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

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Message-ID: <20210509155033.GB36905@ranger.igk.intel.com>
Date:   Sun, 9 May 2021 17:50:33 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Zvi Effron <zeffron@...tgames.com>,
        T K Sourabh <sourabhtk37@...il.com>,
        Xdp <xdp-newbies@...r.kernel.org>,
        intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
        magnus.karlsson@...el.com, kuba@...nel.org
Subject: Re: Dropped packets mapping IRQs for adjusted queue counts on i40e

On Thu, May 06, 2021 at 12:29:40PM +0200, Toke Høiland-Jørgensen wrote:
> Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:
> 
> > On Wed, May 05, 2021 at 01:01:28PM -0700, Jesse Brandeburg wrote:
> >> Zvi Effron wrote:
> >> 
> >> > On Tue, May 4, 2021 at 4:07 PM Zvi Effron <zeffron@...tgames.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
> >
> > Hey Zvi, sorry for the lack of assistance, there has been statutory free
> > time in Poland and today i'm in the birthday mode, but we managed to
> > discuss the issue with Magnus and we feel like we could have a solution
> > for that, more below.
> >
> >> 
> >> 
> >> It seems like for Intel drivers, igc, ixgbe, i40e, ice all have
> >> this problem.
> >> 
> >> Notably, igb, fixes it like I would expect.
> >
> > igb is correct but I think that we would like to avoid the introduction of
> > locking for higher speed NICs in XDP data path.
> >
> > We talked with Magnus that for i40e and ice that have lots of HW
> > resources, we could always create the xdp_rings array of num_online_cpus()
> > size and use smp_processor_id() for accesses, regardless of the user's
> > changes to queue count.
> 
> What is "lots"? Systems with hundreds of CPUs exist (and I seem to
> recall an issue with just such a system on Intel hardware(?)). Also,
> what if num_online_cpus() changes?

"Lots" is 16k for ice. For i40e datasheet tells that it's only 1536 for
whole device, so I back off from the statement that i40e has a lot of
resources :)

Also, s/num_online_cpus()/num_possible_cpus().

> 
> > This way the smp_processor_id() provides the serialization by itself as
> > we're under napi on a given cpu, so there's no need for locking
> > introduction - there is a per-cpu XDP ring provided. If we would stick to
> > the approach where you adjust the size of xdp_rings down to the shrinked
> > Rx queue count and use a smp_processor_id() % vsi->num_queue_pairs formula
> > then we could have a resource contention. Say that you did on a 16 core
> > system:
> > $ ethtool -L eth0 combined 2
> >
> > and then mapped the q0 to cpu1 and q1 to cpu 11. Both queues will grab the
> > xdp_rings[1], so we would have to introduce the locking.
> >
> > Proposed approach would just result with more Tx queues packed onto Tx
> > ring container of queue vector.
> >
> > Thoughts? Any concerns? Should we have a 'fallback' mode if we would be
> > out of queues?
> 
> Yes, please :)

How to have a fallback (in drivers that need it) in a way that wouldn't
hurt the scenario where queue per cpu requirement is satisfied?

> 
> -Toke
>