On Thu, May 06, 2021 at 12:29:40PM +0200, Toke Høiland-Jørgensen wrote: > Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx> 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@xxxxxxxxxxxxx> 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 >
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