Re: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with virtio-blk (also 4.12 stable)

[Christian Borntraeger]

Independent from the issues with the dasd disks, this also seem to not enable
additional hardware queues.

with cpus 0,1 (and 248 cpus max)
I get cpus 0 and 2-247 attached to hardware contect 0 and I get
cpu 1 for hardware context 1. 

If I now add a cpu this does not change anything. hardware context 2,3,4
etc all have no CPU and hardware context 0 keeps sitting on all cpus (except 1).




On 12/07/2017 10:20 AM, Christian Borntraeger wrote:
> 
> 
> On 12/07/2017 12:29 AM, Christoph Hellwig wrote:
>> On Wed, Dec 06, 2017 at 01:25:11PM +0100, Christian Borntraeger wrote:
>> t > commit 11b2025c3326f7096ceb588c3117c7883850c068    -> bad
>>>     blk-mq: create a blk_mq_ctx for each possible CPU
>>> does not boot on DASD and 
>>> commit 9c6ae239e01ae9a9f8657f05c55c4372e9fc8bcc    -> good
>>>    genirq/affinity: assign vectors to all possible CPUs
>>> does boot with DASD disks.
>>>
>>> Also adding Stefan Haberland if he has an idea why this fails on DASD and adding Martin (for the
>>> s390 irq handling code).
>>
>> That is interesting as it really isn't related to interrupts at all,
>> it just ensures that possible CPUs are set in ->cpumask.
>>
>> I guess we'd really want:
>>
>> e005655c389e3d25bf3e43f71611ec12f3012de0
>> "blk-mq: only select online CPUs in blk_mq_hctx_next_cpu"
>>
>> before this commit, but it seems like the whole stack didn't work for
>> your either.
>>
>> I wonder if there is some weird thing about nr_cpu_ids in s390?
> 
> The problem starts as soon as NR_CPUS is larger than the number
> of real CPUs.
> 
> Aquestions Wouldnt your change in blk_mq_hctx_next_cpu fail if there is more than 1 non-online cpu:
> 
> e.g. dont we need something like (whitespace and indent damaged)
> 
> @@ -1241,11 +1241,11 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>         if (--hctx->next_cpu_batch <= 0) {
>                 int next_cpu;
>  
> +               do  {
>                 next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
> -               if (!cpu_online(next_cpu))
> -                       next_cpu = cpumask_next(next_cpu, hctx->cpumask);
>                 if (next_cpu >= nr_cpu_ids)
>                         next_cpu = cpumask_first(hctx->cpumask);
> +               } while (!cpu_online(next_cpu));
>  
>                 hctx->next_cpu = next_cpu;
>                 hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
> 
> it does not fix the issue, though (and it would be pretty inefficient for large NR_CPUS)
> 
>