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

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Message-ID: <CANn89iKfsYgQmF1AM4zb1M6nnrAqEGJ+nmgJ_53T2NV812ZAVA@mail.gmail.com>
Date:   Mon, 13 Mar 2017 16:44:19 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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 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.

Should I had, very loudly ?


>> 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 ?

Nice to hear Alexei.