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

[Shakeel Butt <shakeel.butt@linux.dev>]

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 <shakeel.butt@linux.dev>
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 <shakeel.butt@linux.dev>
---
 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)
-- 
2.53.0-Meta