On Fri, Jun 26, 2009 at 05:54:10PM +0200, Jarek Poplawski wrote:

On Fri, Jun 26, 2009 at 08:30:10AM -0700, Paul E. McKenney wrote:

quoted

On Fri, Jun 26, 2009 at 05:10:52PM +0200, Jarek Poplawski wrote:

quoted

On Fri, Jun 26, 2009 at 03:52:55PM +0200, Robert Olsson wrote:

quoted

Jarek Poplawski writes:

 Thanks, 

 Should be worth testing so we synchronize_rcu instead of doing call_rcu's
 

Alas take 2 (nor 1) doesn't compile, so here it is again.

So the idea is to balance memory and latency, so that large changes
(those affecting the root node) get at least one synchronize_rcu(),
while smaller changes just use call_rcu(), correct?  This means that
the amount of memory awaiting an RCU grace period is limited, but
the algorithm avoids per-node synchronize_rcu() overhead.

If I understand the goal correctly, looks good!  (Give or take my
limited understanding of fib_trie and is usage, of course.)

The goal is practically to replace all call_rcu() during
trie_rebalance() with synchronize_rcu() (except some freeing after
ENOMEM). I guess currently (<= 2.6.30) call_rcu() can free this
memory after trie_rebalance() has finished, that's why there were
problems with enabled preemption. So this patch tries to do/force
this a bit earlier - at least before the top/largest node is
rebalanced.

On the other hand, we could probably stay with call_rcu() plus only
one synchronize_rcu() before the top node's resize() if you think it's
enough here?

Thanks,
Jarek P.

quoted

							Thanx, Paul

quoted

Thanks,
Jarek P.

--- (take 3 - for testing)

 net/ipv4/fib_trie.c |   30 ++++++++++++++++++++++++------
 1 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 012cf5a..1a4c4b7 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c

@@ -366,6 +366,17 @@ static void __tnode_vfree(struct work_struct *arg)
 	vfree(tn);
 }

+static void __tnode_free(struct tnode *tn)
+{
+	size_t size = sizeof(struct tnode) +
+		      (sizeof(struct node *) << tn->bits);
+
+	if (size <= PAGE_SIZE)
+		kfree(tn);
+	else
+		vfree(tn);
+}
+
 static void __tnode_free_rcu(struct rcu_head *head)
 {
 	struct tnode *tn = container_of(head, struct tnode, rcu);

@@ -402,7 +413,7 @@ static void tnode_free_flush(void)
 	while ((tn = tnode_free_head)) {
 		tnode_free_head = tn->tnode_free;
 		tn->tnode_free = NULL;
-		tnode_free(tn);
+		__tnode_free(tn);
 	}
 }

@@ -1021,18 +1032,25 @@ static void trie_rebalance(struct trie *t, struct tnode *tn)
 				      (struct node *)tn, wasfull);

 		tp = node_parent((struct node *) tn);
-		tnode_free_flush();
 		if (!tp)
 			break;
 		tn = tp;
 	}

+	if (tnode_free_head) {
+		synchronize_rcu();
+		tnode_free_flush();
+	}
+
 	/* Handle last (top) tnode */
-	if (IS_TNODE(tn))
+	if (IS_TNODE(tn)) {
 		tn = (struct tnode *)resize(t, (struct tnode *)tn);
-
-	rcu_assign_pointer(t->trie, (struct node *)tn);
-	tnode_free_flush();
+		rcu_assign_pointer(t->trie, (struct node *)tn);
+		synchronize_rcu();
+		tnode_free_flush();
+	} else {
+		rcu_assign_pointer(t->trie, (struct node *)tn);
+	}

 	return;
 }
--

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