netdev - Re: [PATCH v6 12/12] net/mlx4_en: add prefetch in xdp rx path

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Date:	Sun, 10 Jul 2016 15:50:10 -0500
From:	Tom Herbert <tom@...bertland.com>
To:	Brenden Blanco <bblanco@...mgrid.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Martin KaFai Lau <kafai@...com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Ari Saha <as754m@....com>, Or Gerlitz <gerlitz.or@...il.com>,
	john fastabend <john.fastabend@...il.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Thomas Graf <tgraf@...g.ch>,
	Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v6 12/12] net/mlx4_en: add prefetch in xdp rx path

> This particular patch in the series is meant to be standalone exactly
> for this reason. I don't pretend to assert that this optimization will
> work for everybody, or even for a future version of me with different
> hardware. But, it passes my internal criteria for usefulness:
> 1. It provides a measurable gain in the experiments that I have at hand
> 2. The code is easy to review
> 3. The change does not negatively impact non-XDP users
>
> I would love to have a solution for all mlx4 driver users, but this
> patch set is focused on a different goal. So, without munging a
> different set of changes for the universal use case, and probably
> violating criteria #2 or #3, I went with what you see.
>
> In hopes of not derailing the whole patch series, what is an actionable
> next step for this patch #12?
> Ideas:
> Pick a safer N? (I saw improvements with N=1 as well)
> Drop this patch?
>
As Alexei mentioned the right prefetch model may be dependent on
workload. For instance, the XDP program for an ILA router is is far
less code path than packets going through TCP so it makes sense that
we would want different prefetch characteristics to optimize for each
case. Can we make this a configurable knob for each RX queue to allow
that?

> One thing I definitely don't want to do is go into the weeds trying to
> get a universal prefetch logic in order to merge the XDP framework, even
> though I agree the net result would benefit everybody.

Agreed, a salient point of XDP is that it's _not_ a generic mechanism
meant for all applications. We don't want to sacrifice performance for
generality.

Tom