[PATCH 9/9] fs/epoll: Use faster rb_first_cached()

Davidlohr Bueso
2017-06-29T17:17:45Z

... 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.

Signed-off-by: Davidlohr Bueso <dbueso@suse.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