> -----Original Message-----
> From: Ming Lei [mailto:ming....@redhat.com]
> Sent: Sunday, February 11, 2018 11:01 AM
> To: Kashyap Desai
> Cc: Hannes Reinecke; Jens Axboe; linux-block@vger.kernel.org; Christoph
> Hellwig; Mike Snitzer; linux-s...@vger.kernel.org; Arun Easi; Omar
Sandoval;
> Martin K . Petersen; James Bottomley; Christoph Hellwig; Don Brace;
Peter
> Rivera; Paolo Bonzini; Laurence Oberman
> Subject: Re: [PATCH 0/5] blk-mq/scsi-mq: support global tags & introduce
> force_blk_mq
>
> On Sat, Feb 10, 2018 at 09:00:57AM +0800, Ming Lei wrote:
> > Hi Kashyap,
> >
> > On Fri, Feb 09, 2018 at 02:12:16PM +0530, Kashyap Desai wrote:
> > > > -----Original Message-----
> > > > From: Ming Lei [mailto:ming....@redhat.com]
> > > > Sent: Friday, February 9, 2018 11:01 AM
> > > > To: Kashyap Desai
> > > > Cc: Hannes Reinecke; Jens Axboe; linux-block@vger.kernel.org;
> > > > Christoph Hellwig; Mike Snitzer; linux-s...@vger.kernel.org; Arun
> > > > Easi; Omar
> > > Sandoval;
> > > > Martin K . Petersen; James Bottomley; Christoph Hellwig; Don
> > > > Brace;
> > > Peter
> > > > Rivera; Paolo Bonzini; Laurence Oberman
> > > > Subject: Re: [PATCH 0/5] blk-mq/scsi-mq: support global tags &
> > > > introduce force_blk_mq
> > > >
> > > > On Fri, Feb 09, 2018 at 10:28:23AM +0530, Kashyap Desai wrote:
> > > > > > -----Original Message-----
> > > > > > From: Ming Lei [mailto:ming....@redhat.com]
> > > > > > Sent: Thursday, February 8, 2018 10:23 PM
> > > > > > To: Hannes Reinecke
> > > > > > Cc: Kashyap Desai; Jens Axboe; linux-block@vger.kernel.org;
> > > > > > Christoph Hellwig; Mike Snitzer; linux-s...@vger.kernel.org;
> > > > > > Arun Easi; Omar
> > > > > Sandoval;
> > > > > > Martin K . Petersen; James Bottomley; Christoph Hellwig; Don
> > > > > > Brace;
> > > > > Peter
> > > > > > Rivera; Paolo Bonzini; Laurence Oberman
> > > > > > Subject: Re: [PATCH 0/5] blk-mq/scsi-mq: support global tags &
> > > > > > introduce force_blk_mq
> > > > > >
> > > > > > On Thu, Feb 08, 2018 at 08:00:29AM +0100, Hannes Reinecke
wrote:
> > > > > > > On 02/07/2018 03:14 PM, Kashyap Desai wrote:
> > > > > > > >> -----Original Message-----
> > > > > > > >> From: Ming Lei [mailto:ming....@redhat.com]
> > > > > > > >> Sent: Wednesday, February 7, 2018 5:53 PM
> > > > > > > >> To: Hannes Reinecke
> > > > > > > >> Cc: Kashyap Desai; Jens Axboe;
> > > > > > > >> linux-block@vger.kernel.org; Christoph Hellwig; Mike
> > > > > > > >> Snitzer; linux-s...@vger.kernel.org; Arun Easi; Omar
> > > > > > > > Sandoval;
> > > > > > > >> Martin K . Petersen; James Bottomley; Christoph Hellwig;
> > > > > > > >> Don Brace;
> > > > > > > > Peter
> > > > > > > >> Rivera; Paolo Bonzini; Laurence Oberman
> > > > > > > >> Subject: Re: [PATCH 0/5] blk-mq/scsi-mq: support global
> > > > > > > >> tags & introduce force_blk_mq
> > > > > > > >>
> > > > > > > >> On Wed, Feb 07, 2018 at 07:50:21AM +0100, Hannes Reinecke
> > > wrote:
> > > > > > > >>> Hi all,
> > > > > > > >>>
> > > > > > > >>> [ .. ]
> > > > > > > >>>>>
> > > > > > > >>>>> Could you share us your patch for enabling
> > > > > > > >>>>> global_tags/MQ on
> > > > > > > >>>> megaraid_sas
> > > > > > > >>>>> so that I can reproduce your test?
