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

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Date:   Wed, 07 Sep 2016 08:32:59 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
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, 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.

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)