Date:   Sat, 27 Jul 2019 19:35:42 +0800
From:   Cheng Jian <cj.chengjian@...wei.com>
To:     <cj.chengjian@...wei.com>, <xiexiuqi@...wei.com>,
        <viro@...iv.linux.org.uk>, <houtao1@...wei.com>
CC:     <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH] epoll: optimize epmutex in ep_free and eventpoll_release_file

We are optimizing the Request-Per-Second of nginx http server,
and we found that acquiring epmutex in eventpoll_release_file()
will become a bottleneck under the one-request-per-connection
scenario.

Optimize the epmutex with a smaller granularity. Introduce
an ref-counter to eventpoll and free eventpoll by rcu, using rcu
and list_first_or_null_rcu() to iterate file->f_ep_links instead
of epmutex.

The following are some details of the scenario:

HTTP server (nginx):
	* under ARM64 with 64 cores
	* 64 worker processes, each worker is binded to a specific CPU
	* keepalive_requests = 1 in nginx.conf: nginx will close the
	  connection fd after a reply is send
HTTP client[benchmark] (wrk):
	* under x86-64 with 48 cores
	* 16 threads, 64 connections per-thread

Before the patch, the RPS measured by wrk is ~220K, after applying
the patch the RPS is ~240K. We also measure the overhead of
eventpoll_release_file() and its children by perf: 29% before and
2% after.

Link : https://lkml.org/lkml/2017/10/28/81

Signed-off-by: Cheng Jian <cj.chengjian@...wei.com>
Signed-off-by: Hou Tao <houtao1@...wei.com>
---
 fs/eventpoll.c | 106 ++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 92 insertions(+), 14 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index d7f1f5011fac..dc81f1c4fbaa 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -38,6 +38,7 @@
 #include <linux/compat.h>
 #include <linux/rculist.h>
 #include <net/busy_poll.h>
+#include <linux/refcount.h>
 
 /*
  * LOCKING:
@@ -225,6 +226,11 @@ struct eventpoll {
 	/* used to track busy poll napi_id */
 	unsigned int napi_id;
 #endif
+
+	/* used to ensure the validity of eventpoll when release file */
+	refcount_t ref;
+	/* used to free itself */
+	struct rcu_head rcu;
 };
 
 /* Wait structure used by the poll hooks */
@@ -809,6 +815,32 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
 	return 0;
 }
 
+static void ep_rcu_free(struct rcu_head *head)
+{
+	struct eventpoll *ep = container_of(head, struct eventpoll, rcu);
+
+	kfree(ep);
+}
+
+static void eventpoll_put_ep(struct eventpoll *ep)
+{
+	if (refcount_dec_and_test(&ep->ref)) {
+		mutex_destroy(&ep->mtx);
+		free_uid(ep->user);
+		wakeup_source_unregister(ep->ws);
+
+		call_rcu(&ep->rcu, ep_rcu_free);
+	}
+}
+
+static struct eventpoll *eventpoll_get_ep(struct eventpoll *ep)
+{
+	if (refcount_inc_not_zero(&ep->ref))
+		return ep;
+	else
+		return NULL;
+}
+
 static void ep_free(struct eventpoll *ep)
 {
 	struct rb_node *rbp;
@@ -826,11 +858,11 @@ static void ep_free(struct eventpoll *ep)
 	 * anymore. The only hit might come from eventpoll_release_file() but
 	 * holding "epmutex" is sufficient here.
 	 */
-	mutex_lock(&epmutex);
 
 	/*
 	 * Walks through the whole tree by unregistering poll callbacks.
 	 */
+	mutex_lock(&ep->mtx);
 	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 
@@ -846,7 +878,6 @@ static void ep_free(struct eventpoll *ep)
 	 * We do not need to lock ep->mtx, either, we only do it to prevent
 	 * a lockdep warning.
 	 */
-	mutex_lock(&ep->mtx);
 	while ((rbp = rb_first_cached(&ep->rbr)) != NULL) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		ep_remove(ep, epi);
@@ -854,11 +885,19 @@ static void ep_free(struct eventpoll *ep)
 	}
 	mutex_unlock(&ep->mtx);
 
