linux-kernel - Re: [PATCH 17/17] block/cfq: cache rightmost rb

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Date:   Wed, 19 Jul 2017 09:59:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     akpm@...ux-foundation.org, mingo@...nel.org, peterz@...radead.org,
        jack@...e.cz, torvalds@...ux-foundation.org,
        kirill.shutemov@...ux.intel.com, hch@...radead.org,
        ldufour@...ux.vnet.ibm.com, mhocko@...e.com,
        mgorman@...hsingularity.net, linux-kernel@...r.kernel.org,
        axboe@...com, linux-block@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 17/17] block/cfq: cache rightmost rb_node

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@...com
> Cc: linux-block@...r.kernel.org
> Signed-off-by: Davidlohr Bueso <dbueso@...e.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 <jack@...e.com>
SUSE Labs, CR