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 (file)
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -434,6 +434,69 @@ static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
                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 @@ static void update_queue(struct sem_array *sma, int semnum)
 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 @@ again:
 
                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 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
        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;
        }