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

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Message-ID: <87h7jaq1dd.fsf@toke.dk>
Date:   Mon, 10 May 2021 13:22:54 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.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

Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:

> 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().

OK, even 1536 is more than I expected; I figured it would be way lower,
which is why you were suggesting to use num_online_cpus() instead; but
yeah, num_possible_cpus() is obviously better, then :)

>> > 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?

Well, it should be possible to detect this at setup time, right? Not too
familiar with the driver code, but would it be possible to statically
dispatch to an entirely different code path if this happens?

-Toke