[PATCH 6.4 182/800] mm: move mm_count into its own cache line

Greg Kroah-Hartman
2023-07-16T20:02:42Z

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

[ Upstream commit c1753fd02a0058ea43cbb31ab26d25be2f6cfe08 ]

The mm_struct mm_count field is frequently updated by mmgrab/mmdrop
performed by context switch. This causes false-sharing for surrounding
mm_struct fields which are read-mostly.

This has been observed on a 2sockets/112core/224cpu Intel Sapphire Rapids
server running hackbench, and by the kernel test robot will-it-scale
testcase.

Move the mm_count field into its own cache line to prevent false-sharing
with other mm_struct fields.

Move mm_count to the first field of mm_struct to minimize the amount of
padding required: rather than adding padding before and after the mm_count
field, padding is only added after mm_count.

Note that I noticed this odd comment in mm_struct:

commit 2e3025434a6b ("mm: relocate 'write_protect_seq' in struct mm_struct")

/*
* With some kernel config, the current mmap_lock's offset
* inside 'mm_struct' is at 0x120, which is very optimal, as
* its two hot fields 'count' and 'owner' sit in 2 different
* cachelines, and when mmap_lock is highly contended, both
* of the 2 fields will be accessed frequently, current layout
* will help to reduce cache bouncing.
*
* So please be careful with adding new fields before
* mmap_lock, which can easily push the 2 fields into one
* cacheline.
*/
struct rw_semaphore mmap_lock;

This comment is rather odd for a few reasons:

- It requires addition/removal of mm_struct fields to carefully consider
field alignment of _other_ fields,
- It expresses the wish to keep an "optimal" alignment for a specific
kernel config.

I suspect that the author of this comment may want to revisit this topic
and perhaps introduce a split-struct approach for struct rw_semaphore,
if the need is to place various fields of this structure in different
cache lines.

Link: https://lkml.kernel.org/r/20230515143536.114960-1-mathieu.desnoyers@efficios.com
Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
Fixes: af7f588d8f73 ("sched: Introduce per-memory-map concurrency ID")
Link: https://lore.kernel.org/lkml/7a0c1db1-103d-d518-ed96-1584a28fbf32@efficios.com
Reported-by: kernel test robot <yujie.liu@intel.com>
Link: https://lore.kernel.org/oe-lkp/202305151017.27581d75-yujie.liu@intel.com
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Olivier Dion <odion@efficios.com>
Cc: <michael.christie@oracle.com>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/mm_types.h | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 306a3d1a0fa65..de10fc797c8e9 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -583,6 +583,21 @@ struct mm_cid {
struct kioctx_table;
struct mm_struct {
struct {
+ /*
+ * Fields which are often written to are placed in a separate
+ * cache line.
+ */
+ struct {
+ /**
+ * @mm_count: The number of references to &struct
+ * mm_struct (@mm_users count as 1).
+ *
+ * Use mmgrab()/mmdrop() to modify. When this drops to
+ * 0, the &struct mm_struct is freed.
+ */
+ atomic_t mm_count;
+ } ____cacheline_aligned_in_smp;
+
struct maple_tree mm_mt;
#ifdef CONFIG_MMU
unsigned long (*get_unmapped_area) (struct file *filp,
@@ -620,14 +635,6 @@ struct mm_struct {
*/
atomic_t mm_users;

- /**
- * @mm_count: The number of references to &struct mm_struct
- * (@mm_users count as 1).
- *
- * Use mmgrab()/mmdrop() to modify. When this drops to 0, the
- * &struct mm_struct is freed.
- */
- atomic_t mm_count;
#ifdef CONFIG_SCHED_MM_CID
/**
* @pcpu_cid: Per-cpu current cid.
--
2.39.2