[RCU] adds a prefetch() in rcu_do_batch()

[Eric Dumazet <dada1@cosmosbay.com>]

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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 patches 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: Eric Dumazet <dada1@cosmosbay.com>

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--- linux-2.6.19-rc6/kernel/rcupdate.c	2006-11-16 05:03:40.000000000 +0100
+++ linux-2.6.19-rc6-ed/kernel/rcupdate.c	2006-11-22 15:12:09.000000000 +0100
@@ -235,12 +235,14 @@ static void rcu_do_batch(struct rcu_data
 
 	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;