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From: Shakeel Butt To: Harry Yoo Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Alexandre Ghiti , Joshua Hahn , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot Subject: Re: [PATCH 4/4] memcg: multi objcg charge support Message-ID: References: <20260520053123.2709959-1-shakeel.butt@linux.dev> <20260520053123.2709959-5-shakeel.butt@linux.dev> <4e20f643-6983-4b6e-b12d-c6c4eb20ae0c@kernel.org> 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: <4e20f643-6983-4b6e-b12d-c6c4eb20ae0c@kernel.org> X-Migadu-Flow: FLOW_OUT 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 > > 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 > > Tested-by: kernel test robot > > --- > > @@ -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().