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

Jens Axboe
2025-04-22T14:29:15Z

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.

--
Jens Axboe