[PATCH 5.12 110/178] mm: relocate write_protect_seq in struct mm_struct

Greg Kroah-Hartman
2021-06-21T16:48:00Z

From: Feng Tang <feng.tang@intel.com>

[ Upstream commit 2e3025434a6ba090c85871a1d4080ff784109e1f ]

0day robot reported a 9.2% regression for will-it-scale mmap1 test
case[1], caused by commit 57efa1fe5957 ("mm/gup: prevent gup_fast from
racing with COW during fork").

Further debug shows the regression is due to that commit changes the
offset of hot fields 'mmap_lock' inside structure 'mm_struct', thus some
cache alignment changes.

>From the perf data, the contention for 'mmap_lock' is very severe and
takes around 95% cpu cycles, and it is a rw_semaphore

struct rw_semaphore {
atomic_long_t count; /* 8 bytes */
atomic_long_t owner; /* 8 bytes */
struct optimistic_spin_queue osq; /* spinner MCS lock */
...

Before commit 57efa1fe5957 adds the 'write_protect_seq', it happens to
have a very optimal cache alignment layout, as Linus explained:

"and before the addition of the 'write_protect_seq' field, the
mmap_sem was at offset 120 in 'struct mm_struct'.

Which meant that count and owner were in two different cachelines,
and then when you have contention and spend time in
rwsem_down_write_slowpath(), this is probably *exactly* the kind
of layout you want.

Because first the rwsem_write_trylock() will do a cmpxchg on the
first cacheline (for the optimistic fast-path), and then in the
case of contention, rwsem_down_write_slowpath() will just access
the second cacheline.

Which is probably just optimal for a load that spends a lot of
time contended - new waiters touch that first cacheline, and then
they queue themselves up on the second cacheline."

After the commit, the rw_semaphore is at offset 128, which means the
'count' and 'owner' fields are now in the same cacheline, and causes
more cache bouncing.

Currently there are 3 "#ifdef CONFIG_XXX" before 'mmap_lock' which will
affect its offset:

CONFIG_MMU
CONFIG_MEMBARRIER
CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES

The layout above is on 64 bits system with 0day's default kernel config
(similar to RHEL-8.3's config), in which all these 3 options are 'y'.
And the layout can vary with different kernel configs.

Relayouting a structure is usually a double-edged sword, as sometimes it
can helps one case, but hurt other cases. For this case, one solution
is, as the newly added 'write_protect_seq' is a 4 bytes long seqcount_t
(when CONFIG_DEBUG_LOCK_ALLOC=n), placing it into an existing 4 bytes
hole in 'mm_struct' will not change other fields' alignment, while
restoring the regression.

Link: https://lore.kernel.org/lkml/20210525031636.GB7744@xsang-OptiPlex-9020/ [1]
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/mm_types.h | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5aacc1c10a45..8f0fb62e8975 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -445,13 +445,6 @@ struct mm_struct {
*/
atomic_t has_pinned;

- /**
- * @write_protect_seq: Locked when any thread is write
- * protecting pages mapped by this mm to enforce a later COW,
- * for instance during page table copying for fork().
- */
- seqcount_t write_protect_seq;
-
#ifdef CONFIG_MMU
atomic_long_t pgtables_bytes; /* PTE page table pages */
#endif
@@ -460,6 +453,18 @@ struct mm_struct {
spinlock_t page_table_lock; /* Protects page tables and some
* counters
*/
+ /*
+ * 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;

struct list_head mmlist; /* List of maybe swapped mm's. These
@@ -480,7 +485,15 @@ struct mm_struct {
unsigned long stack_vm; /* VM_STACK */
unsigned long def_flags;

+ /**
+ * @write_protect_seq: Locked when any thread is write
+ * protecting pages mapped by this mm to enforce a later COW,
+ * for instance during page table copying for fork().
+ */
+ seqcount_t write_protect_seq;
+
spinlock_t arg_lock; /* protect the below fields */
+
unsigned long start_code, end_code, start_data, end_data;
unsigned long start_brk, brk, start_stack;
unsigned long arg_start, arg_end, env_start, env_end;
--
2.30.2