ipc/sem.c: optimize update_queue() for bulk wakeup calls
The following series of patches tries to fix the spinlock contention
reported by Chris Mason - his benchmark exposes problems of the current
code:
- In the worst case, the algorithm used by update_queue() is O(N^2).
Bulk wake-up calls can enter this worst case. The patch series fix
that.
Note that the benchmark app doesn't expose the problem, it just should
be fixed: Real world apps might do the wake-ups in another order than
perfect FIFO.
- The part of the code that runs within the semaphore array spinlock is
significantly larger than necessary.
The patch series fixes that. This change is responsible for the main
improvement.
- The cacheline with the spinlock is also used for a variable that is
read in the hot path (sem_base) and for a variable that is unnecessarily
written to multiple times (sem_otime). The last step of the series
cacheline-aligns the spinlock.
This patch:
The SysV semaphore code allows to perform multiple operations on all
semaphores in the array as atomic operations. After a modification,
update_queue() checks which of the waiting tasks can complete.
The algorithm that is used to identify the tasks is O(N^2) in the worst
case. For some cases, it is simple to avoid the O(N^2).
The patch adds a detection logic for some cases, especially for the case
of an array where all sleeping tasks are single sembuf operations and a
multi-sembuf operation is used to wake up multiple tasks.
A big database application uses that approach.
The patch fixes wakeup due to semctl(,,SETALL,) - the initial version of
the patch breaks that.
[akpm@linux-foundation.org: make do_smart_update() static]
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Zach Brown <zach.brown@oracle.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Nick Piggin <npiggin@suse.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 dbef95b..81a9c74 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -434,6 +434,69 @@
sma->complex_count--;
}
+/** check_restart(sma, q)
+ * @sma: semaphore array
+ * @q: the operation that just completed
+ *
+ * update_queue is O(N^2) when it restarts scanning the whole queue of
+ * waiting operations. Therefore this function checks if the restart is
+ * really necessary. It is called after a previously waiting operation
+ * was completed.
+ */
+static int check_restart(struct sem_array *sma, struct sem_queue *q)
+{
+ struct sem *curr;
+ struct sem_queue *h;
+
+ /* if the operation didn't modify the array, then no restart */
+ if (q->alter == 0)
+ return 0;
+
+ /* pending complex operations are too difficult to analyse */
+ if (sma->complex_count)
+ return 1;
+
+ /* we were a sleeping complex operation. Too difficult */
+ if (q->nsops > 1)
+ return 1;
+
+ curr = sma->sem_base + q->sops[0].sem_num;
+
+ /* No-one waits on this queue */
+ if (list_empty(&curr->sem_pending))
+ return 0;
+
+ /* the new semaphore value */
+ if (curr->semval) {
+ /* It is impossible that someone waits for the new value:
+ * - q is a previously sleeping simple operation that
+ * altered the array. It must be a decrement, because
+ * simple increments never sleep.
+ * - The value is not 0, thus wait-for-zero won't proceed.
+ * - If there are older (higher priority) decrements
+ * in the queue, then they have observed the original
+ * semval value and couldn't proceed. The operation
+ * decremented to value - thus they won't proceed either.
+ */
+ BUG_ON(q->sops[0].sem_op >= 0);
+ return 0;
+ }
+ /*
+ * semval is 0. Check if there are wait-for-zero semops.
+ * They must be the first entries in the per-semaphore simple queue
+ */
+ h = list_first_entry(&curr->sem_pending, struct sem_queue, simple_list);
+ BUG_ON(h->nsops != 1);
+ BUG_ON(h->sops[0].sem_num != q->sops[0].sem_num);
+
+ /* Yes, there is a wait-for-zero semop. Restart */
+ if (h->sops[0].sem_op == 0)
+ return 1;
+
+ /* Again - no-one is waiting for the new value. */
+ return 0;
+}
+
/**
* update_queue(sma, semnum): Look for tasks that can be completed.
@@ -469,7 +532,7 @@
again:
walk = pending_list->next;
while (walk != pending_list) {
- int error, alter;
+ int error, restart;
q = (struct sem_queue *)((char *)walk - offset);
walk = walk->next;
@@ -494,22 +557,43 @@
unlink_queue(sma, q);
- /*
- * The next operation that must be checked depends on the type
- * of the completed operation:
- * - if the operation modified the array, then restart from the
- * head of the queue and check for threads that might be
- * waiting for the new semaphore values.
- * - if the operation didn't modify the array, then just
- * continue.
- */
- alter = q->alter;
+ if (error)
+ restart = 0;
+ else
+ restart = check_restart(sma, q);
+
wake_up_sem_queue(q, error);
- if (alter && !error)
+ if (restart)
goto again;
}
}
+/** do_smart_update(sma, sops, nsops): Optimized update_queue
+ * @sma: semaphore array
+ * @sops: operations that were performed
+ * @nsops: number of operations
+ *
+ * do_smart_update() does the required called to update_queue, based on the
+ * actual changes that were performed on the semaphore array.
+ */
+static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsops)
+{
+ int i;
+
+ if (sma->complex_count || sops == NULL) {
+ update_queue(sma, -1);
+ return;
+ }
+
+ for (i = 0; i < nsops; i++) {
+ if (sops[i].sem_op > 0 ||
+ (sops[i].sem_op < 0 &&
+ sma->sem_base[sops[i].sem_num].semval == 0))
+ update_queue(sma, sops[i].sem_num);
+ }
+}
+
+
/* The following counts are associated to each semaphore:
* semncnt number of tasks waiting on semval being nonzero
* semzcnt number of tasks waiting on semval being zero
@@ -1225,7 +1309,7 @@
error = try_atomic_semop (sma, sops, nsops, un, task_tgid_vnr(current));
if (error <= 0) {
if (alter && error == 0)
- update_queue(sma, (nsops == 1) ? sops[0].sem_num : -1);
+ do_smart_update(sma, sops, nsops);
goto out_unlock_free;
}