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... 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.c... 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.