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

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Date:   Mon, 18 Sep 2023 10:52:54 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Hayes Wang <hayeswang@...ltek.com>
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>,
        "bjorn@...k.no" <bjorn@...k.no>,
        "pabeni@...hat.com" <pabeni@...hat.com>
Subject: Re: [PATCH net-next resend 1/2] r8152: remove queuing rx packets in driver

On Mon, Sep 18, 2023 at 10:39 AM Hayes Wang <hayeswang@...ltek.com> wrote:
>
> Eric Dumazet <edumazet@...gle.com>
> > Sent: Monday, September 18, 2023 3:55 PM
> [...]
> > >                         urb->actual_length = 0;
> > >                         list_add_tail(&agg->list, next);
> > >                 }
> > > +
> > > +               /* Break if budget is exhausted. */
> >
> > [1] More conventional way to to put this condition at the beginning of
> > the while () loop,
> > because the budget could be zero.
>
> If the budget is zero, the function wouldn't be called.
> a7b8d60b3723 ("r8152: check budget for r8152_poll") avoids it.

Yes, and we could revert  this patch :/

Moving the test at the front of the loop like most other drivers would
have avoided this issue,
and avoided this discussion.

>
> > > +               if (work_done >= budget)
> > > +                       break;
> > >         }
> > >
> > > +       /* Splice the remained list back to rx_done */
> > >         if (!list_empty(&rx_queue)) {
> > >                 spin_lock_irqsave(&tp->rx_lock, flags);
> > > -               list_splice_tail(&rx_queue, &tp->rx_done);
> > > +               list_splice(&rx_queue, &tp->rx_done);
> > >                 spin_unlock_irqrestore(&tp->rx_lock, flags);
> > >         }
> > >
> > >  out1:
> > > -       return work_done;
> > > +       if (work_done > budget)
> >
> > This (work_done >budget) condition would never be true if point [1] is
> > addressed.
>
> A bulk transfer may contain many packets, so the work_done may be more than budget.
> That is why I queue the packets in the driver before this patch.
> For example, if a bulk transfer contains 70 packet and budget is 64,
> napi_gro_receive would be called for the first 64 packets and 6 packets would
> be queued in driver for next schedule. After this patch, napi_gro_receive() would
> be called for the 70 packets, even the budget is 64. And the remained bulk transfers
> would be handled for next schedule.

A comment would be nice. NAPI logic should look the same in all drivers.

If a driver has some peculiarities, comments would help to maintain
the code in the long run.

>
> > > +               return budget;
> > > +       else
> > > +               return work_done;
> > >  }
>
> Best Regards,
> Hayes
>