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 <manfred@colorfullife.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 31 insertions, 6 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index 6291257ee049..51352e1bfff9 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -99,6 +99,7 @@ struct sem {
                                         /* 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. */
@@ -911,8 +912,14 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
                         }
                 }
         }
-        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();
+                }
+        }
 }
 
 
@@ -1058,6 +1065,21 @@ static unsigned long copy_semid_to_user(void __user *buf, struct semid64_ds *in,
         }
 }
 
+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)
 {
@@ -1131,9 +1153,9 @@ static int semctl_nolock(struct ipc_namespace *ns, int semid,
                         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;
@@ -2025,6 +2047,9 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
 {
         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",
@@ -2036,7 +2061,7 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
                           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