Re: [PATCH 0/3] Suppress undesirable hung task warnings.

[Julian Sun <sunjunchao@bytedance.com>]

On 9/22/25 7:38 PM, Lance Yang wrote:

Hi, Lance

Thanks for your review and comments.

> Hi Julian
> 
> Thanks for the patch series!
> 
> On 2025/9/22 17:41, Julian Sun wrote:
>> As suggested by Andrew Morton in [1], we need a general mechanism
>> that allows the hung task detector to ignore unnecessary hung
> 
> Yep, I understand the goal is to suppress what can be a benign hung task
> warning during memcg teardown.
> 
>> tasks. This patch set implements this functionality.
>>
>> Patch 1 introduces a PF_DONT_HUNG flag. The hung task detector will
>> ignores all tasks that have the PF_DONT_HUNG flag set.
> 
> However, I'm concerned that the PF_DONT_HUNG flag is a bit too powerful
> and might mask real, underlying hangs.

The flag takes effect only when wait_event_no_hung() or 
wb_wait_for_completion_no_hung() is called, and its effect is limited to 
a single wait event, without affecting subsequent wait events. So AFAICS 
it will not mask real hang warnings.>
>>
>> Patch 2 introduces wait_event_no_hung() and 
>> wb_wait_for_completion_no_hung(),
>> which enable the hung task detector to ignore hung tasks caused by these
>> wait events.
> 
> Instead of making the detector ignore the task, what if we just change
> the waiting mechanism? Looking at wb_wait_for_completion(), we could
> introduce a new helper that internally uses wait_event_timeout() in a
> loop.
> 
> Something simple like this:
> 
> void wb_wait_for_completion_no_hung(struct wb_completion *done)
> {
>          atomic_dec(&done->cnt);
>          while (atomic_read(&done->cnt))
>                  wait_event_timeout(*done->waitq, !atomic_read(&done- 
>  >cnt), timeout);
> }
> 
> The periodic wake-ups from wait_event_timeout() would naturally prevent
> the detector from complaining about slow but eventually completing 
> writeback.

Yeah, this could definitely eliminate the hung task warning complained here.
However what I aim to provide is a general mechanism for waiting on 
events. Of course, we could use code similar to the following, but this 
would introduce additional overhead from waking tasks and multiple 
operations on wq_head—something I don't want to introduce.

+#define wait_event_no_hung(wq_head, condition) \
+do {                   \
+       while (!(condition))    \
+               wait_event_timeout(wq_head, condition, timeout); \
+}

But I can try this approach or do not introcude wait_event_no_hung() if 
you want.>
>>
>> Patch 3 uses wb_wait_for_completion_no_hung() in the final phase of memcg
>> teardown to eliminate the hung task warning.
>>
>> Julian Sun (3):
>>    sched: Introduce a new flag PF_DONT_HUNG.
>>    writeback: Introduce wb_wait_for_completion_no_hung().
>>    memcg: Don't trigger hung task when memcg is releasing.
>>
>>   fs/fs-writeback.c           | 15 +++++++++++++++
>>   include/linux/backing-dev.h |  1 +
>>   include/linux/sched.h       | 12 +++++++++++-
>>   include/linux/wait.h        | 15 +++++++++++++++
>>   kernel/hung_task.c          |  6 ++++++
>>   mm/memcontrol.c             |  2 +-
>>   6 files changed, 49 insertions(+), 2 deletions(-)
>>
> 

Thanks,
-- 
Julian Sun <sunjunchao@bytedance.com>