On Tue 18-07-17 18:46:03, Davidlohr Bueso wrote:
> For the same reasons we already cache the leftmost pointer,
> apply the same optimization for rb_last() calls. Users must
> explicitly do this as rb_root_cached only deals with the
> smallest node.
>
> Cc: ax...@fb.com
> Cc: linux-bl...@vger.kernel.org
> Signed-off-by: Davidlohr Bueso <dbu...@suse.de>
Hum, as I'm reading the code, here we have about 1:1 ratio of cached
lookups and tree insert / delete where we have to maintain the cached
value. Is the optimization worth it in such case?
Honza
> ---
> This is part of the rbtree internal caching series:
> https://lwn.net/Articles/726809/
>
> block/cfq-iosched.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 92c31683a2bb..57ec45fd4590 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -94,11 +94,13 @@ struct cfq_ttime {
> */
> struct cfq_rb_root {
> struct rb_root_cached rb;
> + struct rb_node *rb_rightmost;
> unsigned count;
> u64 min_vdisktime;
> struct cfq_ttime ttime;
> };
> #define CFQ_RB_ROOT (struct cfq_rb_root) { .rb = RB_ROOT_CACHED, \
> + .rb_rightmost = NULL, \
> .ttime = {.last_end_request = ktime_get_ns(),},}
>
> /*
> @@ -1183,6 +1185,9 @@ static struct cfq_group *cfq_rb_first_group(struct
> cfq_rb_root *root)
>
> static void cfq_rb_erase(struct rb_node *n, struct cfq_rb_root *root)
> {
> + if (root->rb_rightmost == n)
> + root->rb_rightmost = rb_next(n);
> +
> rb_erase_cached(n, &root->rb);
> RB_CLEAR_NODE(n);
>
> @@ -1239,20 +1244,24 @@ __cfq_group_service_tree_add(struct cfq_rb_root *st,
> struct cfq_group *cfqg)
> struct rb_node *parent = NULL;
> struct cfq_group *__cfqg;
> s64 key = cfqg_key(st, cfqg);
> - bool leftmost = true;
> + bool leftmost = true, rightmost = true;
>
> while (*node != NULL) {
> parent = *node;
> __cfqg = rb_entry_cfqg(parent);
>
> - if (key < cfqg_key(st, __cfqg))
> + if (key < cfqg_key(st, __cfqg)) {
> node = &parent->rb_left;
> - else {
> + rightmost = false;
> + } else {
> node = &parent->rb_right;
> leftmost = false;
> }
> }
>
> + if (rightmost)
> + st->rb_rightmost = &cfqg->rb_node;
> +
> rb_link_node(&cfqg->rb_node, parent, node);
> rb_insert_color_cached(&cfqg->rb_node, &st->rb, leftmost);
> }
> @@ -1355,7 +1364,7 @@ cfq_group_notify_queue_add(struct cfq_data *cfqd,
> struct cfq_group *cfqg)
> * so that groups get lesser vtime based on their weights, so that
> * if group does not loose all if it was not continuously backlogged.
> */
> - n = rb_last(&st->rb.rb_root);
> + n = st->rb_rightmost;
> if (n) {
> __cfqg = rb_entry_cfqg(n);
> cfqg->vdisktime = __cfqg->vdisktime +
> @@ -2204,7 +2213,7 @@ static void cfq_service_tree_add(struct cfq_data *cfqd,
> struct cfq_queue *cfqq,
> st = st_for(cfqq->cfqg, cfqq_class(cfqq), cfqq_type(cfqq));
> if (cfq_class_idle(cfqq)) {
> rb_key = CFQ_IDLE_DELAY;
> - parent = rb_last(&st->rb.rb_root);
> + parent = st->rb_rightmost;
> if (parent && parent != &cfqq->rb_node) {
> __cfqq = rb_entry(parent, struct cfq_queue, rb_node);
> rb_key += __cfqq->rb_key;
> --
> 2.12.0
>
--
Jan Kara <j...@suse.com>
SUSE Labs, CR
