ipc/sem.c: replace shared sem_otime with per-semaphore value

sem_otime contains the time of the last semaphore operation that
completed successfully. Every operation updates this value, thus access
from multiple cpus can cause thrashing.

Therefore the patch replaces the variable with a per-semaphore variable.
The per-array sem_otime is only calculated when required.

No performance improvement on a single-socket i3 - only important for
larger systems.

Signed-off-by: Manfred Spraul <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/sem.h

···
 struct sem_array {
 	struct kern_ipc_perm	____cacheline_aligned_in_smp
 				sem_perm;	/* permissions .. see ipc.h */
-	time_t			sem_otime;	/* last semop time */
 	time_t			sem_ctime;	/* last change time */
 	struct sem		*sem_base;	/* ptr to first semaphore in array */
 	struct list_head	pending_alter;	/* pending operations */

ipc/sem.c

···
 					/* that alter the semaphore */
 	struct list_head pending_const; /* pending single-sop operations */
 					/* that do not alter the semaphore*/
+	time_t	sem_otime;	/* candidate for sem_otime */
 } ____cacheline_aligned_in_smp;
 
 /* One queue for each sleeping process in the system. */
···
 			}
 		}
 	}
-	if (otime)
-		sma->sem_otime = get_seconds();
+	if (otime) {
+		if (sops == NULL) {
+			sma->sem_base[0].sem_otime = get_seconds();
+		} else {
+			sma->sem_base[sops[0].sem_num].sem_otime =
+								get_seconds();
+		}
+	}
 }
 
 
···
 	}
 }
 
+static time_t get_semotime(struct sem_array *sma)
+{
+	int i;
+	time_t res;
+
+	res = sma->sem_base[0].sem_otime;
+	for (i = 1; i < sma->sem_nsems; i++) {
+		time_t to = sma->sem_base[i].sem_otime;
+
+		if (to > res)
+			res = to;
+	}
+	return res;
+}
+
 static int semctl_nolock(struct ipc_namespace *ns, int semid,
 			 int cmd, int version, void __user *p)
 {
···
 			goto out_unlock;
 
 		kernel_to_ipc64_perm(&sma->sem_perm, &tbuf.sem_perm);
-		tbuf.sem_otime  = sma->sem_otime;
-		tbuf.sem_ctime  = sma->sem_ctime;
-		tbuf.sem_nsems  = sma->sem_nsems;
+		tbuf.sem_otime = get_semotime(sma);
+		tbuf.sem_ctime = sma->sem_ctime;
+		tbuf.sem_nsems = sma->sem_nsems;
 		rcu_read_unlock();
 		if (copy_semid_to_user(p, &tbuf, version))
 			return -EFAULT;
···
 {
 	struct user_namespace *user_ns = seq_user_ns(s);
 	struct sem_array *sma = it;
+	time_t sem_otime;
+
+	sem_otime = get_semotime(sma);
 
 	return seq_printf(s,
 			  "%10d %10d  %4o %10u %5u %5u %5u %5u %10lu %10lu\n",
···
 			  from_kgid_munged(user_ns, sma->sem_perm.gid),
 			  from_kuid_munged(user_ns, sma->sem_perm.cuid),
 			  from_kgid_munged(user_ns, sma->sem_perm.cgid),
-			  sma->sem_otime,
+			  sem_otime,
 			  sma->sem_ctime);
 }
 #endif