Re: Hard and soft lockups with FIO and LTP runs on a large system

[Bharata B Rao]

On 17-Jul-24 3:12 PM, Vlastimil Babka wrote:
> On 7/3/24 5:11 PM, Bharata B Rao wrote:
> > Many soft and hard lockups are seen with upstream kernel when running a
> > bunch of tests that include FIO and LTP filesystem test on 10 NVME
> > disks. The lockups can appear anywhere between 2 to 48 hours. Originally
> > this was reported on a large customer VM instance with passthrough NVME
> > disks on older kernels(v5.4 based). However, similar problems were
> > reproduced when running the tests on bare metal with latest upstream
> > kernel (v6.10-rc3). Other lockups with different signatures are seen but
> > in this report, only those related to MM area are being discussed.
> > Also note that the subsequent description is related to the lockups in
> > bare metal upstream (and not VM).
> >
> > The general observation is that the problem usually surfaces when the
> > system free memory goes very low and page cache/buffer consumption hits
> > the ceiling. Most of the times the two contended locks are lruvec and
> > inode->i_lock spinlocks.
> >
> > - Could this be a scalability issue in LRU list handling and/or page
> > cache invalidation typical to a large system configuration?
>
> Seems to me it could be (except that ZONE_DMA corner case) a general
> scalability issue in that you tweak some part of the kernel and the
> contention moves elsewhere. At least in MM we have per-node locks so this
> means 256 CPUs per lock? It used to be that there were not that many
> (cores/threads) per a physical CPU and its NUMA node, so many cpus would
> mean also more NUMA nodes where the locks contention would distribute among
> them. I think you could try fakenuma to create these nodes artificially and
> see if it helps for the MM part. But if the contention moves to e.g. an
> inode lock, I'm not sure what to do about that then.

See below...

>
<SNIP>
> > 3) AMD has a BIOS setting called NPS (Nodes per socket), using which a
> > socket can be further partitioned into smaller NUMA nodes. With NPS=4,
> > there will be four NUMA nodes in one socket, and hence 8 NUMA nodes in
> > the system. This was done to check if having more number of kswapd
> > threads working on lesser number of folios per node would make a
> > difference. However here too, multiple  soft lockups were seen (in
> > clear_shadow_entry() as seen in MGLRU case). No hard lockups were observed.

These are some softlockups seen with NPS4 mode.

watchdog: BUG: soft lockup - CPU#315 stuck for 11s! [kworker/315:1H:5153]
CPU: 315 PID: 5153 Comm: kworker/315:1H Kdump: loaded Not tainted 
6.10.0-rc3-enbprftw #12
Workqueue: kblockd blk_mq_run_work_fn
RIP: 0010:handle_softirqs+0x70/0x2f0
Call Trace:
  <IRQ>
  __irq_exit_rcu+0x68/0x90
  irq_exit_rcu+0x12/0x20
  sysvec_apic_timer_interrupt+0x85/0xb0
  </IRQ>
  <TASK>
  asm_sysvec_apic_timer_interrupt+0x1f/0x30
RIP: 0010:iommu_dma_map_page+0xca/0x2c0
  dma_map_page_attrs+0x20d/0x2a0
  nvme_prep_rq.part.0+0x63d/0x940 [nvme]
  nvme_queue_rq+0x82/0x210 [nvme]
  blk_mq_dispatch_rq_list+0x289/0x6d0
  __blk_mq_sched_dispatch_requests+0x142/0x5f0
  blk_mq_sched_dispatch_requests+0x36/0x70
  blk_mq_run_work_fn+0x73/0x90
  process_one_work+0x185/0x3d0
  worker_thread+0x2ce/0x3e0
  kthread+0xe5/0x120
  ret_from_fork+0x3d/0x60
  ret_from_fork_asm+0x1a/0x30


watchdog: BUG: soft lockup - CPU#0 stuck for 11s! [fio:19820]
CPU: 0 PID: 19820 Comm: fio Kdump: loaded Tainted: G             L 
6.10.0-rc3-enbprftw #12
RIP: 0010:native_queued_spin_lock_slowpath+0x2b8/0x300
Call Trace:
  <IRQ>
  </IRQ>
  <TASK>
  _raw_spin_lock+0x2d/0x40
  clear_shadow_entry+0x3d/0x100
  mapping_try_invalidate+0x11b/0x1e0
  invalidate_mapping_pages+0x14/0x20
  invalidate_bdev+0x40/0x50
  blkdev_common_ioctl+0x5f7/0xa90
  blkdev_ioctl+0x10d/0x270
  __x64_sys_ioctl+0x99/0xd0
  x64_sys_call+0x1219/0x20d0
  do_syscall_64+0x51/0x120
  entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7fc92fc3ec6b
  </TASK>

The above one (clear_shadow_entry) has since been fixed by Yu Zhao and 
fix is in mm tree.

We had seen a couple of scenarios with zone lock contention from page 
free and slab free code paths, as reported here: 
https://lore.kernel.org/linux-mm/b68e43d4-91f2-4481-80a9-d166c0a43584@xxxxxxx/

Would you have any insights on these?

Regards,
Bharata.