Re: [linus:master] [mm] 01b9da291c: stress-ng.switch.ops_per_sec 67.7% regression

May 14, 2026 at 12:46 AM, "Qi Zheng" <[email protected] mailto:[email protected]?to=%22Qi%20Zheng%22%20%3Cqi.zheng%40linux.dev%3E > 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 <[email protected]>
> >  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 <[email protected]>
> >  ---
> >  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.

> 
> Hi kernel-test-robot, could you help check if the patch above fixes the
> issue on your end?
> 

In the meantime, Oliver, can you please help in testing this patch?