-	mutex_unlock(&epmutex);
-	mutex_destroy(&ep->mtx);
-	free_uid(ep->user);
-	wakeup_source_unregister(ep->ws);
-	kfree(ep);
+	/*
+	 * ep will not been added to visited_list, because ep_ctrl()
+	 * can not get its reference and can not reference it by the
+	 * corresponding epitem. The only possible operation is list_del_init,
+	 * so it's OK to use list_empty_careful() here.
+	 */
+	if (!list_empty_careful(&ep->visited_list_link)) {
+		mutex_lock(&epmutex);
+		list_del_init(&ep->visited_list_link);
+		mutex_unlock(&epmutex);
+	}
+
+	eventpoll_put_ep(ep);
 }
 
 static int ep_eventpoll_release(struct inode *inode, struct file *file)
@@ -985,7 +1024,7 @@ static const struct file_operations eventpoll_fops = {
 void eventpoll_release_file(struct file *file)
 {
 	struct eventpoll *ep;
-	struct epitem *epi, *next;
+	struct epitem *epi;
 
 	/*
 	 * We don't want to get "file->f_lock" because it is not
@@ -1000,14 +1039,51 @@ void eventpoll_release_file(struct file *file)
 	 *
 	 * Besides, ep_remove() acquires the lock, so we can't hold it here.
 	 */
-	mutex_lock(&epmutex);
-	list_for_each_entry_safe(epi, next, &file->f_ep_links, fllink) {
-		ep = epi->ep;
+	rcu_read_lock();
+	while (true) {
+		epi = list_first_or_null_rcu(&file->f_ep_links,
+				struct epitem, fllink);
+		if (!epi)
+			break;
+
+		ep = eventpoll_get_ep(epi->ep);
+		/* Current epi had been removed by ep_free() */
+		if (!ep)
+			continue;
+		rcu_read_unlock();
+
 		mutex_lock_nested(&ep->mtx, 0);
-		ep_remove(ep, epi);
+		/*
+		 * If rb_first_cached() returns NULL, it means that
+		 * the current epi had been removed by ep_free().
+		 * To prevent epi from double-freeing, check the
+		 * condition before invoking ep_remove().
+		 * If eventpoll_release_file() frees epi firstly,
+		 * the epi will not be freed again because the epi
+		 * must have been removed from ep->rbr when ep_free()
+		 * is invoked.
+		 */
+		if (rb_first_cached(&ep->rbr))
+			ep_remove(ep, epi);
 		mutex_unlock(&ep->mtx);
+
+		eventpoll_put_ep(ep);
+
+		rcu_read_lock();
+	}
+	rcu_read_unlock();
+
+	/*
+	 * The file can not been added to tfile_check_list again, because
+	 * (1) refcnt has been zero, ep_ctrl() can no longer get its reference
+	 * (2) related ep items have been removed, ep_loop_check_proc() can not
+	 *     get the file by ep->rbr.
+	 */
+	if (!list_empty_careful(&file->f_tfile_llink)) {
+		mutex_lock(&epmutex);
+		list_del_init(&file->f_tfile_llink);
+		mutex_unlock(&epmutex);
 	}
-	mutex_unlock(&epmutex);
 }
 
 static int ep_alloc(struct eventpoll **pep)
@@ -1030,6 +1106,8 @@ static int ep_alloc(struct eventpoll **pep)
 	ep->rbr = RB_ROOT_CACHED;
 	ep->ovflist = EP_UNACTIVE_PTR;
 	ep->user = user;
+	INIT_LIST_HEAD(&ep->visited_list_link);
+	refcount_set(&ep->ref, 1);
 
 	*pep = ep;
 
@@ -2018,7 +2096,7 @@ static int ep_loop_check(struct eventpoll *ep, struct file *file)
 	list_for_each_entry_safe(ep_cur, ep_next, &visited_list,
 							visited_list_link) {
 		ep_cur->visited = 0;
-		list_del(&ep_cur->visited_list_link);
+		list_del_init(&ep_cur->visited_list_link);
 	}
 	return ret;
 }
-- 
2.20.1

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