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

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Aug 22, 2025 at 02:00:10AM +0800, Julian Sun wrote:
...
> Do you mean logic like this?
> 
>     for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++)
>         wb_wait_for_completion(&memcg->cgwb_frn[i].done);
>     kfree(memcg);
> 
> But there still exist task hang issues as long as
> wb_wait_for_completion() exists.

Ah, right. I was just thinking about the workqueue being stalled. The
problem is that the wait itself is too long.

> I think the scope of impact of the current changes should be
> manageable. I have checked all the other places where wb_queue_work()
> is called, and their free_done values are all 0, and I also tested
> this patch with the reproducer in [1] with kasan and kmemleak enabled.
> The test result looks fine, so this should not have a significant
> impact.
> What do you think?

My source of reluctance is that it's a peculiar situation where flushing of
a cgroup takes that long due to hard throttling and the self-freeing
mechanism isn't the prettiest thing. Do you think you can do the same thing
through custom waitq wakeup function?

Thanks.

-- 
tejun