linux-kernel - [PATCH 14/17] fs/epoll: use faster rb_first

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Date:   Tue, 18 Jul 2017 18:46:00 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     akpm@...ux-foundation.org
Cc:     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, dave@...olabs.net,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 14/17] fs/epoll: use faster rb_first_cached()

... such that we can avoid the tree walks to get the
node with the smallest key. Semantically the same,
as the previously used rb_first(), but O(1). The
main overhead is the extra footprint for the cached
rb_node pointer, which should not matter for epoll.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 fs/eventpoll.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index e767e4389cb1..c56e842bc9cc 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -205,7 +205,7 @@ struct eventpoll {
 	struct list_head rdllist;
 
 	/* RB tree root used to store monitored fd structs */
-	struct rb_root rbr;
+	struct rb_root_cached rbr;
 
 	/*
 	 * This is a single linked list that chains all the "struct epitem" that
@@ -791,7 +791,7 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
 	list_del_rcu(&epi->fllink);
 	spin_unlock(&file->f_lock);
 
-	rb_erase(&epi->rbn, &ep->rbr);
+	rb_erase_cached(&epi->rbn, &ep->rbr);
 
 	spin_lock_irqsave(&ep->lock, flags);
 	if (ep_is_linked(&epi->rdllink))
@@ -835,7 +835,7 @@ static void ep_free(struct eventpoll *ep)
 	/*
 	 * Walks through the whole tree by unregistering poll callbacks.
 	 */
-	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
+	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 
 		ep_unregister_pollwait(ep, epi);
@@ -851,7 +851,7 @@ static void ep_free(struct eventpoll *ep)
 	 * a lockdep warning.
 	 */
 	mutex_lock(&ep->mtx);
-	while ((rbp = rb_first(&ep->rbr)) != NULL) {
+	while ((rbp = rb_first_cached(&ep->rbr)) != NULL) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		ep_remove(ep, epi);
 		cond_resched();
@@ -958,7 +958,7 @@ static void ep_show_fdinfo(struct seq_file *m, struct file *f)
 	struct rb_node *rbp;
 
 	mutex_lock(&ep->mtx);
-	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
+	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
 		struct epitem *epi = rb_entry(rbp, struct epitem, rbn);
 		struct inode *inode = file_inode(epi->ffd.file);
 
@@ -1035,7 +1035,7 @@ static int ep_alloc(struct eventpoll **pep)
 	init_waitqueue_head(&ep->wq);
 	init_waitqueue_head(&ep->poll_wait);
 	INIT_LIST_HEAD(&ep->rdllist);
-	ep->rbr = RB_ROOT;
+	ep->rbr = RB_ROOT_CACHED;
 	ep->ovflist = EP_UNACTIVE_PTR;
 	ep->user = user;
 
@@ -1061,7 +1061,7 @@ static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
 	struct epoll_filefd ffd;
 
 	ep_set_ffd(&ffd, file, fd);
-	for (rbp = ep->rbr.rb_node; rbp; ) {
+	for (rbp = ep->rbr.rb_root.rb_node; rbp; ) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		kcmp = ep_cmp_ffd(&ffd, &epi->ffd);
 		if (kcmp > 0)
@@ -1083,7 +1083,7 @@ static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long t
 	struct rb_node *rbp;
 	struct epitem *epi;
 
-	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
+	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		if (epi->ffd.fd == tfd) {
 			if (toff == 0)
@@ -1263,20 +1263,22 @@ static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
 static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
 {
 	int kcmp;
-	struct rb_node **p = &ep->rbr.rb_node, *parent = NULL;
+	struct rb_node **p = &ep->rbr.rb_root.rb_node, *parent = NULL;
 	struct epitem *epic;
+	bool leftmost = true;
 
 	while (*p) {
 		parent = *p;
 		epic = rb_entry(parent, struct epitem, rbn);
 		kcmp = ep_cmp_ffd(&epi->ffd, &epic->ffd);
-		if (kcmp > 0)
+		if (kcmp > 0) {
 			p = &parent->rb_right;
-		else
+			leftmost = false;
+		} else
 			p = &parent->rb_left;
 	}
 	rb_link_node(&epi->rbn, parent, p);
-	rb_insert_color(&epi->rbn, &ep->rbr);
+	rb_insert_color_cached(&epi->rbn, &ep->rbr, leftmost);
 }
 
 
@@ -1520,7 +1522,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
 	list_del_rcu(&epi->fllink);
 	spin_unlock(&tfile->f_lock);
 
-	rb_erase(&epi->rbn, &ep->rbr);
+	rb_erase_cached(&epi->rbn, &ep->rbr);
 
 error_unregister:
 	ep_unregister_pollwait(ep, epi);
@@ -1868,7 +1870,7 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
 	mutex_lock_nested(&ep->mtx, call_nests + 1);
 	ep->visited = 1;
 	list_add(&ep->visited_list_link, &visited_list);
-	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
+	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		if (unlikely(is_file_epoll(epi->ffd.file))) {
 			ep_tovisit = epi->ffd.file->private_data;
-- 
2.12.0