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From: Shakeel Butt To: Qi Zheng Cc: kernel test robot , oe-lkp@lists.linux.dev, lkp@intel.com, linux-kernel@vger.kernel.org, Andrew Morton , David Carlier , Allen Pais , Axel Rasmussen , Baoquan He , Chengming Zhou , Chen Ridong , David Hildenbrand , Hamza Mahfooz , Harry Yoo , Hugh Dickins , Imran Khan , Johannes Weiner , Kamalesh Babulal , Lance Yang , Liam Howlett , Lorenzo Stoakes , Michal Hocko , Michal =?utf-8?Q?Koutn=C3=BD?= , Mike Rapoport , Muchun Song , Muchun Song , Nhat Pham , Roman Gushchin , Suren Baghdasaryan , Usama Arif , Vlastimil Babka , Wei Xu , Yosry Ahmed , Yuanchu Xie , Zi Yan , Usama Arif , cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: [linus:master] [mm] 01b9da291c: stress-ng.switch.ops_per_sec 67.7% regression Message-ID: References: <202605121641.b6a60cb0-lkp@intel.com> <0e1b8994-944d-4dda-8966-3cd43661796d@linux.dev> <46e9f5cf-34cb-466d-a53a-5778768af4d9@linux.dev> <93b7c3f206f158e7387cbb5f0bf5845b59b93053@linux.dev> <19693be6-7132-446e-b3fc-b7e9f56e5949@linux.dev> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <19693be6-7132-446e-b3fc-b7e9f56e5949@linux.dev> X-Migadu-Flow: FLOW_OUT On Fri, May 15, 2026 at 03:37:22PM +0800, Qi Zheng wrote: > Hi Shakeel, > > On 5/14/26 9:40 PM, Shakeel Butt wrote: > > May 14, 2026 at 12:46 AM, "Qi Zheng" wrote: > > > > > > > > > > On 5/13/26 10:27 PM, Shakeel Butt wrote: > > > > > > > > > > > On Wed, May 13, 2026 at 06:49:45AM -0700, Shakeel Butt wrote: > > > > > > > > > > > > > > On Wed, May 13, 2026 at 10:10:34AM +0800, Qi Zheng wrote: > > > > > > > > > On 5/13/26 12:03 AM, Shakeel Butt wrote: > > > > On Tue, May 12, 2026 at 08:56:52PM +0800, kernel test robot wrote: > > > > > > > > Hello, > > > > > > > > kernel test robot noticed a 67.7% regression of stress-ng.switch.ops_per_sec on: > > > > > > > > commit: 01b9da291c4969354807b52956f4aae1f41b4924 ("mm: memcontrol: convert objcg to be per-memcg per-node type") > > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master > > > > > > > > This is most probably due to shuffling of struct mem_cgroup and struct > > > > mem_cgroup_per_node members. > > > > > > > > Another possibility is that after objcg was split into per-node, the > > > > slab accounting fast path is still designed assuming only one current > > > > objcg per CPU: > > > > > > > > struct obj_stock_pcp { > > > > struct obj_cgroup *cached_objcg; > > > > }; > > > > > > > > So it's may cause the following thrashing: > > > > > > > > CPU stock cached = memcg/node0 objcg > > > > free object tagged = memcg/node1 objcg > > > > => __refill_obj_stock --> objcg mismatch > > > > => drain_obj_stock() > > > > => cache switches to node1 objcg > > > > > > > > next local allocation tagged = node0 objcg > > > > => mismatch again > > > > => drain_obj_stock() > > > > > > > > > > > > > > Actually I think this is the issue, we have ping pong threads running on > > > > > different nodes where though theu are in same cgroup but their current->obcg is > > > > > for local node and thus this ping pong is thrashing the per-cpu objcg stock. > > > > > > > > > > The easier fix would be to compare objcg->memcg instead of just objcg during > > > > > draining and caching. In addition we can add support for multiple objcg per-cpu > > > > > stock caching. > > > > > > > > > Something like the following: > > > > From d756abe831a905d6fe32bad9a984fc619dafb7e0 Mon Sep 17 00:00:00 2001 > > > > From: Shakeel Butt > > > > Date: Wed, 13 May 2026 07:24:55 -0700 > > > > Subject: [PATCH] mm/memcontrol: skip obj_stock drain when refilled objcg > > > > shares memcg > > > > Signed-off-by: Shakeel Butt > > > > --- > > > > mm/memcontrol.c | 14 +++++++++++++- > > > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > > index d978e18b9b2d..01ed7a8e18ac 100644 > > > > --- a/mm/memcontrol.c > > > > +++ b/mm/memcontrol.c > > > > @@ -3318,6 +3318,7 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, > > > > unsigned int nr_bytes, > > > > bool allow_uncharge) > > > > { > > > > + struct obj_cgroup *cached; > > > > unsigned int nr_pages = 0; > > > > > if (!stock) { > > > > @@ -3327,7 +3328,18 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, > > > > goto out; > > > > } > > > > > - if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */ > > > > + cached = READ_ONCE(stock->cached_objcg); > > > > + if (cached != objcg && > > > > + (!cached || obj_cgroup_memcg(cached) != obj_cgroup_memcg(objcg))) { > > > > drain_obj_stock(stock); > > > > obj_cgroup_get(objcg); > > > > stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes) > > > > > > > This change looks like it should be able to fix the ping-pong issue, but > > > I stiil haven't reproduced the performance regression locally. I'll > > > continue testing it. > > > > Same here, couldn't reproduce locally. It seems like we had to craft a scenario > > where the pair pingpong threads get their current->objcg from different nodes. > > I will try that. > > I still haven't been able to reproduce the LKP results locally, but I > used an AI bot to generate a pingpong test case (pasted at the end) and > automatically ran the test on a physical machine. The results are as > follows: > > parent: 8285917d6f > bad: 01b9da291c > fix: 01b9da291c + stock patch > > | kernel | mq_ops/sec mean | vs parent | drain_obj_stock / round | > |--------|-----------------|-----------|-------------------------| > | parent | 9.743M | baseline | ~0 | > | bad | 7.821M | -19.73% | ~11.16M | > | fix | 9.274M | -4.81% | ~0 | > > Probing the drain_obj_stock() calls confirms that the fix restores the > frequency to the parent's baseline. > > And it seems that besides __refill_obj_stock(), we should also modify > __consume_obj_stock()? > Thanks a lot Qi. I will send the formal patch and will add your Debugged-by if you don't mind.