netdev - Re: Bad XDP performance with mlx5

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Date:   Tue, 4 Jun 2019 11:15:54 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Tom Barbette <barbette@....se>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        "toke@...hat.com" <toke@...hat.com>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        Leon Romanovsky <leonro@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>, brouer@...hat.com,
        Björn Töpel 
        <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Bad XDP performance with mlx5

On Tue, 4 Jun 2019 09:28:22 +0200
Tom Barbette <barbette@....se> wrote:

> Thanks Jesper for looking into this!
> 
> I don't think I will be of much help further on this matter. My take
> out would be: as a first-time user looking into XDP after watching a
> dozen of XDP talks, I would have expected XDP default settings to be
> identical to SKB, so I don't have to watch out for a set of
> per-driver parameter checklist to avoid increasing my CPU consumption
> by 15% when inserting "a super efficient and light BPF program". But
> I understand it's not that easy...

The gap should not be this large, but as I demonstrated it was primarily
because you hit an unfortunate interaction with TCP and how the mlx5
driver does page-caching (p.s. we are working on removing this driver
local recycle-cache).
  When loading an XDP/eBPF-prog then the driver change the underlying RX
memory model, which waste memory to gain packets-per-sec speed, but TCP
sees this memory waste and gives us a penalty.

It is important to understand, that XDP is not optimized for TCP.  XDP
is designed and optimized for L2-L3 handling of packets (TCP is L4).
Before XDP these L2-L3 use-cases were "slow", because the kernel
netstack assumes a L4/socket use-case (full SKB), when less was really
needed.

This is actually another good example of why XDP programs per RX-queue,
will be useful (notice: which is not implemented upstream, yet...).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer