On Mon, 10 May 2021 13:22:54 +0200 Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx> writes: > > > 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(). > > 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? The ndo_xdp_xmit call is a function pointer. Thus, if it happens at this level, then at setup time the driver can simply change the NDO to use a TX-queue-locked variant. I actually consider it a bug that i40e allow this misconfig to happen. The ixgbe driver solves the problem by rejecting XDP attach if the system have more CPUs than TXQs available. IMHO it is a better solution to add shard'ed/partitioned TXQ-locking when this situation happens, instead of denying XDP attach. Since the original XDP-redirect the ndo_xdp_xmit call have gotten bulking added, thus the locking will be amortized over the bulk. One question is how do we inform the end-user that XDP will be using a slightly slower TXQ-locking scheme? Given we have no XDP-features exposed, I suggest a simple kernel log message, which we already have for other XDP situations when the MTU is too large, or TSO is enabled. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
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