> > > > > > > >>>>>
> > > > > > > >>>>>> See below perf top data. "bt_iter" is consuming 4
> > > > > > > >>>>>> times more
> > > > > CPU.
> > > > > > > >>>>>
> > > > > > > >>>>> Could you share us what the IOPS/CPU utilization
> > > > > > > >>>>> effect is after
> > > > > > > >>>> applying the
> > > > > > > >>>>> patch V2? And your test script?
> > > > > > > >>>> Regarding CPU utilization, I need to test one more
time.
> > > > > > > >>>> Currently system is in used.
> > > > > > > >>>>
> > > > > > > >>>> I run below fio test on total 24 SSDs expander
attached.
> > > > > > > >>>>
> > > > > > > >>>> numactl -N 1 fio jbod.fio --rw=randread --iodepth=64
> > > > > > > >>>> --bs=4k --ioengine=libaio --rw=randread
> > > > > > > >>>>
> > > > > > > >>>> Performance dropped from 1.6 M IOPs to 770K IOPs.
> > > > > > > >>>>
> > > > > > > >>> This is basically what we've seen with earlier
iterations.
> > > > > > > >>
> > > > > > > >> Hi Hannes,
> > > > > > > >>
> > > > > > > >> As I mentioned in another mail[1], Kashyap's patch has a
> > > > > > > >> big issue,
> > > > > > > > which
> > > > > > > >> causes only reply queue 0 used.
> > > > > > > >>
> > > > > > > >> [1] https://marc.info/?l=linux-scsi&m=151793204014631&w=2
> > > > > > > >>
> > > > > > > >> So could you guys run your performance test again after
> > > > > > > >> fixing the
> > > > > > > > patch?
> > > > > > > >
> > > > > > > > Ming -
> > > > > > > >
> > > > > > > > I tried after change you requested.  Performance drop is
> > > > > > > > still
> > > > > unresolved.
> > > > > > > > From 1.6 M IOPS to 770K IOPS.
> > > > > > > >
> > > > > > > > See below data. All 24 reply queue is in used correctly.
> > > > > > > >
> > > > > > > > IRQs / 1 second(s)
> > > > > > > > IRQ#  TOTAL  NODE0   NODE1  NAME
> > > > > > > >  360  16422      0   16422  IR-PCI-MSI 70254653-edge
megasas
> > > > > > > >  364  15980      0   15980  IR-PCI-MSI 70254657-edge
megasas
> > > > > > > >  362  15979      0   15979  IR-PCI-MSI 70254655-edge
megasas
> > > > > > > >  345  15696      0   15696  IR-PCI-MSI 70254638-edge
megasas
> > > > > > > >  341  15659      0   15659  IR-PCI-MSI 70254634-edge
megasas
> > > > > > > >  369  15656      0   15656  IR-PCI-MSI 70254662-edge
megasas
> > > > > > > >  359  15650      0   15650  IR-PCI-MSI 70254652-edge
megasas
> > > > > > > >  358  15596      0   15596  IR-PCI-MSI 70254651-edge
megasas
> > > > > > > >  350  15574      0   15574  IR-PCI-MSI 70254643-edge
megasas
> > > > > > > >  342  15532      0   15532  IR-PCI-MSI 70254635-edge
megasas
> > > > > > > >  344  15527      0   15527  IR-PCI-MSI 70254637-edge
megasas
> > > > > > > >  346  15485      0   15485  IR-PCI-MSI 70254639-edge
megasas
> > > > > > > >  361  15482      0   15482  IR-PCI-MSI 70254654-edge
megasas
> > > > > > > >  348  15467      0   15467  IR-PCI-MSI 70254641-edge
megasas
> > > > > > > >  368  15463      0   15463  IR-PCI-MSI 70254661-edge
megasas
> > > > > > > >  354  15420      0   15420  IR-PCI-MSI 70254647-edge
megasas
> > > > > > > >  351  15378      0   15378  IR-PCI-MSI 70254644-edge
megasas
> > > > > > > >  352  15377      0   15377  IR-PCI-MSI 70254645-edge
megasas
> > > > > > > >  356  15348      0   15348  IR-PCI-MSI 70254649-edge
megasas
> > > > > > > >  337  15344      0   15344  IR-PCI-MSI 70254630-edge
megasas
