On 4/22/25 8:10 AM, ??? wrote: > On Tue, Apr 22, 2025 at 9:35?PM Jens Axboe <axboe@xxxxxxxxx> wrote: >> >> 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 > Sorry,The issue occurs very infrequently, and I can't manually > reproduce it. It's not very elegant, but for corner cases, it seems > necessary to make some compromises. I'm going to see if I can create one. Not sure I fully understand the issue yet, but I'd be surprised if there isn't a more appropriate and elegant solution rather than exposing the io-wq guts and having userfaultfd manipulate them. That really should not be necessary. -- Jens Axboe
