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 11:50 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 4/22/25 8:29 AM, Jens Axboe wrote:
> > On 4/22/25 8:18 AM, ??? wrote:
> >> On Tue, Apr 22, 2025 at 10:13?PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>
> >>> On 4/22/25 8:10 AM, ??? wrote:
> >>>> 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.
> >>>
> >>> 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
> >> Thanks.I'm looking forward to your good news.
> >
> > Well, let's hope there is! In any case, your patches could be
> > considerably improved if you did:
> >
> > void set_userfault_flag_for_ioworker(void)
> > {
> >       struct io_worker *worker;
> >       if (!(current->flags & PF_IO_WORKER))
> >               return;
> >       worker = current->worker_private;
> >       set_bit(IO_WORKER_F_FAULT, &worker->flags);
> > }
> >
> > void clear_userfault_flag_for_ioworker(void)
> > {
> >       struct io_worker *worker;
> >       if (!(current->flags & PF_IO_WORKER))
> >               return;
> >       worker = current->worker_private;
> >       clear_bit(IO_WORKER_F_FAULT, &worker->flags);
> > }
> >
> > and then userfaultfd would not need any odd checking, or needing io-wq
> > related structures public. That'd drastically cut down on the size of
> > them, and make it a bit more palatable.
>
> Forgot to ask, what kernel are you running on?
>
> --
> Jens Axboe
Thanks Jens It is linux-image-6.8.0-1026-gcp