Re: [PATCH] blk-mq: only run mapped hw queues in blk_mq_run_hw_queues()

[Ming Lei <ming.lei@redhat.com>]

On Fri, Apr 06, 2018 at 10:51:28AM +0200, Christian Borntraeger wrote:
> 
> 
> On 04/06/2018 10:41 AM, Ming Lei wrote:
> > On Thu, Apr 05, 2018 at 07:39:56PM +0200, Christian Borntraeger wrote:
> >>
> >>
> >> On 04/05/2018 06:11 PM, Ming Lei wrote:
> >>>>
> >>>> Could you please apply the following patch and provide the dmesg boot log?
> >>>
> >>> And please post out the 'lscpu' log together from the test machine too.
> >>
> >> attached.
> >>
> >> As I said before this seems to go way with CONFIG_NR_CPUS=64 or smaller.
> >> We have 282 nr_cpu_ids here (max 141CPUs on that z13 with SMT2) but only 8 Cores
> >> == 16 threads.
> > 
> > OK, thanks!
> > 
> > The most weird thing is that hctx->next_cpu is computed as 512 since
> > nr_cpu_id is 282, and hctx->next_cpu should have pointed to one of
> > possible CPU.
> > 
> > Looks like it is a s390 specific issue, since I can setup one queue
> > which has same mapping with yours:
> > 
> > 	- nr_cpu_id is 282
> > 	- CPU 0~15 is online
> > 	- 64 queues null_blk
> > 	- still run all hw queues in .complete handler
> > 
> > But can't reproduce this issue at all.
> > 
> > So please test the following patch, which may tell us why hctx->next_cpu
> > is computed wrong:
> 
> I see things like
> 
> [    8.196907] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196910] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196912] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196913] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196914] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196915] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196916] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196916] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196917] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> [    8.196918] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> 
> which is exactly what happens if the find and and operation fails (returns size of bitmap).

Given both 'cpu_online_mask' and 'hctx->cpumask' are shown as correct
in your previous debug log, it means the following function returns
totally wrong result on S390.

	cpumask_first_and(hctx->cpumask, cpu_online_mask);

The debugfs log shows that each hctx->cpumask includes one online
CPU(0~15).

So looks it isn't one issue in block MQ core.

Thanks,
Ming