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

[Alexander Duyck]

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@xxxxxxxxxx>
> ---
>  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.