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

On Tue, 15 Dec 2020 18:49:55 +0000
Edward Cree [off-list ref] wrote:

On 15/12/2020 09:43, Jesper Dangaard Brouer wrote:

quoted

On Mon, 14 Dec 2020 17:29:06 -0800
Ivan Babrou [off-list ref] wrote:
  

quoted

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.

So, what I hear is that this fix is just pampering over the real issue.

I suggest that you/we detect the situation, and have a code path that
will take a lock (per 16 packets bulk) and solve the issue.

If you care about maximum performance you can implement this via
changing the ndo_xdp_xmit pointer to the fallback function when needed,
to avoid having a to check for the fallback mode in the fast-path.

(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@gmail.com>

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