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

[Lance Yang <lance.yang@linux.dev>]

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.

> 
> 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.

> 
> 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(-)
>