[PATCH v2 1/3] mutex: Make more scalable by doing less atomic operations

From: Waiman Long
Date: Mon Apr 15 2013 - 10:39:17 EST

In the __mutex_lock_common() function, an initial entry into
the lock slow path will cause two atomic_xchg instructions to be
issued. Together with the atomic decrement in the fast path, a total
of three atomic read-modify-write instructions will be issued in
rapid succession. This can cause a lot of cache bouncing when many
tasks are trying to acquire the mutex at the same time.

This patch will reduce the number of atomic_xchg instructions used by
checking the counter value first before issuing the instruction. The
atomic_read() function is just a simple memory read. The atomic_xchg()
function, on the other hand, can be up to 2 order of magnitude or even
more in cost when compared with atomic_read(). By using atomic_read()
to check the value first before calling atomic_xchg(), we can avoid a
lot of unnecessary cache coherency traffic. The only downside with this
change is that a task on the slow path will have a tiny bit
less chance of getting the mutex when competing with another task
in the fast path.

The same is true for the atomic_cmpxchg() function in the
mutex-spin-on-owner loop. So an atomic_read() is also performed before
calling atomic_cmpxchg().

The mutex locking and unlocking code for the x86 architecture can allow
any negative number to be used in the mutex count to indicate that some
tasks are waiting for the mutex. I am not so sure if that is the case
for the other architectures. So the default is to avoid atomic_xchg()
if the count has already been set to -1. For x86, the check is modified
to include all negative numbers to cover a larger case.

The following table shows the jobs per minutes (JPM) scalability data
on an 8-node 80-core Westmere box with a 3.7.10 kernel. The numactl
command is used to restrict the running of the high_systime workloads
to 1/2/4/8 nodes with hyperthreading on and off.

+-----------------+-----------+------------+----------+
| Configuration | Mean JPM | Mean JPM | % Change |
| | w/o patch | with patch | |
+-----------------+-----------------------------------+
| | User Range 1100 - 2000 |
+-----------------+-----------------------------------+
| 8 nodes, HT on | 36980 | 148590 | +301.8% |
| 8 nodes, HT off | 42799 | 145011 | +238.8% |
| 4 nodes, HT on | 61318 | 118445 | +51.1% |
| 4 nodes, HT off | 158481 | 158592 | +0.1% |
| 2 nodes, HT on | 180602 | 173967 | -3.7% |
| 2 nodes, HT off | 198409 | 198073 | -0.2% |
| 1 node , HT on | 149042 | 147671 | -0.9% |
| 1 node , HT off | 126036 | 126533 | +0.4% |
+-----------------+-----------------------------------+
| | User Range 200 - 1000 |
+-----------------+-----------------------------------+
| 8 nodes, HT on | 41525 | 122349 | +194.6% |
| 8 nodes, HT off | 49866 | 124032 | +148.7% |
| 4 nodes, HT on | 66409 | 106984 | +61.1% |
| 4 nodes, HT off | 119880 | 130508 | +8.9% |
| 2 nodes, HT on | 138003 | 133948 | -2.9% |
| 2 nodes, HT off | 132792 | 131997 | -0.6% |
| 1 node , HT on | 116593 | 115859 | -0.6% |
| 1 node , HT off | 104499 | 104597 | +0.1% |
+-----------------+------------+-----------+----------+

At low user range 10-100, the JPM differences were within +/-1%. So
they are not that interesting.

AIM7 benchmark run has a pretty large run-to-run variance due to random
nature of the subtests executed. So a difference of less than +-5%
may not be really significant.

This patch improves high_systime workload performance at 4 nodes
and up by maintaining transaction rates without significant drop-off
at high node count. The patch has practically no impact on 1 and 2
nodes system.

The table below shows the percentage time (as reported by perf
record -a -s -g) spent on the __mutex_lock_slowpath() function by
the high_systime workload at 1500 users for 2/4/8-node configurations
with hyperthreading off.

+---------------+-----------------+------------------+---------+
| Configuration | %Time w/o patch | %Time with patch | %Change |
+---------------+-----------------+------------------+---------+
| 8 nodes | 65.34% | 0.69% | -99% |
| 4 nodes | 8.70% | 1.02% | -88% |
| 2 nodes | 0.41% | 0.32% | -22% |
+---------------+-----------------+------------------+---------+

It is obvious that the dramatic performance improvement at 8
nodes was due to the drastic cut in the time spent within the
__mutex_lock_slowpath() function.

The table below show the improvements in other AIM7 workloads (at 8
nodes, hyperthreading off).

+--------------+---------------+----------------+-----------------+
| Workload | mean % change | mean % change | mean % change |
| | 10-100 users | 200-1000 users | 1100-2000 users |
+--------------+---------------+----------------+-----------------+
| alltests | +0.6% | +104.2% | +185.9% |
| five_sec | +1.9% | +0.9% | +0.9% |
| fserver | +1.4% | -7.7% | +5.1% |
| new_fserver | -0.5% | +3.2% | +3.1% |
| shared | +13.1% | +146.1% | +181.5% |
| short | +7.4% | +5.0% | +4.2% |
+--------------+---------------+----------------+-----------------+

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
Reviewed-by: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
---
arch/x86/include/asm/mutex.h | 10 ++++++++++
kernel/mutex.c | 19 ++++++++++++++++---
2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/mutex.h b/arch/x86/include/asm/mutex.h
index 7d3a482..bc2a0b0 100644
--- a/arch/x86/include/asm/mutex.h
+++ b/arch/x86/include/asm/mutex.h
@@ -3,3 +3,13 @@
#else
# include <asm/mutex_64.h>
#endif
+
+#ifndef __ASM_MUTEX_H
+#define __ASM_MUTEX_H
+/*
+ * For the x86 architecture, it allows any negative number (besides -1) in
+ * the mutex count to indicate that some other threads are waiting on the
+ * mutex.
+ */
+#define __ARCH_ALLOW_ANY_NEGATIVE_MUTEX_COUNT 1
+#endif
diff --git a/kernel/mutex.c b/kernel/mutex.c
index 52f2301..e3496df 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -37,6 +37,17 @@
# include <asm/mutex.h>
#endif

+/*
+ * A mutex count of -1 indicates that waiters are sleeping waiting for the
+ * mutex. Some architectures can allow any negative number, not just -1, for
+ * this purpose.
+ */
+#ifdef __ARCH_ALLOW_ANY_NEGATIVE_MUTEX_COUNT
+#define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) >= 0)
+#else
+#define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) != -1)
+#endif
+
void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
@@ -171,7 +182,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
if (owner && !mutex_spin_on_owner(lock, owner))
break;

- if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
+ if ((atomic_read(&lock->count) == 1) &&
+ (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
lock_acquired(&lock->dep_map, ip);
mutex_set_owner(lock);
preempt_enable();
@@ -205,7 +217,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
list_add_tail(&waiter.list, &lock->wait_list);
waiter.task = task;

- if (atomic_xchg(&lock->count, -1) == 1)
+ if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
goto done;

lock_contended(&lock->dep_map, ip);
@@ -220,7 +232,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
* that when we release the lock, we properly wake up the
* other waiters:
*/
- if (atomic_xchg(&lock->count, -1) == 1)
+ if (MUTEX_SHOW_NO_WAITER(lock) &&
+ (atomic_xchg(&lock->count, -1) == 1))
break;

/*
--
1.7.1

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