netdev - Re: [Intel-wired-lan] [PATCH iwl-net] idpf: enable WB_ON

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Date: Thu, 14 Dec 2023 13:59:57 +0100
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Michal Kubiak <michal.kubiak@...el.com>
Cc: Joshua Hay <joshua.a.hay@...el.com>, intel-wired-lan@...ts.osuosl.org,
 maciej.fijalkowski@...el.com, emil.s.tantilov@...el.com,
 larysa.zaremba@...el.com, netdev@...r.kernel.org,
 aleksander.lobakin@...el.com, alan.brady@...el.com,
 Przemek Kitszel <przemyslaw.kitszel@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-net] idpf: enable WB_ON_ITR

Lieber Michal,


Am 13.12.23 um 14:23 schrieb Michal Kubiak:
> On Tue, Dec 12, 2023 at 05:50:55PM +0100, Paul Menzel wrote:

>> On 12/12/23 15:55, Michal Kubiak wrote:
>>> From: Joshua Hay <joshua.a.hay@...el.com>
>>>
>>> Tell hardware to writeback completed descriptors even when interrupts
>>
>> Should you resend, the verb is spelled with a space: write back.
> 
> Sure, I will fix it.

Thanks.

>>> are disabled. Otherwise, descriptors might not be written back until
>>> the hardware can flush a full cacheline of descriptors. This can cause
>>> unnecessary delays when traffic is light (or even trigger Tx queue
>>> timeout).
>>
>> How can the problem be reproduced and the patch be verified?

[…]

> To be honest, I have noticed the problem during the implementation of
> AF_XDP feature for IDPF driver. In my scenario, I had 2 Tx queues:
>   - regular LAN Tx queue
>   - and XDP Tx queue
> added to the same q_vector attached to the same NAPI, so those 2 Tx
> queues were handled in the same NAPI poll loop.
> Then, when I started a huge Tx zero-copy trafic using AF_XDP (on the XDP
> queue), and, at the same time, tried to xmit a few packets using the second
> (non-XDP) queue (e.g. with scapy), I was getting the Tx timeout on that regular
> LAN Tx queue.
> That is why I decided to upstream this fix. With disabled writebacks,
> there is no chance to get the completion descriptor for the queue where
> the traffic is much lighter.
> 
> I have never tried to reproduce the scenario described by Joshua
> in his original patch ("unnecessary delays when traffic is light").

Understood. Maybe you could add a summary of the above to the commit 
message or just copy and paste. I guess, it’s better than nothing. And 
thank you for upstreaming this.


Kind regards,

Paul