linux-kernel - Re: [PATCH 1/3] sfc: revert "reduce the number of requested xdp ev queues"

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Date:   Fri, 9 Jul 2021 15:07:32 +0100
From:   Edward Cree <ecree.xilinx@...il.com>
To:     Íñigo Huguet <ihuguet@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, ivan@...udflare.com,
        ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
        john.fastabend@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sfc: revert "reduce the number of requested xdp ev
 queues"

On 08/07/2021 13:14, Íñigo Huguet wrote:
> In my opinion, there is no reason to make that distinction between
> normal traffic and XDP traffic.
> [...]
> If the user wants to prevent XDP from mixing with normal traffic, just
> not attaching an XDP program to the interface, or not using
> XDP_TX/REDIRECT in it would be enough. Probably I don't understand
> what you want to say here.

I think it's less about that and more about avoiding lock contention.
If two sources (XDP and the regular stack) are both trying to use a TXQ,
 and contending for a lock, it's possible that the resulting total
 throughput could be far less than either source alone would get if it
 had exclusive use of a queue.
There don't really seem to be any good answers to this; any CPU in the
 system can initiate an XDP_REDIRECT at any time and if they can't each
 get a queue to themselves then I don't see how the arbitration can be
 performant.  (There is the middle-ground possibility of TXQs shared by
 multiple XDP CPUs but not shared with the regular stack, in which case
 if only a subset of CPUs are actually handling RX on the device(s) with
 an XDP_REDIRECTing program it may be possible to avoid contention if
 the core-to-XDP-TXQ mapping can be carefully configured.)

-ed