> > > > > > > >  343  15320      0   15320  IR-PCI-MSI 70254636-edge
megasas
> > > > > > > >  355  15266      0   15266  IR-PCI-MSI 70254648-edge
megasas
> > > > > > > >  335  15247      0   15247  IR-PCI-MSI 70254628-edge
megasas
> > > > > > > >  363  15233      0   15233  IR-PCI-MSI 70254656-edge
megasas
> > > > > > > >
> > > > > > > >
> > > > > > > > Average:        CPU      %usr     %nice      %sys
%iowait
> > > > > %steal
> > > > > > > > %irq     %soft    %guest    %gnice     %idle
> > > > > > > > Average:         18      3.80      0.00     14.78
10.08
> > > > > 0.00
> > > > > > > > 0.00      4.01      0.00      0.00     67.33
> > > > > > > > Average:         19      3.26      0.00     15.35
10.62
> > > > > 0.00
> > > > > > > > 0.00      4.03      0.00      0.00     66.74
> > > > > > > > Average:         20      3.42      0.00     14.57
10.67
> > > > > 0.00
> > > > > > > > 0.00      3.84      0.00      0.00     67.50
> > > > > > > > Average:         21      3.19      0.00     15.60
10.75
> > > > > 0.00
> > > > > > > > 0.00      4.16      0.00      0.00     66.30
> > > > > > > > Average:         22      3.58      0.00     15.15
10.66
> > > > > 0.00
> > > > > > > > 0.00      3.51      0.00      0.00     67.11
> > > > > > > > Average:         23      3.34      0.00     15.36
10.63
> > > > > 0.00
> > > > > > > > 0.00      4.17      0.00      0.00     66.50
> > > > > > > > Average:         24      3.50      0.00     14.58
10.93
> > > > > 0.00
> > > > > > > > 0.00      3.85      0.00      0.00     67.13
> > > > > > > > Average:         25      3.20      0.00     14.68
10.86
> > > > > 0.00
> > > > > > > > 0.00      4.31      0.00      0.00     66.95
> > > > > > > > Average:         26      3.27      0.00     14.80
10.70
> > > > > 0.00
> > > > > > > > 0.00      3.68      0.00      0.00     67.55
> > > > > > > > Average:         27      3.58      0.00     15.36
10.80
> > > > > 0.00
> > > > > > > > 0.00      3.79      0.00      0.00     66.48
> > > > > > > > Average:         28      3.46      0.00     15.17
10.46
> > > > > 0.00
> > > > > > > > 0.00      3.32      0.00      0.00     67.59
> > > > > > > > Average:         29      3.34      0.00     14.42
10.72
> > > > > 0.00
> > > > > > > > 0.00      3.34      0.00      0.00     68.18
> > > > > > > > Average:         30      3.34      0.00     15.08
10.70
> > > > > 0.00
> > > > > > > > 0.00      3.89      0.00      0.00     66.99
> > > > > > > > Average:         31      3.26      0.00     15.33
10.47
> > > > > 0.00
> > > > > > > > 0.00      3.33      0.00      0.00     67.61
> > > > > > > > Average:         32      3.21      0.00     14.80
10.61
> > > > > 0.00
> > > > > > > > 0.00      3.70      0.00      0.00     67.67
> > > > > > > > Average:         33      3.40      0.00     13.88
10.55
> > > > > 0.00
> > > > > > > > 0.00      4.02      0.00      0.00     68.15
> > > > > > > > Average:         34      3.74      0.00     17.41
10.61
> > > > > 0.00
> > > > > > > > 0.00      4.51      0.00      0.00     63.73
> > > > > > > > Average:         35      3.35      0.00     14.37
10.74
> > > > > 0.00
> > > > > > > > 0.00      3.84      0.00      0.00     67.71
> > > > > > > > Average:         36      0.54      0.00      1.77
0.00
> > > > > 0.00
> > > > > > > > 0.00      0.00      0.00      0.00     97.69
> > > > > > > > ..
