BUG at IP: blk_mq_get_request+0x23e/0x390 on 4.16.0-rc7

[Sagi Grimberg]

>> My device exposes nr_hw_queues which is not higher than num_online_cpus
>> so I want to connect all hctxs with hope that they will be used.
> 
> The issue is that CPU online & offline can happen any time, and after
> blk-mq removes CPU hotplug handler, there is no way to remap queue
> when CPU topo is changed.
> 
> For example:
> 
> 1) after nr_hw_queues is set as num_online_cpus() and hw queues
> are initialized, then some of CPUs become offline, and the issue
> reported by Zhang Yi is triggered, but in this case, we should fail
> the allocation since 1:1 mapping doesn't need to use this inactive
> hw queue.

Normal cpu offlining is fine, as the hctxs are already connected. When
we reset the controller and re-establish the queues, the issue triggers
because we call blk_mq_alloc_request_hctx.

The question is, for this particular issue, given that the request
execution is guaranteed to run from an online cpu, will the below work?
--
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 75336848f7a7..81ced3096433 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -444,6 +444,10 @@ struct request *blk_mq_alloc_request_hctx(struct 
request_queue *q,
                 return ERR_PTR(-EXDEV);
         }
         cpu = cpumask_first_and(alloc_data.hctx->cpumask, cpu_online_mask);
+       if (cpu >= nr_cpu_ids) {
+               pr_warn("no online cpu for hctx %d\n", hctx_idx);
+               cpu = cpumask_first(alloc_data.hctx->cpumask);
+       }
         alloc_data.ctx = __blk_mq_get_ctx(q, cpu);

         rq = blk_mq_get_request(q, NULL, op, &alloc_data);
--

> 2) when nr_hw_queues is set as num_online_cpus(), there may be
> much less online CPUs, so the hw queue number can be initialized as
> much smaller, then performance is degraded much even if some CPUs
> become online later.

That is correct, when the controller will be reset though, more queues
will be added to the system. I agree it would be good if we can change
stuff dynamically.

> So the current policy is to map all possible CPUs for handing CPU
> hotplug, and if you want to get 1:1 mapping between hw queue and
> online CPU, the nr_hw_queues can be set as num_possible_cpus.

Having nr_hw_queues == num_possible_cpus cannot work as it requires
establishing an RDMA queue-pair with a set of HW resources both on
the host side _and_ on the controller side, which are idle.

> Please see commit 16ccfff28976130 (nvme: pci: pass max vectors as
> num_possible_cpus() to pci_alloc_irq_vectors).

Yes, I am aware of this patch, however I not sure it'll be a good idea
for nvmf as it takes resources from both the host and the target for
for cpus that may never come online...

> It will waste some memory resource just like percpu variable, but it
> simplifies the queue mapping logic a lot, and can support both hard
> and soft CPU online/offline without CPU hotplug handler, which may
> cause very complicated queue dependency issue.

Yes, but these some memory resources are becoming an issue when it
takes HW (RDMA) resources on the local device and on the target device.

>> I agree we don't want to connect hctx which doesn't have an online
>> cpu, that's redundant, but this is not the case here.
> 
> OK, I will explain below, and it can be fixed by the following patch too:
> 
> https://marc.info/?l=linux-block&m=152318093725257&w=2
> 

I agree this patch is good!

