Keeping them as independent
paths seems to be a good approach.

Tom

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