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
