netdev - Re: [PATCH RFC 11/11] net/mlx5e: XDP TX xmit more

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Date:   Wed, 7 Sep 2016 23:09:53 +0300
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        iovisor-dev <iovisor-dev@...ts.iovisor.org>,
        Linux Netdev List <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tom Herbert <tom@...bertland.com>,
        Martin KaFai Lau <kafai@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH RFC 11/11] net/mlx5e: XDP TX xmit more

On Wed, Sep 7, 2016 at 9:19 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2016-09-07 at 19:57 +0300, Saeed Mahameed wrote:
>
>> Jesper has a similar Idea to make the qdisc think it is under
>> pressure, when the device
>> TX ring is idle most of the time, i think his idea can come in handy here.
>> I am not fully involved in the details, maybe he can elaborate more.
>>
>> But if it works, it will be transparent to napi, and xmit more will
>> happen by design.
>
> I do not think qdisc is relevant here.
>
> Right now, skb->xmit_more is set only by qdisc layer (and pktgen tool),
> because only this layer can know if more packets are to come.
>
>
> What I am saying is that regardless of skb->xmit_more being set or not,
> (for example if no qdisc is even used)
> a NAPI driver can arm a bit asking the doorbell being sent at the end of
> NAPI.
>
> I am not saying this must be done, only that the idea could be extended
> to non XDP world, if we care enough.
>

Yes, and i am just trying to suggest Ideas that do not require
communication between RX (NAPI) and TX.

The problem here is the synchronization (TX doorbell from RX) which is
not as simple as atomic operation for some drivers.

How about RX bulking ? it also can help here, since for the forwarding
case, the forwarding path will be able to
process bulk of RX SKBs and can bulk xmit the portion of SKBs that
will be forwarded.

As Jesper suggested, Let's talk in Netdev1.2 at jesper's session. ( if
you are joining of course).

Thanks
Saeed.