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: axboe@xxxxxx > Cc: linux-block@xxxxxxxxxxxxxxx > Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx> 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 <jack@xxxxxxxx> SUSE Labs, CR
