Commit 2e302543 authored Jun 11, 2021 by Feng Tang Committed by Linus Torvalds Jun 12, 2021

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 57efa1fe ("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 57efa1fe 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>

parent 43cb5d49

include/linux/mm_types.h

+20 −7

Original line number	Diff line number	Diff line
		@@ -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;