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    Subject[PATCH 17/17] block/cfq: cache rightmost rb_node
    Date
    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@fb.com
    Cc: linux-block@vger.kernel.org
    Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
    ---
    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
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