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Wed, 05 Nov 2025 21:35:17 -0800 (PST) Message-ID: Date: Wed, 5 Nov 2025 21:35:14 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v2] mm/memcontrol: Flush stats when write stat file To: Leon Huang Fu , shakeel.butt@linux.dev Cc: akpm@linux-foundation.org, cgroups@vger.kernel.org, corbet@lwn.net, hannes@cmpxchg.org, jack@suse.cz, joel.granados@kernel.org, kyle.meyer@hpe.com, lance.yang@linux.dev, laoar.shao@gmail.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mclapinski@google.com, mhocko@kernel.org, muchun.song@linux.dev, roman.gushchin@linux.dev, yosry.ahmed@linux.dev References: <6kh6hle2xp75hrtikasequ7qvfyginz7pyttltx6pkli26iir5@oqjmglatjg22> <20251106033045.41607-1-leon.huangfu@shopee.com> Content-Language: en-US From: JP Kobryn In-Reply-To: <20251106033045.41607-1-leon.huangfu@shopee.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/5/25 7:30 PM, Leon Huang Fu wrote: > On Thu, Nov 6, 2025 at 9:19 AM Shakeel Butt wrote: >> >> +Yosry, JP >> >> On Wed, Nov 05, 2025 at 03:49:16PM +0800, Leon Huang Fu wrote: >>> On high-core count systems, memory cgroup statistics can become stale >>> due to per-CPU caching and deferred aggregation. Monitoring tools and >>> management applications sometimes need guaranteed up-to-date statistics >>> at specific points in time to make accurate decisions. >> >> Can you explain a bit more on your environment where you are seeing >> stale stats? More specifically, how often the management applications >> are reading the memcg stats and if these applications are reading memcg >> stats for each nodes of the cgroup tree. >> >> We force flush all the memcg stats at root level every 2 seconds but it >> seems like that is not enough for your case. I am fine with an explicit >> way for users to flush the memcg stats. In that way only users who want >> to has to pay for the flush cost. >> > > Thanks for the feedback. I encountered this issue while running the LTP > memcontrol02 test case [1] on a 256-core server with the 6.6.y kernel on XFS, > where it consistently failed. > > I was aware that Yosry had improved the memory statistics refresh mechanism > in "mm: memcg: subtree stats flushing and thresholds" [2], so I attempted to > backport that patchset to 6.6.y [3]. However, even on the 6.15.0-061500-generic > kernel with those improvements, the test still fails intermittently on XFS. > I'm not against this change, but it might be worth testing on a 6.16 or later kernel. There were some changes that could affect your measurements. One is that flushing was isolated to individual subsystems [0] and the other is that updating stats became lockless [1]. [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/kernel/cgroup/rstat.c?h=v6.18-rc4&id=5da3bfa029d6809e192d112f39fca4dbe0137aaf [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/kernel/cgroup/rstat.c?h=v6.18-rc4&id=36df6e3dbd7e7b074e55fec080012184e2fa3a46