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 a4fff7d7abe5..4eb38918da8f 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -448,13 +448,6 @@ 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 @@ -463,6 +456,18 @@ 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 @@ -483,7 +488,15 @@ 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;