From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753097AbdGSH7o (ORCPT ); Wed, 19 Jul 2017 03:59:44 -0400 Received: from mx2.suse.de ([195.135.220.15]:54075 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751683AbdGSH7n (ORCPT ); Wed, 19 Jul 2017 03:59:43 -0400 Date: Wed, 19 Jul 2017 09:59:40 +0200 From: Jan Kara To: Davidlohr Bueso Cc: akpm@linux-foundation.org, mingo@kernel.org, peterz@infradead.org, jack@suse.cz, torvalds@linux-foundation.org, kirill.shutemov@linux.intel.com, hch@infradead.org, ldufour@linux.vnet.ibm.com, mhocko@suse.com, mgorman@techsingularity.net, linux-kernel@vger.kernel.org, axboe@fb.com, linux-block@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH 17/17] block/cfq: cache rightmost rb_node Message-ID: <20170719075940.GC12546@quack2.suse.cz> References: <20170719014603.19029-1-dave@stgolabs.net> <20170719014603.19029-18-dave@stgolabs.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170719014603.19029-18-dave@stgolabs.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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@fb.com > Cc: linux-block@vger.kernel.org > Signed-off-by: Davidlohr Bueso 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 SUSE Labs, CR