mm: move mm_count into its own cache line

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>

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 306a3d1..de10fc7 100644 (file)
--- 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.