Re: [External] Re: [PATCH 0/3] memcg, writeback: Don't wait writeback completion

[Giorgi Tchankvetadze <giorgitchankvetadze1997@gmail.com>]

`memory.stat:writeback` already gives you the exact real-time count of 
pages still waiting to finish write-back for that cgroup, and it is 
updated atomically in the hot path (`set_page_writeback()` / 
`end_page_writeback()`). Reading it is just an `atomic_long_read()` (or 
per-cpu equivalent), so the extra CPU cost of exposing it is essentially 
zero. I was thinking that extra additional info would help us

Thanks,
Giorgi

On 8/21/2025 6:37 AM, Julian Sun wrote:
> Hi, thanks for your review.
> 
> On Wed, Aug 20, 2025 at 8:17 PM Giorgi Tchankvetadze
> <giorgitchankvetadze1997@gmail.com> wrote:
>>
>> Could we add wb_pending_pages to memory.events?
>> Very cheap and useful.
>> A single atomic counter is already kept internally; exposing it is one
>> line in memcontrol.c plus one line in the ABI doc.
> 
> Not sure what do you mean by wb_pending_pages? Another counter besides
> existing MEMCG_LOW MEMCG_HIGH MEMCG_MAX, etc.? And AFAIK there's no
> pending pages in this patch set. Could you give more details?
> 
> Thanks,
>>
>>
>> On 8/20/2025 3:19 PM, Julian Sun wrote:
>>> This patch series aims to eliminate task hangs in mem_cgroup_css_free()
>>> caused by calling wb_wait_for_completion().
>>> This is because there may be a large number of writeback tasks in the
>>> foreign memcg, involving millions of pages, and the situation is
>>> exacerbated by WBT rate limiting—potentially leading to task hangs
>>> lasting several hours.
>>>
>>> Patch 1 is preparatory work and involves no functional changes.
>>> Patch 2 implements the automatic release of wb_completion.
>>> Patch 3 removes wb_wait_for_completion() from mem_cgroup_css_free().
>>>
>>>
>>> Julian Sun (3):
>>>     writeback: Rename wb_writeback_work->auto_free to free_work.
>>>     writeback: Add wb_writeback_work->free_done
>>>     memcg: Don't wait writeback completion when release memcg.
>>>
>>>    fs/fs-writeback.c                | 22 ++++++++++++++--------
>>>    include/linux/backing-dev-defs.h |  6 ++++++
>>>    include/linux/memcontrol.h       |  2 +-
>>>    mm/memcontrol.c                  | 29 ++++++++++++++++++++---------
>>>    4 files changed, 41 insertions(+), 18 deletions(-)
>>>
>>