netdev - Re: [PATCH net-next] mlx4: Better use of order-0 pages in RX path

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Date:   Mon, 13 Mar 2017 16:51:19 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net-next] mlx4: Better use of order-0 pages in RX path

On Mon, Mar 13, 2017 at 04:44:19PM -0700, Eric Dumazet wrote:
> On Mon, Mar 13, 2017 at 4:40 PM, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> > On Mon, Mar 13, 2017 at 04:28:04PM -0700, Eric Dumazet wrote:
> >> On Mon, Mar 13, 2017 at 4:21 PM, Alexei Starovoitov
> >> <alexei.starovoitov@...il.com> wrote:
> >> >
> >> > is it once in the beginning only? If so then why that
> >> > 'if (!ring->page_cache.index)' check is done for every packet?
> >>
> >>
> >>
> >> You did not really read the patch, otherwise you would not ask these questions.
> >
> > please explain. I see
> > +  if (!ring->page_cache.index) {
> > +          npage = mlx4_alloc_page(priv, ring,
> > which is done for every packet that goes via XDP_TX.
> >
> 
> Well, we do for all packets, even on hosts not running XDP:
> 
> if (xdp_prog) { ...
> 
> ...
> 
> Then :
> 
> if (doorbell_pending))
>      mlx4_en_xmit_doorbell(priv->tx_ring[TX_XDP][cq->ring]);
> 
> And nobody complained of few additional instructions.

it's not the additional 'if' I'm concerned about it's the allocation
after 'if', since you didn't explain clearly when it's executed.

> >> Test it, and if you find a regression, shout loudly.
> >
> > that's not how it works. It's a job of submitter to prove
> > that additional code doesn't cause regressions especially
> > when there are legitimate concerns.
> 
> I have no easy way to test XDP. I  have never used it and am not
> planning to use it any time soon.
> 
> Does it mean I no longer can participate to linux dev ?

when you're touching the most performance sensitive piece of XDP in
the driver then yes, you have to show that XDP doesn't regress.
Especially since it's trivial to test.
Two mlx4 serves, pktgen and xdp2 is enough.