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

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Message-ID: <CALzJLG9iVpS2qH5Ryc_DtEjrQMhcKD+qrLrGn=vet=_9N8eXPw@mail.gmail.com>
Date:   Wed, 7 Sep 2016 19:57:19 +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 6:32 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2016-09-07 at 18:08 +0300, Saeed Mahameed wrote:
>> On Wed, Sep 7, 2016 at 5:41 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Wed, 2016-09-07 at 15:42 +0300, Saeed Mahameed wrote:
>> >> Previously we rang XDP SQ doorbell on every forwarded XDP packet.
>> >>
>> >> Here we introduce a xmit more like mechanism that will queue up more
>> >> than one packet into SQ (up to RX napi budget) w/o notifying the hardware.
>> >>
>> >> Once RX napi budget is consumed and we exit napi RX loop, we will
>> >> flush (doorbell) all XDP looped packets in case there are such.
>> >
>> > Why is this idea depends on XDP ?
>> >
>> > It looks like we could apply it to any driver having one IRQ servicing
>> > one RX and one TX, without XDP being involved.
>> >
>>
>> Yes but it is more complicated than XDP case, where the RX ring posts
>> the TX descriptors and once done
>> the RX ring hits the doorbell once for all the TX descriptors it
>> posted, and it is the only possible place to hit a doorbell
>> for XDP TX ring.
>>
>> For regular TX and RX ring sharing the same IRQ, there is no such
>> simple connection between them, and hitting a doorbell
>> from RX ring napi would race with xmit ndo function of the TX ring.
>>
>> How do you synchronize in such case ?
>> isn't the existing xmit more mechanism sufficient enough ?
>
> Only if a qdisc is present and pressure is high enough.
>
> But in a forwarding setup, we likely receive at a lower rate than the
> NIC can transmit.
>

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.

> A simple cmpxchg could be used to synchronize the thing, if we really
> cared about doorbell cost. (Ie if the cost of this cmpxchg() is way
> smaller than doorbell one)
>