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

[姜智伟 <qq282012236@gmail.com>]

On Tue, Apr 22, 2025 at 9:35 PM Jens Axboe <axboe@kernel.dk> 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.