Re: Dropped packets mapping IRQs for adjusted queue counts on i40e

[Jesse Brandeburg <jesse.brandeburg@intel.com>]

Zvi Effron wrote:

> On Tue, May 4, 2021 at 4:07 PM Zvi Effron <zeffron@riotgames.com> wrote:
> > I'm suspecting it's something with how XDP_REDIRECT is implemented in
> > the i40e driver, but I don't know if this is a) cross driver behavior,
> > b) expected behavior, or c) a bug.
> I think I've found the issue, and it appears to be specific to i40e
> (and maybe other drivers, too, but not XDP itself).
> 
> When performing the XDP xmit, i40e uses the smp_processor_id() to
> select the tx queue (see
> https://elixir.bootlin.com/linux/v5.12.1/source/drivers/net/ethernet/intel/i40e/i40e_txrx.c#L3846).
> I'm not 100% clear on how the CPU is selected (since we don't use
> cores 0 and 1), we end up on a core whose id is higher than any
> available queue.
> 
> I'm going to try to modify our IRQ mappings to test this.
> 
> If I'm correct, this feels like a bug to me, since it requires a user
> to understand low level driver details to do IRQ remapping, which is a
> bit higher level. But if it's intended, we'll just have to figure out
> how to work around this. (Unfortunately, using split tx and rx queues
> is not possible with i40e, so that easy solution is unavailable.)
> 
> --Zvi


It seems like for Intel drivers, igc, ixgbe, i40e, ice all have
this problem.

Notably, igb, fixes it like I would expect.

Let's talk about it over on intel-wired-lan and cc netdev.

Jesse