netdev - Re: [PATCH v24 01/20] net: Introduce direct data placement tcp offload

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Date: Tue, 30 Apr 2024 14:54:13 +0300
From: Sagi Grimberg <sagi@...mberg.me>
To: Aurelien Aptel <aaptel@...dia.com>, linux-nvme@...ts.infradead.org,
 netdev@...r.kernel.org, hch@....de, kbusch@...nel.org, axboe@...com,
 chaitanyak@...dia.com, davem@...emloft.net, kuba@...nel.org
Cc: Boris Pismenny <borisp@...dia.com>, aurelien.aptel@...il.com,
 smalin@...dia.com, malin1024@...il.com, ogerlitz@...dia.com,
 yorayz@...dia.com, galshalom@...dia.com, mgurtovoy@...dia.com,
 edumazet@...gle.com, pabeni@...hat.com, dsahern@...nel.org, ast@...nel.org,
 jacob.e.keller@...el.com
Subject: Re: [PATCH v24 01/20] net: Introduce direct data placement tcp
 offload



On 29/04/2024 14:35, Aurelien Aptel wrote:
> Sagi Grimberg <sagi@...mberg.me> writes:
>> This is not simply a steering rule that can be overwritten at any point?
> No, unlike steering rules, the offload resources cannot be moved to a
> different queue.
>
> In order to move it we will need to re-create the queue and the
> resources assigned to it.  We will consider to improve the HW/FW/SW to
> allow this in future versions.

Well, you cannot rely on the fact that the application will be pinned to a
specific cpu core. That may be the case by accident, but you must not and
cannot assume it.

Even today, nvme-tcp has an option to run from an unbound wq context,
where queue->io_cpu is set to WORK_CPU_UNBOUND. What are you going
to do there?

nvme-tcp may handle rx side directly from .data_ready() in the future, what
will the offload do in that case?

>
>> I was simply referring to the fact that you set config->io_cpu from
>> sk->sk_incoming_cpu
>> and then you pass sk (and config) to .sk_add, so why does this
>> assignment need to
>> exist here and not below the interface down at the driver?
> You're correct, it doesn't need to exist *if* we use sk->incoming_cpu,
> which at the time it is used, is the wrong value.
> The right value for cfg->io_cpu is nvme_queue->io_cpu.

io_cpu may or may not mean anything. You cannot rely on it, nor dictate it.

>
> So either:
> - we do that and thus keep cfg->io_cpu.
> - or we remove cfg->io_cpu, and we offload the socket from
>    nvme_tcp_io_work() where the io_cpu is implicitly going to be
>    the current CPU.
What do you mean offload the socket from nvme_tcp_io_work? I do not
understand what this means.