When running the fio test on a 448-core AMD server + a NVME disk,
a soft lockup or a hard lockup call trace is shown:

[soft lockup]
watchdog: BUG: soft lockup - CPU#126 stuck for 23s! [swapper/126:0]
RIP: 0010:_raw_spin_unlock_irqrestore+0x21/0x50
...
Call Trace:
 <IRQ>
 fq_flush_timeout+0x7d/0xd0
 ? __pfx_fq_flush_timeout+0x10/0x10
 call_timer_fn+0x2e/0x150
 run_timer_softirq+0x48a/0x560
 ? __pfx_fq_flush_timeout+0x10/0x10
 ? clockevents_program_event+0xaf/0x130
 __do_softirq+0xf1/0x335
 irq_exit_rcu+0x9f/0xd0
 sysvec_apic_timer_interrupt+0xb4/0xd0
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x1f/0x30
...

Obvisouly, fq_flush_timeout spends over 20 seconds. Here is ftrace log:

               |  fq_flush_timeout() {
               |    fq_ring_free() {
               |      put_pages_list() {
   0.170 us    |        free_unref_page_list();
   0.810 us    |      }
               |      free_iova_fast() {
               |        free_iova() {
 * 85622.66 us |          _raw_spin_lock_irqsave();
   2.860 us    |          remove_iova();
   0.600 us    |          _raw_spin_unlock_irqrestore();
   0.470 us    |          lock_info_report();
   2.420 us    |          free_iova_mem.part.0();
 * 85638.27 us |        }
 * 85638.84 us |      }
               |      put_pages_list() {
   0.230 us    |        free_unref_page_list();
   0.470 us    |      }
   ...            ...
 $ 31017069 us |  }

Most of cores are under lock contention for acquiring iova_rbtree_lock due
to the iova flush queue mechanism.

[hard lockup]
NMI watchdog: Watchdog detected hard LOCKUP on cpu 351
RIP: 0010:native_queued_spin_lock_slowpath+0x2d8/0x330

Call Trace:
 <IRQ>
 _raw_spin_lock_irqsave+0x4f/0x60
 free_iova+0x27/0xd0
 free_iova_fast+0x4d/0x1d0
 fq_ring_free+0x9b/0x150
 iommu_dma_free_iova+0xb4/0x2e0
 __iommu_dma_unmap+0x10b/0x140
 iommu_dma_unmap_sg+0x90/0x110
 dma_unmap_sg_attrs+0x4a/0x50
 nvme_unmap_data+0x5d/0x120 [nvme]
 nvme_pci_complete_batch+0x77/0xc0 [nvme]
 nvme_irq+0x2ee/0x350 [nvme]
 ? __pfx_nvme_pci_complete_batch+0x10/0x10 [nvme]
 __handle_irq_event_percpu+0x53/0x1a0
 handle_irq_event_percpu+0x19/0x60
 handle_irq_event+0x3d/0x60
 handle_edge_irq+0xb3/0x210
 __common_interrupt+0x7f/0x150
 common_interrupt+0xc5/0xf0
 </IRQ>
 <TASK>
 asm_common_interrupt+0x2b/0x40
...

ftrace shows fq_ring_free spends over 10 seconds [1]. Again, most of
cores are under lock contention for acquiring iova_rbtree_lock due
to the iova flush queue mechanism.

[Root Cause]
The root cause is that the max_hw_sectors_kb of nvme disk (mdts=10)
is 4096kb, which streaming DMA mappings cannot benefit from the
scalable IOVA mechanism introduced by the commit 9257b4a2
("iommu/iova: introduce per-cpu caching to iova allocation") if
the length is greater than 128kb.

To fix the lock contention issue, clamp max_hw_sectors based on
DMA optimized limitation in order to leverage scalable IOVA mechanism.

Note: The issue does not happen with another NVME disk (mdts = 5
and max_hw_sectors_kb = 128)

[1] https://gist.github.com/AdrianHuang/bf8ec7338204837631fbdaed25d19cc4Suggested-by: Keith Busch <kbusch@kernel.org>
Reported-and-tested-by: Jiwei Sun <sunjw10@lenovo.com>
Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>