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 Changes since v1: - Optimized the code under Jens Axboe's suggestion to reduce the exposure of IO worker structure. fs/userfaultfd.c | 4 ++++ io_uring/io-wq.c | 35 +++++++++++++++++++++++++++++------ io_uring/io-wq.h | 12 ++++++++++-- 3 files changed, 43 insertions(+), 8 deletions(-) -- 2.34.1