>>>>> Or I may understand you wrong, :-)
>>>>
>>>> In the report we connected 40 hctxs (which was exactly the number of
>>>> online cpus), after Yi removed 3 cpus, we tried to connect 37 hctxs.
>>>> I'm not sure why some hctxs are left without any online cpus.
>>>
>>> That is possible after the following two commits:
>>>
>>> 4b855ad37194 ("blk-mq: Create hctx for each present CPU)
>>> 20e4d8139319 (blk-mq: simplify queue mapping & schedule with each possisble CPU)
>>>
>>> And this can be triggered even without putting down any CPUs.
>>>
>>> The blk-mq CPU hotplug handler is removed in 4b855ad37194, and we can't
>>> remap queue any more when CPU topo is changed, so the static & fixed mapping
>>> has to be setup from the beginning.
>>>
>>> Then if there are less enough online CPUs compared with number of hw queues,
>>> some of hctxes can be mapped with all offline CPUs. For example, if one device
>>> has 4 hw queues, but there are only 2 online CPUs and 6 offline CPUs, at most
>>> 2 hw queues are assigned to online CPUs, and the other two are all with offline
>>> CPUs.
>>
>> That is fine, but the problem that I gave in the example below which has
>> nr_hw_queues == num_online_cpus but because of the mapping, we still
>> have unmapped hctxs.
> 
> For FC's case, there may be some hctxs not 'mapped', which is caused by
> blk_mq_map_queues(), but that should one bug.
> 
> So the patch(blk-mq: don't keep offline CPUs mapped to hctx 0)[1] is
> fixing the issue:
> 	
> [1]	https://marc.info/?l=linux-block&m=152318093725257&w=2
> 
> Once this patch is in, any hctx should be mapped by at least one CPU.

I think this will solve the problem Yi is stepping on.

> Then later, the patch(blk-mq: reimplement blk_mq_hw_queue_mapped)[2]
> extends the mapping concept, maybe it should have been renamed as
> blk_mq_hw_queue_active(), will do it in V2.
> 
> [2] https://marc.info/?l=linux-block&m=152318099625268&w=2

This is also a good patch.

...

>> But when we reset the controller, we call blk_mq_update_nr_hw_queues()
>> with the current number of nr_hw_queues which never exceeds
>> num_online_cpus. This in turn, remaps the mq_map which results
>> in unmapped queues because of the mapping function, not because we
>> have more hctx than online cpus...
> 
> As I mentioned, num_online_cpus() isn't one stable variable, and it
> can change any time.

Correct, but I'm afraid num_possible_cpus might not work either.

> After patch(blk-mq: don't keep offline CPUs mapped to hctx 0) is in,
> there won't be unmapped queue any more.

Yes.

>> An easy fix, is to allocate num_present_cpus queues, and only connect
>> the oneline ones, but as you said, we have unused resources this way.
> 
> Yeah, it should be num_possible_cpus queues because physical CPU hotplug
> is needed to be supported for KVM or S390, or even some X86_64 system.

num_present_cpus is a waste of resources (as I said, both on the host
and on the target), but num_possible_cpus is even worse as this is
all cpus that _can_ be populated.

>> We also have an issue with blk_mq_rdma_map_queues with the only
>> device that supports it because it doesn't use managed affinity (code
>> was reverted) and can have irq affinity redirected in case of cpu
>> offlining...
> 
> That can be one corner case, looks I have to re-consider the patch
> (blk-mq: remove code for dealing with remapping queue), which may cause
> regression for this RDMA case, but I guess CPU hotplug may break this
> case easily.
> 
> [3] https://marc.info/?l=linux-block&m=152318100625284&w=2
> 
> Also this case will make blk-mq's queue mapping much complicated,
> could you provide one link about the reason for reverting managed affinity
> on this device?

The problem was that users reported a regression because now
/proc/irp/$IRQ/smp_affinity is immutable. Looks like netdev users do
this on a regular basis (and also rely on irq_banacer at times) while
nvme users (and other HBAs) do not care about it.

Thread starts here:
https://www.spinics.net/lists/netdev/msg464301.html

> Recently we fix quite a few issues on managed affinity, maybe the
> original issue for RDMA affinity has been addressed already.

That is not specific to RDMA affinity, its because RDMA devices are
also network devices and people want to apply their irq affinity
scripts on it like their used to with other devices.

>> The goal here I think, should be to allocate just enough queues (not
>> more than the number online cpus) and spread it 1x1 with online cpus,
>> and also make sure to allocate completion vectors that align to online
>> cpus. I just need to figure out how to do that...
> 
> I think we have to support CPU hotplug, so your goal may be hard to
> reach if you don't want to waste memory resource.

Well, not so much if I make blk_mq_rdma_map_queues do the right thing?

As I said, for the first go, I'd like to fix the mapping for the simple
case where we map the queues with some cpus offlined. Having queues
being added dynamically is a different story and I agree would require
more work.