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From: "Shakeel Butt" Message-ID: <93b7c3f206f158e7387cbb5f0bf5845b59b93053@linux.dev> TLS-Required: No Subject: Re: [linus:master] [mm] 01b9da291c: stress-ng.switch.ops_per_sec 67.7% regression To: "Qi Zheng" , "kernel test robot" Cc: 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" , "=?utf-8?B?TWljaGFsIEtvdXRuw70=?=" , "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 In-Reply-To: <46e9f5cf-34cb-466d-a53a-5778768af4d9@linux.dev> References: <202605121641.b6a60cb0-lkp@intel.com> <0e1b8994-944d-4dda-8966-3cd43661796d@linux.dev> <46e9f5cf-34cb-466d-a53a-5778768af4d9@linux.dev> X-Migadu-Flow: FLOW_OUT May 14, 2026 at 12:46 AM, "Qi Zheng" wrote: >=20 >=20On 5/13/26 10:27 PM, Shakeel Butt wrote: >=20 >=20>=20 >=20> On Wed, May 13, 2026 at 06:49:45AM -0700, Shakeel Butt wrote: > >=20 >=20> >=20 >=20> > On Wed, May 13, 2026 at 10:10:34AM +0800, Qi Zheng wrote: > > >=20 >=20> On 5/13/26 12:03 AM, Shakeel Butt wrote: > > On Tue, May 12, 2026 at 08:56:52PM +0800, kernel test robot wrote: > >=20 >=20> Hello, > >=20 >=20> kernel test robot noticed a 67.7% regression of stress-ng.switch.o= ps_per_sec on: > >=20 >=20> commit: 01b9da291c4969354807b52956f4aae1f41b4924 ("mm: memcontrol:= convert objcg to be per-memcg per-node type") > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git mast= er > >=20 >=20> This is most probably due to shuffling of struct mem_cgroup and st= ruct > > mem_cgroup_per_node members. > >=20 >=20> Another possibility is that after objcg was split into per-node, t= he > > slab accounting fast path is still designed assuming only one curren= t > > objcg per CPU: > >=20 >=20> struct obj_stock_pcp { > > struct obj_cgroup *cached_objcg; > > }; > >=20 >=20> So it's may cause the following thrashing: > >=20 >=20> CPU stock cached =3D memcg/node0 objcg > > free object tagged =3D memcg/node1 objcg > > =3D> __refill_obj_stock --> objcg mismatch > > =3D> drain_obj_stock() > > =3D> cache switches to node1 objcg > >=20 >=20> next local allocation tagged =3D node0 objcg > > =3D> mismatch again > > =3D> drain_obj_stock() > >=20 >=20> >=20 >=20> > Actually I think this is the issue, we have ping pong threads run= ning on > > > different nodes where though theu are in same cgroup but their cur= rent->obcg is > > > for local node and thus this ping pong is thrashing the per-cpu ob= jcg stock. > > >=20 >=20> > 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. > > >=20 >=20> Something like the following: > > From d756abe831a905d6fe32bad9a984fc619dafb7e0 Mon Sep 17 00:00:00 20= 01 > > From: Shakeel Butt > > Date: Wed, 13 May 2026 07:24:55 -0700 > > Subject: [PATCH] mm/memcontrol: skip obj_stock drain when refilled o= bjcg > > 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_cgro= up *objcg, > > unsigned int nr_bytes, > > bool allow_uncharge) > > { > > + struct obj_cgroup *cached; > > unsigned int nr_pages =3D 0; > > > if (!stock) { > > @@ -3327,7 +3328,18 @@ static void __refill_obj_stock(struct obj_cgr= oup *objcg, > > goto out; > > } > > > - if (READ_ONCE(stock->cached_objcg) !=3D objcg) { /* reset if nec= essary */ > > + cached =3D READ_ONCE(stock->cached_objcg); > > + if (cached !=3D objcg && > > + (!cached || obj_cgroup_memcg(cached) !=3D obj_cgroup_memcg(objcg))= ) { > > drain_obj_stock(stock); > > obj_cgroup_get(objcg); > > stock->nr_bytes =3D atomic_read(&objcg->nr_charged_bytes) > >=20 >=20This 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 sc= enario where the pair pingpong threads get their current->objcg from different n= odes. I will try that. >=20 >=20Hi kernel-test-robot, could you help check if the patch above fixes t= he > issue on your end? >=20 In=20the meantime, Oliver, can you please help in testing this patch?