> > > > > > > > Average:         54      3.60      0.00     15.17
10.39
> > > > > 0.00
> > > > > > > > 0.00      4.22      0.00      0.00     66.62
> > > > > > > > Average:         55      3.33      0.00     14.85
10.55
> > > > > 0.00
> > > > > > > > 0.00      3.96      0.00      0.00     67.31
> > > > > > > > Average:         56      3.40      0.00     15.19
10.54
> > > > > 0.00
> > > > > > > > 0.00      3.74      0.00      0.00     67.13
> > > > > > > > Average:         57      3.41      0.00     13.98
10.78
> > > > > 0.00
> > > > > > > > 0.00      4.10      0.00      0.00     67.73
> > > > > > > > Average:         58      3.32      0.00     15.16
10.52
> > > > > 0.00
> > > > > > > > 0.00      4.01      0.00      0.00     66.99
> > > > > > > > Average:         59      3.17      0.00     15.80
10.35
> > > > > 0.00
> > > > > > > > 0.00      3.86      0.00      0.00     66.80
> > > > > > > > Average:         60      3.00      0.00     14.63
10.59
> > > > > 0.00
> > > > > > > > 0.00      3.97      0.00      0.00     67.80
> > > > > > > > Average:         61      3.34      0.00     14.70
10.66
> > > > > 0.00
> > > > > > > > 0.00      4.32      0.00      0.00     66.97
> > > > > > > > Average:         62      3.34      0.00     15.29
10.56
> > > > > 0.00
> > > > > > > > 0.00      3.89      0.00      0.00     66.92
> > > > > > > > Average:         63      3.29      0.00     14.51
10.72
> > > > > 0.00
> > > > > > > > 0.00      3.85      0.00      0.00     67.62
> > > > > > > > Average:         64      3.48      0.00     15.31
10.65
> > > > > 0.00
> > > > > > > > 0.00      3.97      0.00      0.00     66.60
> > > > > > > > Average:         65      3.34      0.00     14.36
10.80
> > > > > 0.00
> > > > > > > > 0.00      4.11      0.00      0.00     67.39
> > > > > > > > Average:         66      3.13      0.00     14.94
10.70
> > > > > 0.00
> > > > > > > > 0.00      4.10      0.00      0.00     67.13
> > > > > > > > Average:         67      3.06      0.00     15.56
10.69
> > > > > 0.00
> > > > > > > > 0.00      3.82      0.00      0.00     66.88
> > > > > > > > Average:         68      3.33      0.00     14.98
10.61
> > > > > 0.00
> > > > > > > > 0.00      3.81      0.00      0.00     67.27
> > > > > > > > Average:         69      3.20      0.00     15.43
10.70
> > > > > 0.00
> > > > > > > > 0.00      3.82      0.00      0.00     66.85
> > > > > > > > Average:         70      3.34      0.00     17.14
10.59
> > > > > 0.00
> > > > > > > > 0.00      3.00      0.00      0.00     65.92
> > > > > > > > Average:         71      3.41      0.00     14.94
10.56
> > > > > 0.00
> > > > > > > > 0.00      3.41      0.00      0.00     67.69
> > > > > > > >
> > > > > > > > Perf top -
> > > > > > > >
> > > > > > > >   64.33%  [kernel]            [k] bt_iter
> > > > > > > >    4.86%  [kernel]            [k]
blk_mq_queue_tag_busy_iter
> > > > > > > >    4.23%  [kernel]            [k] _find_next_bit
> > > > > > > >    2.40%  [kernel]            [k]
> > > native_queued_spin_lock_slowpath
> > > > > > > >    1.09%  [kernel]            [k] sbitmap_any_bit_set
> > > > > > > >    0.71%  [kernel]            [k] sbitmap_queue_clear
> > > > > > > >    0.63%  [kernel]            [k] find_next_bit
> > > > > > > >    0.54%  [kernel]            [k] _raw_spin_lock_irqsave
> > > > > > > >
> > > > > > > Ah. So we're spending quite some time in trying to find a
> > > > > > > free
> > > tag.
> > > > > > > I guess this is due to every queue starting at the same
> > > > > > > position trying to find a free tag, which inevitably leads
to a
> contention.
> > > > > >
> > > > > > IMO, the above trace means that blk_mq_in_flight() may be the
> > > > > bottleneck,
> > > > > > and looks not related with tag allocation.
> > > > > >
> > > > > > Kashyap, could you run your performance test again after
> > > > > > disabling
> > > > > iostat by
> > > > > > the following command on all test devices and killing all
> > > > > > utilities
> > > > > which may
> > > > > > read iostat(/proc/diskstats, ...)?
