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? > 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 :) -Toke
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