Re: [PATCH 4/4] memcg: multi objcg charge support

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

On Wed, May 20, 2026 at 06:35:30PM +0900, Harry Yoo wrote:
> 
> 
> On 5/20/26 2:31 PM, Shakeel Butt wrote:
> > Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
> > per-node type") split a memcg's single obj_cgroup into one per NUMA
> > node so that reparenting LRU folios can take per-node lru locks. As a
> > side effect, the per-CPU obj_stock_pcp -- which caches exactly one
> > cached_objcg -- thrashes on workloads where threads of the same memcg
> > run on different NUMA nodes. The kernel test robot reported a 67.7%
> > regression on stress-ng.switch.ops_per_sec from this pattern.
> > 
> > Mirror the multi-slot pattern already used by memcg_stock_pcp: turn
> > nr_bytes and cached_objcg into NR_OBJ_STOCK-element arrays, scan all
> > slots on consume/refill/account, prefer empty slots when inserting,
> > and evict a random slot only when full. With multiple slots a CPU can
> > hold the per-node objcg variants of one memcg plus a few siblings
> > without ever forcing a drain.
> > 
> > A single int8_t index records which slot the cached slab stats belong
> > to; the stats are flushed on slot or pgdat change. With NR_OBJ_STOCK
> > = 5 the layout (verified with pahole) is:
> > 
> >    offset 0  : lock(1) + index(1) + node_id(2) + slab stats(4) = 8B
> >    offset 8  : nr_bytes[5]                                     = 10B
> >    offset 18 : padding                                         = 6B
> >    offset 24 : cached[5]                                       = 40B
> >    offset 64 : (line 2) work_struct + flags (cold)
> > 
> > so consume_obj_stock, refill_obj_stock and the slab account path each
> > touch exactly one 64-byte cache line on non-debug 64-bit builds.
> > 
> > Reported-by: kernel test robot <oliver.sang@intel.com>
> > Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com
> > Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > Tested-by: kernel test robot <oliver.sang@intel.com>
> > ---
> > @@ -3350,19 +3405,45 @@ static void __refill_obj_stock(struct obj_cgroup *objcg,
> >   		goto out;
> >   	}
> > -	stock_nr_bytes = stock->nr_bytes;
> > -	if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */
> > -		drain_obj_stock(stock);
> > +	for (i = 0; i < NR_OBJ_STOCK; ++i) {
> > +		struct obj_cgroup *cached = READ_ONCE(stock->cached[i]);
> > +
> > +		if (!cached) {
> > +			if (empty_slot == -1)
> > +				empty_slot = i;
> > +			continue;
> > +		}
> > +		if (cached == objcg) {
> > +			slot = i;
> > +			break;
> > +		}
> > +	}
> > +
> > +	if (slot == -1) {
> > +		slot = empty_slot;
> > +		if (slot == -1) {
> > +			slot = get_random_u32_below(NR_OBJ_STOCK);
> 
> It would break kmalloc_nolock() because _get_random_bytes() uses a spinlock.
> perhaps prandom_u32_state() should be sufficient in this case.
> 
> Is there a reason why it uses random eviction, unlike multi-memcg percpu
> charge cache?

Oh I didn't know and actually we are already using get_random_u32_below() in
refill_stock(). So, it need fixing as well. That would be a separate patch.

I will explore prandom_u32_state().