Re: [PATCH] blk-mq: update hctx->cpumask at cpu-hotplug(Internet mail)

[Dongli Zhang]

On 6/25/19 10:27 AM, Ming Lei wrote:
> On Tue, Jun 25, 2019 at 02:14:46AM +0000, wenbinzeng(æææ) wrote:
>> Hi Ming,
>>
>>> -----Original Message-----
>>> From: Ming Lei <ming.lei@xxxxxxxxxx>
>>> Sent: Tuesday, June 25, 2019 9:55 AM
>>> To: Wenbin Zeng <wenbin.zeng@xxxxxxxxx>
>>> Cc: axboe@xxxxxxxxx; keith.busch@xxxxxxxxx; hare@xxxxxxxx; osandov@xxxxxx;
>>> sagi@xxxxxxxxxxx; bvanassche@xxxxxxx; linux-block@xxxxxxxxxxxxxxx;
>>> linux-kernel@xxxxxxxxxxxxxxx; wenbinzeng(æææ) <wenbinzeng@xxxxxxxxxxx>
>>> Subject: Re: [PATCH] blk-mq: update hctx->cpumask at cpu-hotplug(Internet mail)
>>>
>>> On Mon, Jun 24, 2019 at 11:24:07PM +0800, Wenbin Zeng wrote:
>>>> Currently hctx->cpumask is not updated when hot-plugging new cpus,
>>>> as there are many chances kblockd_mod_delayed_work_on() getting
>>>> called with WORK_CPU_UNBOUND, workqueue blk_mq_run_work_fn may run
>>>
>>> There are only two cases in which WORK_CPU_UNBOUND is applied:
>>>
>>> 1) single hw queue
>>>
>>> 2) multiple hw queue, and all CPUs in this hctx become offline
>>>
>>> For 1), all CPUs can be found in hctx->cpumask.
>>>
>>>> on the newly-plugged cpus, consequently __blk_mq_run_hw_queue()
>>>> reporting excessive "run queue from wrong CPU" messages because
>>>> cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) returns false.
>>>
>>> The message means CPU hotplug race is triggered.
>>>
>>> Yeah, there is big problem in blk_mq_hctx_notify_dead() which is called
>>> after one CPU is dead, but still run this hw queue to dispatch request,
>>> and all CPUs in this hctx might become offline.
>>>
>>> We have some discussion before on this issue:
>>>
>>> https://lore.kernel.org/linux-block/CACVXFVN729SgFQGUgmu1iN7P6Mv5+puE78STz8hj
>>> 9J5bS828Ng@xxxxxxxxxxxxxx/
>>>
>>
>> There is another scenario, you can reproduce it by hot-plugging cpus to kvm guests via qemu monitor (I believe virsh setvcpus --live can do the same thing), for example:
>> (qemu) cpu-add 1
>> (qemu) cpu-add 2
>> (qemu) cpu-add 3
>>
>> In such scenario, cpu 1, 2 and 3 are not visible at boot, hctx->cpumask doesn't get synced when these cpus are added.

Here is how I play with it with the most recent qemu and linux.

Boot VM with 1 out of 4 vcpu online.

# qemu-system-x86_64 -hda disk.img \
-smp 1,maxcpus=4 \
-m 4096M -enable-kvm \
-device nvme,drive=lightnvme,serial=deadbeaf1 \
-drive file=nvme.img,if=none,id=lightnvme \
-vnc :0 \
-kernel /.../mainline-linux/arch/x86_64/boot/bzImage \
-append "root=/dev/sda1 init=/sbin/init text" \
-monitor stdio -net nic -net user,hostfwd=tcp::5022-:22


As Ming mentioned, after boot:

# cat /proc/cpuinfo  | grep processor
processor	: 0

# cat /sys/block/nvme0n1/mq/0/cpu_list
0
# cat /sys/block/nvme0n1/mq/1/cpu_list
1
# cat /sys/block/nvme0n1/mq/2/cpu_list
2
# cat /sys/block/nvme0n1/mq/3/cpu_list
3

# cat /proc/interrupts | grep nvme
 24:         11   PCI-MSI 65536-edge      nvme0q0
 25:         78   PCI-MSI 65537-edge      nvme0q1
 26:          0   PCI-MSI 65538-edge      nvme0q2
 27:          0   PCI-MSI 65539-edge      nvme0q3
 28:          0   PCI-MSI 65540-edge      nvme0q4

I hotplug with "device_add
qemu64-x86_64-cpu,id=core1,socket-id=1,core-id=0,thread-id=0" in qemu monitor.

Dongli Zhang

> 
> It is CPU cold-plug, we suppose to support it.
> 
> The new added CPUs should be visible to hctx, since we spread queues
> among all possible CPUs(), please see blk_mq_map_queues() and
> irq_build_affinity_masks(), which is like static allocation on CPU
> resources.
> 
> Otherwise, you might use an old kernel or there is bug somewhere.
> 
>>
>>>>
>>>> This patch added a cpu-hotplug handler into blk-mq, updating
>>>> hctx->cpumask at cpu-hotplug.
>>>
>>> This way isn't correct, hctx->cpumask should be kept as sync with
>>> queue mapping.
>>
>> Please advise what should I do to deal with the above situation? Thanks a lot.
> 
> As I shared in last email, there is one approach discussed, which seems
> doable.
> 
> Thanks,
> Ming
>