linux-kernel - [PATCH v6.1 03/27] af_unix: Run GC on only one CPU.

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Message-ID: <20250521152920.1116756-4-lee@kernel.org>
Date: Wed, 21 May 2025 16:27:02 +0100
From: Lee Jones <lee@...nel.org>
To: lee@...nel.org,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	Kuniyuki Iwashima <kuniyu@...zon.com>,
	Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
	Jens Axboe <axboe@...nel.dk>,
	Sasha Levin <sashal@...nel.org>,
	Michal Luczaj <mhal@...x.co>,
	Rao Shoaib <Rao.Shoaib@...cle.com>,
	Pavel Begunkov <asml.silence@...il.com>,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Cc: stable@...r.kernel.org
Subject: [PATCH v6.1 03/27] af_unix: Run GC on only one CPU.

From: Kuniyuki Iwashima <kuniyu@...zon.com>

[ Upstream commit 8b90a9f819dc2a06baae4ec1a64d875e53b824ec ]

If more than 16000 inflight AF_UNIX sockets exist and the garbage
collector is not running, unix_(dgram|stream)_sendmsg() call unix_gc().
Also, they wait for unix_gc() to complete.

In unix_gc(), all inflight AF_UNIX sockets are traversed at least once,
and more if they are the GC candidate.  Thus, sendmsg() significantly
slows down with too many inflight AF_UNIX sockets.

There is a small window to invoke multiple unix_gc() instances, which
will then be blocked by the same spinlock except for one.

Let's convert unix_gc() to use struct work so that it will not consume
CPUs unnecessarily.

Note WRITE_ONCE(gc_in_progress, true) is moved before running GC.
If we leave the WRITE_ONCE() as is and use the following test to
call flush_work(), a process might not call it.

    CPU 0                                     CPU 1
    ---                                       ---
                                              start work and call __unix_gc()
    if (work_pending(&unix_gc_work) ||        <-- false
        READ_ONCE(gc_in_progress))            <-- false
            flush_work();                     <-- missed!
	                                      WRITE_ONCE(gc_in_progress, true)

Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
Link: https://lore.kernel.org/r/20240123170856.41348-5-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
(cherry picked from commit 8b90a9f819dc2a06baae4ec1a64d875e53b824ec)
Signed-off-by: Lee Jones <lee@...nel.org>
---
 net/unix/garbage.c | 54 +++++++++++++++++++++++-----------------------
 1 file changed, 27 insertions(+), 27 deletions(-)

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 438f5d9b9173..bf628bfb6d35 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -86,7 +86,6 @@
 /* Internal data structures and random procedures: */
 
 static LIST_HEAD(gc_candidates);
-static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
 
 static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 			  struct sk_buff_head *hitlist)
@@ -182,23 +181,8 @@ static void inc_inflight_move_tail(struct unix_sock *u)
 }
 
 static bool gc_in_progress;
-#define UNIX_INFLIGHT_TRIGGER_GC 16000
-
-void wait_for_unix_gc(void)
-{
-	/* If number of inflight sockets is insane,
-	 * force a garbage collect right now.
-	 * Paired with the WRITE_ONCE() in unix_inflight(),
-	 * unix_notinflight() and gc_in_progress().
-	 */
-	if (READ_ONCE(unix_tot_inflight) > UNIX_INFLIGHT_TRIGGER_GC &&
-	    !READ_ONCE(gc_in_progress))
-		unix_gc();
-	wait_event(unix_gc_wait, !READ_ONCE(gc_in_progress));
-}
 
-/* The external entry point: unix_gc() */
-void unix_gc(void)
+static void __unix_gc(struct work_struct *work)
 {
 	struct sk_buff *next_skb, *skb;
 	struct unix_sock *u;
@@ -209,13 +193,6 @@ void unix_gc(void)
 
 	spin_lock(&unix_gc_lock);
 
-	/* Avoid a recursive GC. */
-	if (gc_in_progress)
-		goto out;
-
-	/* Paired with READ_ONCE() in wait_for_unix_gc(). */
-	WRITE_ONCE(gc_in_progress, true);
-
 	/* First, select candidates for garbage collection.  Only
 	 * in-flight sockets are considered, and from those only ones
 	 * which don't have any external reference.
@@ -346,8 +323,31 @@ void unix_gc(void)
 	/* Paired with READ_ONCE() in wait_for_unix_gc(). */
 	WRITE_ONCE(gc_in_progress, false);
 
-	wake_up(&unix_gc_wait);
-
- out:
 	spin_unlock(&unix_gc_lock);
 }
+
+static DECLARE_WORK(unix_gc_work, __unix_gc);
+
+void unix_gc(void)
+{
+	WRITE_ONCE(gc_in_progress, true);
+	queue_work(system_unbound_wq, &unix_gc_work);
+}
+
+#define UNIX_INFLIGHT_TRIGGER_GC 16000
+
+void wait_for_unix_gc(void)
+{
+	/* If number of inflight sockets is insane,
+	 * force a garbage collect right now.
+	 *
+	 * Paired with the WRITE_ONCE() in unix_inflight(),
+	 * unix_notinflight(), and __unix_gc().
+	 */
+	if (READ_ONCE(unix_tot_inflight) > UNIX_INFLIGHT_TRIGGER_GC &&
+	    !READ_ONCE(gc_in_progress))
+		unix_gc();
+
+	if (READ_ONCE(gc_in_progress))
+		flush_work(&unix_gc_work);
+}
-- 
2.49.0.1143.g0be31eac6b-goog