Re: [PATCH v6] memcg: Don't wait writeback completion when release memcg.

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 18 Sep 2025 05:29:59 +0800 Julian Sun <sunjunchao@bytedance.com> wrote:

> Recently, we encountered the following hung task:
> 
> INFO: task kworker/4:1:1334558 blocked for more than 1720 seconds.
> [Wed Jul 30 17:47:45 2025] Workqueue: cgroup_destroy css_free_rwork_fn
>
> ...
> 
> The direct cause is that memcg spends a long time waiting for dirty page
> writeback of foreign memcgs during release.
> 
> The root causes are:
>     a. The wb may have multiple writeback tasks, containing millions
>        of dirty pages, as shown below:
> 
> >>> for work in list_for_each_entry("struct wb_writeback_work", \
> 				    wb.work_list.address_of_(), "list"):
> ...     print(work.nr_pages, work.reason, hex(work))
> ...
> 900628  WB_REASON_FOREIGN_FLUSH 0xffff969e8d956b40
> 1116521 WB_REASON_FOREIGN_FLUSH 0xffff9698332a9540
>
> ...
>

I don't think it's particularly harmful that a dedicated worker thread
has to wait for a long time in this fashion.  It doesn't have anything
else to do (does it?) and a blocked kernel thread is cheap.

> 3085016 WB_REASON_FOREIGN_FLUSH 0xffff969f0455e000
> 3035712 WB_REASON_FOREIGN_FLUSH 0xffff969d9bbf4b00
> 
>     b. The writeback might severely throttled by wbt, with a speed
>        possibly less than 100kb/s, leading to a very long writeback time.
>
> ...
> 
>  include/linux/memcontrol.h | 14 +++++++++-
>  mm/memcontrol.c            | 57 ++++++++++++++++++++++++++++++++------
>  2 files changed, 62 insertions(+), 9 deletions(-)

Seems we're adding a bunch of tricky code to fix a non-problem which
the hung-task detector undesirably reports.

Would a better fix be to simply suppress the warning?

I don't think we presently have a touch_hung_task_detector() (do we?)
but it's presumably pretty simple.  And maybe
touch_softlockup_watchdog) should be taught to call that
touch_hung_task_dectector().

Another approach might be to set some flag in the task_struct
instructing the hung task detector to ignore this thread.