netdev - Re: [PATCH v15 06/20] nvme-tcp: Add DDP data-path

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Message-ID: <5b0fcc27-04aa-3ebd-e82a-8df39ed3ef5d@grimberg.me>
Date: Tue, 19 Sep 2023 10:04:08 +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
Subject: Re: [PATCH v15 06/20] nvme-tcp: Add DDP data-path


>>> @@ -1235,6 +1330,9 @@ static int nvme_tcp_try_send_cmd_pdu(struct nvme_tcp_request *req)
>>>        else
>>>                msg.msg_flags |= MSG_EOR;
>>>
>>> +     if (test_bit(NVME_TCP_Q_OFF_DDP, &queue->flags))
>>> +             nvme_tcp_setup_ddp(queue, blk_mq_rq_from_pdu(req));
>>
>> Didn't we agree that setup_ddp should move to setup time and
>> not send time?
> 
> We believe we haven't reached a conclusion last time [1].
> 
> Moving the setup_ddp() call earlier at setup time is less efficient (up
> to ~15% less IOPS) when it does the work on a different CPU.
> 
> 1: https://lore.kernel.org/all/253h6oxvlwd.fsf@nvidia.com/

Can you please explain why? sk_incoming_cpu is updated from the network
recv path while you are arguing that the timing matters before you even
send the pdu. I don't understand why should that matter.