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Fri, 22 Aug 2025 02:29:22 -0700 (PDT) Message-ID: Date: Fri, 22 Aug 2025 13:29:18 +0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [External] Re: [PATCH 0/3] memcg, writeback: Don't wait writeback completion To: Julian Sun Cc: linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, axboe@kernel.dk, tj@kernel.org References: <20250820111940.4105766-1-sunjunchao@bytedance.com> <24119aa3-f6ef-4467-80a0-475989e19625@gmail.com> Content-Language: en-US From: Giorgi Tchankvetadze In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: AFD4880011 X-Stat-Signature: xkeqx6e8aqr5ck5i9tkosxhgq9qct4my X-Rspam-User: X-HE-Tag: 1755854964-640768 X-HE-Meta: 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 gV5UnEE3 fo2Lr/SfoQkB4Hnx6R2euQkBflB/hN8f7N9YBUEe1vCiXCvRktcp/a32mAd5hyAaXs/AAT5xjDScPomRB/+VXtC+7Vcid5RG8gOEpEVZ0UBvJ+imgsG+DI2+AZYHeiP+6PhpiQduZgda95aKFDL6jqS2g/pbpnBr15hlRqcqjfiVKr3mXVsRNVVNivdUfKNB2dKl2w0ZGnLGBVmdZ174Lh40cSDiunOvxKuenkU97orJKh8xeD6768jyZlWweysEYcKxxJjVyF+958vW6OMThzfOto6BYEep6Z789zGdfhOFmAk1L745QwDntVsH0NJTtTcHS0MXg8TMUygQZ1Bou5jY1YbkDCeW66gqf6GTHmqOdDFpHSrwFTQco7obI7cQcZAIsv7Tfl8C6q9Ck42ba0YQ2u5R/m3+xgT7a5r+U0E5s6pvulabQ8XLVv2vpsv7x7odNu2+TSH7LzVYKt17QeLeQ0hiXh9roHr67nKBMnnhEe5A= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: `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 > 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(-) >>> >>