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

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Date:   Mon, 12 Jul 2021 15:52:53 +0100
From:   Edward Cree <ecree.xilinx@...il.com>
To:     Íñigo Huguet <ihuguet@...hat.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:     "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, brouer@...hat.com
Subject: Re: [PATCH 1/3] sfc: revert "reduce the number of requested xdp ev
 queues"

On 12/07/2021 14:40, Íñigo Huguet wrote:
> That's a good idea, which in fact I had already considered, but I had
> (almost) discarded because I still see there 2 problems:
> 1. If there are no free MSI-X vectors remaining at all,
> XDP_TX/REDIRECT will still be disabled.
> 2. If the amount of free MSI-X vectors is little. Then, many CPUs will
> be contending for very few queues/locks, not for normal traffic but
> yes for XDP traffic. If someone wants to intensively use
> XDP_TX/REDIRECT will get a very poor performance, with no option to
> get a better tradeoff between normal and XDP traffic.
[snip]
> So I think that we still need a last resort fallback of sharing TXQs
> with network stack:
> 1. If there are enough resources: 1 queue per CPU for XDP
> 2. If there are not enough resources, but still a fair amount: many
> queues dedicated only to XDP, with (hashed) locking contention
> 3. If there are not free resources, or there are very few: TXQs shared
> for network core and XDP

I think the proper solution to this is to get this policy decision out
 of the kernel, and make the allocation of queues be under the control
 of userspace.  I recall some discussion a couple of years ago about
 "making queues a first-class citizen" for the sake of AF_XDP; this
 seems to be part and parcel of that.
But I don't know what such an interface would/should look like.

-ed