[PATCH] Avoid taking global tasklist_lock for single threadedprocess at getrusage()

Avoid taking the global tasklist_lock when possible, if a process is single
threaded during getrusage().  Any avoidance of tasklist_lock is good for
NUMA boxes (and possibly for large SMPs).  Thanks to Oleg Nesterov for
review and suggestions.
Signed-off-by: Nippun Goel <nippung@calsoftinc.com>
Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Signed-off-by: Shai Fultheim <shai@scalex86.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sys.c

@@ -1677,9 +1677,6 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim)
  * a lot simpler!  (Which we're not doing right now because we're not
  * measuring them yet).
  *
- * This expects to be called with tasklist_lock read-locked or better,
- * and the siglock not locked.  It may momentarily take the siglock.
- *
  * When sampling multiple threads for RUSAGE_SELF, under SMP we might have
  * races with threads incrementing their own counters.  But since word
  * reads are atomic, we either get new values or old values and we don't
@@ -1687,6 +1684,25 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim)
  * the c* fields from p->signal from races with exit.c updating those
  * fields when reaping, so a sample either gets all the additions of a
  * given child after it's reaped, or none so this sample is before reaping.
+ *
+ * tasklist_lock locking optimisation:
+ * If we are current and single threaded, we do not need to take the tasklist
+ * lock or the siglock.  No one else can take our signal_struct away,
+ * no one else can reap the children to update signal->c* counters, and
+ * no one else can race with the signal-> fields.
+ * If we do not take the tasklist_lock, the signal-> fields could be read
+ * out of order while another thread was just exiting. So we place a
+ * read memory barrier when we avoid the lock.  On the writer side,
+ * write memory barrier is implied in  __exit_signal as __exit_signal releases
+ * the siglock spinlock after updating the signal-> fields.
+ *
+ * We don't really need the siglock when we access the non c* fields
+ * of the signal_struct (for RUSAGE_SELF) even in multithreaded
+ * case, since we take the tasklist lock for read and the non c* signal->
+ * fields are updated only in __exit_signal, which is called with
+ * tasklist_lock taken for write, hence these two threads cannot execute
+ * concurrently.
+ *
  */
 
 static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
@@ -1694,13 +1710,23 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
 	struct task_struct *t;
 	unsigned long flags;
 	cputime_t utime, stime;
+	int need_lock = 0;
 
 	memset((char *) r, 0, sizeof *r);
+	utime = stime = cputime_zero;
 
-	if (unlikely(!p->signal))
-		return;
+	if (p != current || !thread_group_empty(p))
+		need_lock = 1;
 
-	utime = stime = cputime_zero;
+	if (need_lock) {
+		read_lock(&tasklist_lock);
+		if (unlikely(!p->signal)) {
+			read_unlock(&tasklist_lock);
+			return;
+		}
+	} else
+		/* See locking comments above */
+		smp_rmb();
 
 	switch (who) {
 		case RUSAGE_BOTH:
@@ -1740,6 +1766,8 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
 			BUG();
 	}
 
+	if (need_lock)
+		read_unlock(&tasklist_lock);
 	cputime_to_timeval(utime, &r->ru_utime);
 	cputime_to_timeval(stime, &r->ru_stime);
 }
@@ -1747,9 +1775,7 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
 int getrusage(struct task_struct *p, int who, struct rusage __user *ru)
 {
 	struct rusage r;
-	read_lock(&tasklist_lock);
 	k_getrusage(p, who, &r);
-	read_unlock(&tasklist_lock);
 	return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0;
 }