From: Feng Tang <feng.tang@intel.com>
To: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Jason Gunthorpe <jgg@nvidia.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
kernel test robot <oliver.sang@intel.com>,
John Hubbard <jhubbard@nvidia.com>
Cc: linux-kernel@vger.kernel.org, lkp@lists.01.org,
Feng Tang <feng.tang@intel.com>, Peter Xu <peterx@redhat.com>
Subject: [PATCH] mm: relocate 'write_protect_seq' in struct mm_struct
Date: Fri, 11 Jun 2021 09:54:42 +0800 [thread overview]
Message-ID: <1623376482-92265-1-git-send-email-feng.tang@intel.com> (raw)
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.
[1]. https://lore.kernel.org/lkml/20210525031636.GB7744@xsang-OptiPlex-9020/
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>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
---
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 5aacc1c..cba6022 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;
--
2.7.4
next reply other threads:[~2021-06-11 1:54 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-11 1:54 Feng Tang [this message]
2021-06-11 17:09 ` [PATCH] mm: relocate 'write_protect_seq' in struct mm_struct Jason Gunthorpe
2021-06-14 3:27 ` Feng Tang
2021-06-15 1:11 ` Feng Tang
2021-06-15 18:52 ` Peter Xu
2021-06-16 1:51 ` Feng Tang
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