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

Ming Lei
2018-04-06T15:40:31Z

On Fri, Apr 06, 2018 at 05:11:53PM +0200, Christian Borntraeger wrote:
>
>
> On 04/06/2018 04:58 PM, Ming Lei wrote:
> > On Fri, Apr 06, 2018 at 04:26:49PM +0200, Christian Borntraeger wrote:
> >>
> >>
> >> On 04/06/2018 03:41 PM, Ming Lei wrote:
> >>> On Fri, Apr 06, 2018 at 12:19:19PM +0200, Christian Borntraeger wrote:
> >>>>
> >>>>
> >>>> On 04/06/2018 11:23 AM, Ming Lei wrote:
> >>>>> 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).
> >>>>
> >>>> Really? the last log (with the latest patch applied shows a lot of contexts
> >>>> that do not have CPUs in 0-15:
> >>>>
> >>>> e.g.
> >>>> [ 4.049828] dump CPUs mapped to this hctx:
> >>>> [ 4.049829] 18
> >>>> [ 4.049829] 82
> >>>> [ 4.049830] 146
> >>>> [ 4.049830] 210
> >>>> [ 4.049831] 274
> >>>
> >>> That won't be an issue, since no IO can be submitted from these offline
> >>> CPUs, then these hctx shouldn't have been run at all.
> >>>
> >>> But hctx->next_cpu can be set as 512 for these inactive hctx in
> >>> blk_mq_map_swqueue(), then please test the attached patch, and if
> >>> hctx->next_cpu is still set as 512, something is still wrong.
> >>
> >>
> >> WIth this patch I no longer see the "run queue from wrong CPU x, hctx active" messages.
> >> your debug code still triggers, though.
> >>
> >> wrong next_cpu 512, blk_mq_hctx_next_cpu, first_and
> >> wrong next_cpu 512, blk_mq_hctx_next_cpu, next_and
> >>
> >> If we would remove the debug code then dmesg would be clean it seems.
> >
> > That is still a bit strange, since for any inactive hctx(without online
> > CPU mapped), blk_mq_run_hw_queue() will check blk_mq_hctx_has_pending()
>
> I think for next_and it is reasonable to see this, as the next_and will return
> 512 after we have used the last one. In fact the code does call first_and in
> that case for a reason, no?

It is possible for dumping 'first_and' when there isn't any online CPUs mapped
to this hctx.

But my question is that for this case, there shouldn't be any IO queued
for this hctx, and blk_mq_hctx_has_pending() has been called to check
that, so blk_mq_hctx_next_cpu() should have only be called when
blk_mq_hctx_has_pending() in blk_mq_run_hw_queue() is true.

Thanks,
Ming