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

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Date: Wed, 13 Dec 2023 14:23:19 +0100
From: Michal Kubiak <michal.kubiak@...el.com>
To: Paul Menzel <pmenzel@...gen.mpg.de>
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

On Tue, Dec 12, 2023 at 05:50:55PM +0100, Paul Menzel wrote:
> Dear Michal, dear Joshua,
> 
> 
> Thank you for your patch.
> 
> 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.

> 
> > 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?
> 
> 
> Kind regards,
> 
> Paul
> 
> 

Hi Paul,

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").

Thanks,
Michal