> > > > > >
> > > > > >     echo 0 > /sys/block/sdN/queue/iostat
> > > > >
> > > > > Ming - After changing iostat = 0 , I see performance issue is
> > > resolved.
> > > > >
> > > > > Below is perf top output after iostats = 0
> > > > >
> > > > >
> > > > >   23.45%  [kernel]             [k] bt_iter
> > > > >    2.27%  [kernel]             [k] blk_mq_queue_tag_busy_iter
> > > > >    2.18%  [kernel]             [k] _find_next_bit
> > > > >    2.06%  [megaraid_sas]       [k] complete_cmd_fusion
> > > > >    1.87%  [kernel]             [k] clflush_cache_range
> > > > >    1.70%  [kernel]             [k] dma_pte_clear_level
> > > > >    1.56%  [kernel]             [k] __domain_mapping
> > > > >    1.55%  [kernel]             [k] sbitmap_queue_clear
> > > > >    1.30%  [kernel]             [k] gup_pgd_range
> > > >
> > > > Hi Kashyap,
> > > >
> > > > Thanks for your test and update.
> > > >
> > > > Looks blk_mq_queue_tag_busy_iter() is still sampled by perf even
> > > > though iostats is disabled, and I guess there may be utilities
> > > > which are
> > > reading iostats
> > > > a bit frequently.
> > >
> > > I  will be doing some more testing and post you my findings.
> >
> > I will find sometime this weekend to see if I can cook a patch to
> > address this issue of io accounting.
>
> Hi Kashyap,
>
> Please test the top 5 patches in the following tree to see if
megaraid_sas's
> performance is OK:
>
>       https://github.com/ming1/linux/commits/v4.15-for-next-global-tags-
> v2
>
> This tree is made by adding these 5 patches against patchset V2.
>

Ming -
I applied 5 patches on top of V2 and behavior is still unchanged. Below is
perf top data. (1000K IOPS)

  34.58%  [kernel]                 [k] bt_iter
   2.96%  [kernel]                 [k] sbitmap_any_bit_set
   2.77%  [kernel]                 [k] bt_iter_global_tags
   1.75%  [megaraid_sas]           [k] complete_cmd_fusion
   1.62%  [kernel]                 [k] sbitmap_queue_clear
   1.62%  [kernel]                 [k] _raw_spin_lock
   1.51%  [kernel]                 [k] blk_mq_run_hw_queue
   1.45%  [kernel]                 [k] gup_pgd_range
   1.31%  [kernel]                 [k] irq_entries_start
   1.29%  fio                      [.] __fio_gettime
   1.13%  [kernel]                 [k] _raw_spin_lock_irqsave
   0.95%  [kernel]                 [k] native_queued_spin_lock_slowpath
   0.92%  [kernel]                 [k] scsi_queue_rq
   0.91%  [kernel]                 [k] blk_mq_run_hw_queues
   0.85%  [kernel]                 [k] blk_mq_get_request
   0.81%  [kernel]                 [k] switch_mm_irqs_off
   0.78%  [megaraid_sas]           [k] megasas_build_io_fusion
   0.77%  [kernel]                 [k] __schedule
   0.73%  [kernel]                 [k] update_load_avg
   0.69%  [kernel]                 [k] fput
   0.65%  [kernel]                 [k] scsi_dispatch_cmd
   0.64%  fio                      [.] fio_libaio_event
   0.53%  [kernel]                 [k] do_io_submit
   0.52%  [kernel]                 [k] read_tsc
   0.51%  [megaraid_sas]           [k] megasas_build_and_issue_cmd_fusion
   0.51%  [kernel]                 [k] scsi_softirq_done
   0.50%  [kernel]                 [k] kobject_put
   0.50%  [kernel]                 [k] cpuidle_enter_state
   0.49%  [kernel]                 [k] native_write_msr
   0.48%  fio                      [.] io_completed

Below is perf top data with iostat=0  (1400K IOPS)

   4.87%  [kernel]                      [k] sbitmap_any_bit_set
   2.93%  [kernel]                      [k] _raw_spin_lock
   2.84%  [megaraid_sas]                [k] complete_cmd_fusion
   2.38%  [kernel]                      [k] irq_entries_start
   2.36%  [kernel]                      [k] gup_pgd_range
   2.35%  [kernel]                      [k] blk_mq_run_hw_queue
   2.30%  [kernel]                      [k] sbitmap_queue_clear
   2.01%  fio                           [.] __fio_gettime
   1.78%  [kernel]                      [k] _raw_spin_lock_irqsave
   1.51%  [kernel]                      [k] scsi_queue_rq
