linux-kernel - Re: [PATCH net-next] sfc: reduce the number of requested xdp ev queues

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Message-ID: <205ba636-f180-3003-a41c-828e1fe1a13b@gmail.com>
Date:   Tue, 15 Dec 2020 18:49:55 +0000
From:   Edward Cree <ecree.xilinx@...il.com>
To:     Jesper Dangaard Brouer <jbrouer@...hat.com>,
        Ivan Babrou <ivan@...udflare.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...udflare.com,
        Martin Habets <habetsm.xilinx@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH net-next] sfc: reduce the number of requested xdp ev
 queues

On 15/12/2020 09:43, Jesper Dangaard Brouer wrote:
> On Mon, 14 Dec 2020 17:29:06 -0800
> Ivan Babrou <ivan@...udflare.com> wrote:
> 
>> Without this change the driver tries to allocate too many queues,
>> breaching the number of available msi-x interrupts on machines
>> with many logical cpus and default adapter settings:
>>
>> Insufficient resources for 12 XDP event queues (24 other channels, max 32)
>>
>> Which in turn triggers EINVAL on XDP processing:
>>
>> sfc 0000:86:00.0 ext0: XDP TX failed (-22)
> 
> I have a similar QA report with XDP_REDIRECT:
>   sfc 0000:05:00.0 ens1f0np0: XDP redirect failed (-22)
> 
> Here we are back to the issue we discussed with ixgbe, that NIC / msi-x
> interrupts hardware resources are not enough on machines with many
> logical cpus.
> 
> After this fix, what will happen if (cpu >= efx->xdp_tx_queue_count) ?
Same as happened before: the "failed -22".  But this fix will make that
 less likely to happen, because it ties more TXQs to each EVQ, and it's
 the EVQs that are in short supply.
(Strictly speaking, I believe the limitation is a software one, that
 comes from the driver's channel structures having been designed a
 decade ago when 32 cpus ought to be enough for anybody... AFAIR the
 hardware is capable of giving us something like 1024 evqs if we ask
 for them, it just might not have that many msi-x vectors for us.)
Anyway, the patch looks correct, so
Acked-by: Edward Cree <ecree.xilinx@...il.com>

-ed