Commit 9de5ab8a authored by Manfred Spraul's avatar Manfred Spraul Committed by Linus Torvalds

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.comSigned-off-by: default avatarManfred 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 27d7be18
......@@ -21,7 +21,7 @@ struct sem_array {
struct list_head list_id; /* undo requests on this array */
int sem_nsems; /* no. of semaphores in array */
int complex_count; /* pending complex operations */
bool complex_mode; /* no parallel simple ops */
unsigned int use_global_lock;/* >0: global lock required */
};
#ifdef CONFIG_SYSVIPC
......
......@@ -158,23 +158,43 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
#define SEMMSL_FAST 256 /* 512 bytes on stack */
#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);
......
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