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

[Jens Axboe <axboe@kernel.dk>]

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