Re: [PATCH 0/2] Fix 100% CPU usage issue in IOU worker threads

Jens Axboe
2025-04-22T13:35:00Z

On 4/22/25 4:45 AM, Zhiwei Jiang wrote:
> In the Firecracker VM scenario, sporadically encountered threads with
> the UN state in the following call stack:
> [<0>] io_wq_put_and_exit+0xa1/0x210
> [<0>] io_uring_clean_tctx+0x8e/0xd0
> [<0>] io_uring_cancel_generic+0x19f/0x370
> [<0>] __io_uring_cancel+0x14/0x20
> [<0>] do_exit+0x17f/0x510
> [<0>] do_group_exit+0x35/0x90
> [<0>] get_signal+0x963/0x970
> [<0>] arch_do_signal_or_restart+0x39/0x120
> [<0>] syscall_exit_to_user_mode+0x206/0x260
> [<0>] do_syscall_64+0x8d/0x170
> [<0>] entry_SYSCALL_64_after_hwframe+0x78/0x80
> The cause is a large number of IOU kernel threads saturating the CPU
> and not exiting. When the issue occurs, CPU usage 100% and can only
> be resolved by rebooting. Each thread's appears as follows:
> iou-wrk-44588 [kernel.kallsyms] [k] ret_from_fork_asm
> iou-wrk-44588 [kernel.kallsyms] [k] ret_from_fork
> iou-wrk-44588 [kernel.kallsyms] [k] io_wq_worker
> iou-wrk-44588 [kernel.kallsyms] [k] io_worker_handle_work
> iou-wrk-44588 [kernel.kallsyms] [k] io_wq_submit_work
> iou-wrk-44588 [kernel.kallsyms] [k] io_issue_sqe
> iou-wrk-44588 [kernel.kallsyms] [k] io_write
> iou-wrk-44588 [kernel.kallsyms] [k] blkdev_write_iter
> iou-wrk-44588 [kernel.kallsyms] [k] iomap_file_buffered_write
> iou-wrk-44588 [kernel.kallsyms] [k] iomap_write_iter
> iou-wrk-44588 [kernel.kallsyms] [k] fault_in_iov_iter_readable
> iou-wrk-44588 [kernel.kallsyms] [k] fault_in_readable
> iou-wrk-44588 [kernel.kallsyms] [k] asm_exc_page_fault
> iou-wrk-44588 [kernel.kallsyms] [k] exc_page_fault
> iou-wrk-44588 [kernel.kallsyms] [k] do_user_addr_fault
> iou-wrk-44588 [kernel.kallsyms] [k] handle_mm_fault
> iou-wrk-44588 [kernel.kallsyms] [k] hugetlb_fault
> iou-wrk-44588 [kernel.kallsyms] [k] hugetlb_no_page
> iou-wrk-44588 [kernel.kallsyms] [k] hugetlb_handle_userfault
> iou-wrk-44588 [kernel.kallsyms] [k] handle_userfault
> iou-wrk-44588 [kernel.kallsyms] [k] schedule
> iou-wrk-44588 [kernel.kallsyms] [k] __schedule
> iou-wrk-44588 [kernel.kallsyms] [k] __raw_spin_unlock_irq
> iou-wrk-44588 [kernel.kallsyms] [k] io_wq_worker_sleeping
>
> I tracked the address that triggered the fault and the related function
> graph, as well as the wake-up side of the user fault, and discovered this
> : In the IOU worker, when fault in a user space page, this space is
> associated with a userfault but does not sleep. This is because during
> scheduling, the judgment in the IOU worker context leads to early return.
> Meanwhile, the listener on the userfaultfd user side never performs a COPY
> to respond, causing the page table entry to remain empty. However, due to
> the early return, it does not sleep and wait to be awakened as in a normal
> user fault, thus continuously faulting at the same address,so CPU loop.
> Therefore, I believe it is necessary to specifically handle user faults by
> setting a new flag to allow schedule function to continue in such cases,
> make sure the thread to sleep.
>
> Patch 1 io_uring: Add new functions to handle user fault scenarios
> Patch 2 userfaultfd: Set the corresponding flag in IOU worker context
>
> fs/userfaultfd.c | 7 ++++++
> io_uring/io-wq.c | 57 +++++++++++++++---------------------------------
> io_uring/io-wq.h | 45 ++++++++++++++++++++++++++++++++++++--
> 3 files changed, 68 insertions(+), 41 deletions(-)

Do you have a test case for this? I don't think the proposed solution is
very elegant, userfaultfd should not need to know about thread workers.
I'll ponder this a bit...

--
Jens Axboe