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

[Bharata B Rao <bharata@amd.com>]

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@amd.com/

Would you have any insights on these?

Regards,
Bharata.