[PATCH] rcu: add a prefetch() in rcu_do_batch()

On some workloads, (for example when lot of close() syscalls are done), RCU
qlen can be quite large, and RCU heads are no longer in cpu cache when
rcu_do_batch() is called.

This patch adds a prefetch() in rcu_do_batch() to give CPU a hint to bring
back cache lines containing 'struct rcu_head's.

Most list manipulations macros include prefetch(), but not open coded ones
(at least with current C compilers :) )

I got a nice speedup on a trivial benchmark (3.48 us per iteration instead
of 3.95 us on a 1.6 GHz Pentium-M)

while (1) { pipe(p); close(fd[0]); close(fd[1]);}
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

kernel/rcupdate.c

@@ -235,12 +235,14 @@ static void rcu_do_batch(struct rcu_data *rdp)
 
 	list = rdp->donelist;
 	while (list) {
-		next = rdp->donelist = list->next;
+		next = list->next;
+		prefetch(next);
 		list->func(list);
 		list = next;
 		if (++count >= rdp->blimit)
 			break;
 	}
+	rdp->donelist = list;
 
 	local_irq_disable();
 	rdp->qlen -= count;