ipc/sem: add hysteresis
sysv sem has two lock modes: One with per-semaphore locks, one lock mode
with a single global lock for the whole array. When switching from the
per-semaphore locks to the global lock, all per-semaphore locks must be
scanned for ongoing operations.
The patch adds a hysteresis for switching from the global lock to the
per semaphore locks. This reduces how often the per-semaphore locks
must be scanned.
Compared to the initial patch, this is a simplified solution: Setting
USE_GLOBAL_LOCK_HYSTERESIS to 1 restores the current behavior.
In theory, a workload with exactly 10 simple sops and then one complex
op now scales a bit worse, but this is pure theory: If there is
concurrency, the it won't be exactly 10:1:10:1:10:1:... If there is no
concurrency, then there is no need for scalability.
Link: http://lkml.kernel.org/r/1476851896-3590-3-git-send-email-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: <1vier1@web.de>
Cc: kernel test robot <xiaolong.ye@intel.com>
Cc: <felixh@informatik.uni-bremen.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/ipc/sem.c b/ipc/sem.c
index fe5db1e..e468cd1 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -159,22 +159,42 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
#define SEMOPM_FAST 64 /* ~ 372 bytes on stack */
/*
+ * Switching from the mode suitable for simple ops
+ * to the mode for complex ops is costly. Therefore:
+ * use some hysteresis
+ */
+#define USE_GLOBAL_LOCK_HYSTERESIS 10
+
+/*
* Locking:
* a) global sem_lock() for read/write
* sem_undo.id_next,
* sem_array.complex_count,
- * sem_array.complex_mode
* sem_array.pending{_alter,_const},
* sem_array.sem_undo
*
* b) global or semaphore sem_lock() for read/write:
* sem_array.sem_base[i].pending_{const,alter}:
- * sem_array.complex_mode (for read)
*
* c) special:
* sem_undo_list.list_proc:
* * undo_list->lock for write
* * rcu for read
+ * use_global_lock:
+ * * global sem_lock() for write
+ * * either local or global sem_lock() for read.
+ *
+ * Memory ordering:
+ * Most ordering is enforced by using spin_lock() and spin_unlock().
+ * The special case is use_global_lock:
+ * Setting it from non-zero to 0 is a RELEASE, this is ensured by
+ * using smp_store_release().
+ * Testing if it is non-zero is an ACQUIRE, this is ensured by using
+ * smp_load_acquire().
+ * Setting it from 0 to non-zero must be ordered with regards to
+ * this smp_load_acquire(), this is guaranteed because the smp_load_acquire()
+ * is inside a spin_lock() and after a write from 0 to non-zero a
+ * spin_lock()+spin_unlock() is done.
*/
#define sc_semmsl sem_ctls[0]
@@ -273,12 +293,16 @@ static void complexmode_enter(struct sem_array *sma)
int i;
struct sem *sem;
- if (sma->complex_mode) {
- /* We are already in complex_mode. Nothing to do */
+ if (sma->use_global_lock > 0) {
+ /*
+ * We are already in global lock mode.
+ * Nothing to do, just reset the
+ * counter until we return to simple mode.
+ */
+ sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
return;
}
-
- sma->complex_mode = true;
+ sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
for (i = 0; i < sma->sem_nsems; i++) {
sem = sma->sem_base + i;
@@ -299,13 +323,17 @@ static void complexmode_tryleave(struct sem_array *sma)
*/
return;
}
- /*
- * Immediately after setting complex_mode to false,
- * a simple op can start. Thus: all memory writes
- * performed by the current operation must be visible
- * before we set complex_mode to false.
- */
- smp_store_release(&sma->complex_mode, false);
+ if (sma->use_global_lock == 1) {
+ /*
+ * Immediately after setting use_global_lock to 0,
+ * a simple op can start. Thus: all memory writes
+ * performed by the current operation must be visible
+ * before we set use_global_lock to 0.
+ */
+ smp_store_release(&sma->use_global_lock, 0);
+ } else {
+ sma->use_global_lock--;
+ }
}
#define SEM_GLOBAL_LOCK (-1)
@@ -335,22 +363,23 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
* Optimized locking is possible if no complex operation
* is either enqueued or processed right now.
*
- * Both facts are tracked by complex_mode.
+ * Both facts are tracked by use_global_mode.
*/
sem = sma->sem_base + sops->sem_num;
/*
- * Initial check for complex_mode. Just an optimization,
+ * Initial check for use_global_lock. Just an optimization,
* no locking, no memory barrier.
*/
- if (!sma->complex_mode) {
+ if (!sma->use_global_lock) {
/*
* It appears that no complex operation is around.
* Acquire the per-semaphore lock.
*/
spin_lock(&sem->lock);
- if (!smp_load_acquire(&sma->complex_mode)) {
+ /* pairs with smp_store_release() */
+ if (!smp_load_acquire(&sma->use_global_lock)) {
/* fast path successful! */
return sops->sem_num;
}
@@ -360,19 +389,26 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
/* slow path: acquire the full lock */
ipc_lock_object(&sma->sem_perm);
- if (sma->complex_count == 0) {
- /* False alarm:
- * There is no complex operation, thus we can switch
- * back to the fast path.
+ if (sma->use_global_lock == 0) {
+ /*
+ * The use_global_lock mode ended while we waited for
+ * sma->sem_perm.lock. Thus we must switch to locking
+ * with sem->lock.
+ * Unlike in the fast path, there is no need to recheck
+ * sma->use_global_lock after we have acquired sem->lock:
+ * We own sma->sem_perm.lock, thus use_global_lock cannot
+ * change.
*/
spin_lock(&sem->lock);
+
ipc_unlock_object(&sma->sem_perm);
return sops->sem_num;
} else {
- /* Not a false alarm, thus complete the sequence for a
- * full lock.
+ /*
+ * Not a false alarm, thus continue to use the global lock
+ * mode. No need for complexmode_enter(), this was done by
+ * the caller that has set use_global_mode to non-zero.
*/
- complexmode_enter(sma);
return SEM_GLOBAL_LOCK;
}
}
@@ -476,7 +512,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
}
sma->complex_count = 0;
- sma->complex_mode = true; /* dropped by sem_unlock below */
+ sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
INIT_LIST_HEAD(&sma->pending_alter);
INIT_LIST_HEAD(&sma->pending_const);
INIT_LIST_HEAD(&sma->list_id);
