[PATCH] mm/memcontrol: avoid false sharing between vmstats and events

[Usama Arif <usama.arif@linux.dev>]

Moving v1 userspace eventfd handling into memcontrol-v1.c shrank
struct vmpressure from 112 to 24 bytes when CONFIG_MEMCG_V1 is disabled.
This moved memory_events_local[MEMCG_SWAP_FAIL] and the hot
vmstats_percpu pointer onto the same cacheline.

The stress-ng mremap stressor exercises MADV_PAGEOUT with swap
disabled, generating about 20 million MEMCG_SWAP_FAIL updates per
60-second run on a 176-CPU test system.  Those writes bounce the line
while memcg statistics paths load vmstats_percpu.

Move cgwb_list into the existing alignment gap and cacheline-align
vmstats_percpu.  This separates the pointer from the event counters
without increasing the size of struct mem_cgroup in the tested
configuration.

The blamed commit reduced median mremap throughput by 4.38% on the
test system with one socket. The patched kernel brings the performance
to within 0.5% of the parent which is within the observed boot-to-boot
spread (up to 1.2%).

Fixes: ea928e9e18da ("mm/vmpressure: move v1 userspace eventfd code into memcontrol-v1.c")
Reported-by: kernel test robot <yi1.lai@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202608131743.c6a7dda4-lkp@intel.com
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 include/linux/memcontrol.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e78bc98ab229b..215e2e87f42b2 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -268,10 +268,15 @@ struct mem_cgroup {
 #endif
 	int kmemcg_id;
 
-	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
-
 #ifdef CONFIG_CGROUP_WRITEBACK
 	struct list_head cgwb_list;
+#endif
+
+	/* Keep the hot per-CPU stats pointer away from memory event counters. */
+	struct memcg_vmstats_percpu __percpu *vmstats_percpu
+		____cacheline_aligned_in_smp;
+
+#ifdef CONFIG_CGROUP_WRITEBACK
 	struct wb_domain cgwb_domain;
 	struct memcg_cgwb_frn cgwb_frn[MEMCG_CGWB_FRN_CNT];
 #endif
-- 
2.53.0-Meta