   1.43%  [kernel]                      [k] blk_mq_run_hw_queues
   1.36%  [kernel]                      [k] fput
   1.32%  [kernel]                      [k] __schedule
   1.31%  [kernel]                      [k] switch_mm_irqs_off
   1.29%  [kernel]                      [k] update_load_avg
   1.25%  [megaraid_sas]                [k] megasas_build_io_fusion
   1.22%  [kernel]                      [k]
native_queued_spin_lock_slowpath
   1.03%  [kernel]                      [k] scsi_dispatch_cmd
   1.03%  [kernel]                      [k] blk_mq_get_request
   0.91%  fio                           [.] fio_libaio_event
   0.89%  [kernel]                      [k] scsi_softirq_done
   0.87%  [kernel]                      [k] kobject_put
   0.86%  [kernel]                      [k] cpuidle_enter_state
   0.84%  fio                           [.] io_completed
   0.83%  [kernel]                      [k] do_io_submit
   0.83%  [megaraid_sas]                [k]
megasas_build_and_issue_cmd_fusion
   0.83%  [kernel]                      [k] __switch_to
   0.82%  [kernel]                      [k] read_tsc
   0.80%  [kernel]                      [k] native_write_msr
   0.76%  [kernel]                      [k] aio_comp


Perf data without V2 patch applied.  (1600K IOPS)

   5.97%  [megaraid_sas]           [k] complete_cmd_fusion
   5.24%  [kernel]                 [k] bt_iter
   3.28%  [kernel]                 [k] _raw_spin_lock
   2.98%  [kernel]                 [k] irq_entries_start
   2.29%  fio                      [.] __fio_gettime
   2.04%  [kernel]                 [k] scsi_queue_rq
   1.92%  [megaraid_sas]           [k] megasas_build_io_fusion
   1.61%  [kernel]                 [k] switch_mm_irqs_off
   1.59%  [megaraid_sas]           [k] megasas_build_and_issue_cmd_fusion
   1.41%  [kernel]                 [k] scsi_dispatch_cmd
   1.33%  [kernel]                 [k] scsi_softirq_done
   1.18%  [kernel]                 [k] gup_pgd_range
   1.18%  [kernel]                 [k] blk_mq_complete_request
   1.13%  [kernel]                 [k] blk_mq_free_request
   1.05%  [kernel]                 [k] do_io_submit
   1.04%  [kernel]                 [k] _find_next_bit
   1.02%  [kernel]                 [k] blk_mq_get_request
   0.95%  [megaraid_sas]           [k] megasas_build_ldio_fusion
   0.95%  [kernel]                 [k] scsi_dec_host_busy
   0.89%  fio                      [.] get_io_u
   0.88%  [kernel]                 [k] entry_SYSCALL_64
   0.84%  [megaraid_sas]           [k] megasas_queue_command
   0.79%  [kernel]                 [k] native_write_msr
   0.77%  [kernel]                 [k] read_tsc
   0.73%  [kernel]                 [k] _raw_spin_lock_irqsave
   0.73%  fio                      [.] fio_libaio_commit
   0.72%  [kernel]                 [k] kmem_cache_alloc
   0.72%  [kernel]                 [k] blkdev_direct_IO
   0.69%  [megaraid_sas]           [k] MR_GetPhyParams
   0.68%  [kernel]                 [k] blk_mq_dequeue_f


> If possible, please provide us the performance data without these
patches and
> with these patches, together with perf trace.
>
> The top 5 patches are for addressing the io accounting issue, and which
> should be the main reason for your performance drop, even lockup in
> megaraid_sas's ISR, IMO.

I think performance drop is different issue. May be a side effect of the
patch set. Even though we fix this perf issue, cpu lock up is completely
different issue.
Regarding cpu lock up, there was similar discussion and folks are finding
irq poll is good method to resolve lockup.  Not sure why NVME driver did
not opted irq_poll, but there was extensive discussion and I am also
seeing cpu lock up mainly due to multiple completion queue/reply queue is
tied to single CPU. We have weighing method in irq poll to quit ISR and
that is the way we can avoid lock-up.
http://lists.infradead.org/pipermail/linux-nvme/2017-January/007724.html

>
> Thanks,
> Ming

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