Re: [RFC 3/4] net/fib: Check budget before should_{inflate,halve}()

Alexander Duyck
2019-04-01T18:20:29Z

On Tue, 2019-03-26 at 15:30 +0000, Dmitry Safonov wrote:
> Those functions are compute-costly, if we're out of budget - better
> omit additional computations.
>
> Signed-off-by: Dmitry Safonov <dima@arista.com>
> ---
> net/ipv4/fib_trie.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
> index d90cf9dfd443..2ce2739e7693 100644
> --- a/net/ipv4/fib_trie.c
> +++ b/net/ipv4/fib_trie.c
> @@ -868,7 +868,7 @@ static struct key_vector *resize(struct trie *t, struct key_vector *tn,
> /* Double as long as the resulting node has a number of
> * nonempty nodes that are above the threshold.
> */
> - while (should_inflate(tp, tn) && *budget) {
> + while (*budget && should_inflate(tp, tn)) {
> tp = inflate(t, tn, budget);
> if (!tp) {
> #ifdef CONFIG_IP_FIB_TRIE_STATS
> @@ -894,7 +894,7 @@ static struct key_vector *resize(struct trie *t, struct key_vector *tn,
> /* Halve as long as the number of empty children in this
> * node is above threshold.
> */
> - while (should_halve(tp, tn) && *budget) {
> + while (*budget && should_halve(tp, tn)) {
> tp = halve(t, tn, budget);
> if (!tp) {
> #ifdef CONFIG_IP_FIB_TRIE_STATS

Based on my comments in the other patches I would say this is a bad
idea.

Really the budget should allow at least 1 pass through the trie to
attempt to either inflate or deflate the node and all children. This
logic is optimizing for the case where *budget is 0 and that should not
occur that often.