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From: Shakeel Butt To: Roman Gushchin Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Muchun Song , Yosry Ahmed , Waiman Long , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team Subject: Re: [PATCH] memcg: no refill for offlined objcg Message-ID: References: <20250410210535.1005312-1-shakeel.butt@linux.dev> <7ia47c3r1sb0.fsf@castle.c.googlers.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7ia47c3r1sb0.fsf@castle.c.googlers.com> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: ABE0D40009 X-Stat-Signature: sj4rptiq1phkhcyxcjjsn3d88wa6t31r X-HE-Tag: 1744656296-178081 X-HE-Meta: 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 h0xIgqry l6oEOUdEstv5jNOS6bOaatpd/a5RBqXKf+mYO/CRWJiFU6CENbvlGMYQTo4JMJq0Y9q5dTk4sUZPeEaS4Y1YfjgF6o28Hi2/343ppncgHF3bDR+Ljqf7HRA8MsKg/lLg4RgED9vnFlZo/ga1n0m1WIQCG2dUIZqXp9oWrr4DAnvPmLkPcVxvtmQ6s+w== 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: On Thu, Apr 10, 2025 at 11:59:47PM +0000, Roman Gushchin wrote: > Shakeel Butt writes: > > > In our fleet, we are observing refill_obj_stock() spending a lot of cpu > > in obj_cgroup_get() and on further inspection it seems like the given > > objcg is offlined and the kernel has to take the slow path i.e. atomic > > operations for objcg reference counting. > > > > Other than expensive atomic operations, refilling stock of an offlined > > objcg is a waster as there will not be new allocations for the offlined > > objcg. In addition, refilling triggers flush of the previous objcg which > > might be used in future. So, let's just avoid refilling the stock with > > the offlined objcg. > > Hm, but on the other side if there are multiple uncharges in a row, > refilling obj stocks might be still cheaper? > Thanks for the review. I looked at the fleet data again and what you are suspecting i.e. multiple objects of same objcg getting freed closeby is possible. I think I should be optimizing/batching at the upper layer. I will be looking at rcu freeing side which is the most obvious batching opportunity. > In general I think that switching to atomic css refcnt on memcg > offlining is a mistake - it makes memory reclaim generally more > expensive. We can simple delay it until the approximate refcnt > number reaches some low value, e.g. 100 objects. This is a good idea. For the memcg, I think after Muchun's LRU reparenting, we should not have zombie refcnt slowdown but for objcg this idea might help. Anyways this is for later.