path: root/include
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authorFeng Tang <>2021-06-11 09:54:42 +0800
committerLinus Torvalds <>2021-06-12 13:28:50 -0700
commit2e3025434a6ba090c85871a1d4080ff784109e1f (patch)
treea001326a73141aca19aa9266492f3be55cc8e4d1 /include
parent43cb5d49a99b3ecd9fef9826899aac948c3048da (diff)
mm: relocate 'write_protect_seq' in struct mm_struct
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: [1] Reported-by: kernel test robot <> Signed-off-by: Feng Tang <> Reviewed-by: John Hubbard <> Reviewed-by: Jason Gunthorpe <> Cc: Peter Xu <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'include')
1 files 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;
atomic_long_t pgtables_bytes; /* PTE page table pages */
@@ -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;