netdev - Re: [PATCH net v2 3/4] bnxt: make sure xmit_more + errors does not miss doorbells

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Date:   Wed, 11 Aug 2021 16:38:42 -0700
From:   Michael Chan <michael.chan@...adcom.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Jeffrey Huang <huangjw@...adcom.com>,
        Eddie Wai <eddie.wai@...adcom.com>,
        Prashant Sreedharan <prashant@...adcom.com>,
        Andrew Gospodarek <gospo@...adcom.com>,
        Netdev <netdev@...r.kernel.org>,
        Edwin Peer <edwin.peer@...adcom.com>
Subject: Re: [PATCH net v2 3/4] bnxt: make sure xmit_more + errors does not
 miss doorbells

On Wed, Aug 11, 2021 at 4:17 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 11 Aug 2021 16:00:52 -0700 Michael Chan wrote:
> > On Wed, Aug 11, 2021 at 3:44 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > On Wed, 11 Aug 2021 15:36:34 -0700 Michael Chan wrote:
> > > > On Wed, Aug 11, 2021 at 2:38 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > > > @@ -367,6 +368,13 @@ static u16 bnxt_xmit_get_cfa_action(struct sk_buff *skb)
> > > > >         return md_dst->u.port_info.port_id;
> > > > >  }
> > > > >
> > > > > +static void bnxt_txr_db_kick(struct bnxt *bp, struct bnxt_tx_ring_info *txr,
> > > > > +                            u16 prod)
> > > > > +{
> > > > > +       bnxt_db_write(bp, &txr->tx_db, prod);
> > > > > +       txr->kick_pending = 0;
> > > > > +}
> > > > > +
> > > > >  static netdev_tx_t bnxt_start_xmit(struct sk_buff *skb, struct net_device *dev)
> > > > >  {
> > > > >         struct bnxt *bp = netdev_priv(dev);
> > > > > @@ -396,6 +404,8 @@ static netdev_tx_t bnxt_start_xmit(struct sk_buff *skb, struct net_device *dev)
> > > > >         free_size = bnxt_tx_avail(bp, txr);
> > > > >         if (unlikely(free_size < skb_shinfo(skb)->nr_frags + 2)) {
> > > > >                 netif_tx_stop_queue(txq);
> > > > > +               if (net_ratelimit() && txr->kick_pending)
> > > > > +                       netif_warn(bp, tx_err, dev, "bnxt: ring busy!\n");
> > > >
> > > > You forgot to remove this.
> > >
> > > I changed my mind. I added the && txr->kick_pending to the condition,
> > > if there is a race and napi starts the queue unnecessarily the kick
> > > can't be pending.
> >
> > I don't understand.  The queue should be stopped if we have <=
> > MAX_SKB_FRAGS + 1 descriptors left.  If there is a race and the queue
> > is awake, the first TX packet may slip through if
> > skb_shinfo(skb)->nr_frags is small and we have enough descriptors for
> > it.  Let's say xmit_more is set for this packet and so kick is
> > pending.  The next packet may not fit anymore and it will hit this
> > check here.
>
> But even if we slip past this check we can only do it once, the check
> at the end of start_xmit() will see we have fewer slots than MAX_FRAGS
> + 2, ring the doorbell and stop.

Yeah, I think you're right.

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