linux-kernel - Re: [PATCH net-next v2 1/2] r8152: remove queuing rx packets in driver

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Message-ID: <50a8ec7dece0100c931fd187e19e14dd1ca1a0e9.camel@redhat.com>
Date:   Thu, 21 Sep 2023 15:39:15 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Hayes Wang <hayeswang@...ltek.com>, Andrew Lunn <andrew@...n.ch>
Cc:     "kuba@...nel.org" <kuba@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        nic_swsd <nic_swsd@...ltek.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "bjorn@...k.no" <bjorn@...k.no>
Subject: Re: [PATCH net-next v2 1/2] r8152: remove queuing rx packets in
 driver

On Tue, 2023-09-19 at 12:26 +0000, Hayes Wang wrote:
> Andrew Lunn <andrew@...n.ch>
> > Sent: Tuesday, September 19, 2023 8:08 PM
> [...]
> > > -     return work_done;
> > > +     if (work_done > budget)
> > > +             return budget;
> > > +     else
> > > +             return work_done;
> > >  }
> > 
> > I don't know NAPI too well. Are there implications of not telling it
> > the truth?
> 
> You could check the reply from Jakub Kicinski, as following link.
> https://www.spinics.net/lists/netdev/msg933846.html
> https://www.spinics.net/lists/netdev/msg933923.html
> 
> If the work_done is more than budget, I should return budget.

One of the key points in Jakub's reply is that the burst must exceed
the budget by a limited number of packets:

"Nothing will explode if we process a few more packets than budget
(assuming budget > 0)"

How many packets can contain at most a single URB?

If that number can be significant greater then the napi budget, I think
we are better off keeping the packet in the queue - otherwise the whole
system latency/stability would be affected.

Cheers,

Paolo