Re: wait_for_unix_gc can cause CPU overload for well behaved programs

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Ivan Babrou <ivan@cloudflare.com>
Date: Thu, 19 Oct 2023 15:35:01 -0700
> Hello,
> 
> We have observed this issue twice (2019 and 2023): a well behaved
> service that doesn't pass any file descriptors around starts to spend
> a ton of CPU time in wait_for_unix_gc.
> 
> The cause of this is that the unix send path unconditionally calls
> wait_for_unix_gc, which is a global garbage collection. If any
> misbehaved program exists on a system, it can force extra work for
> well behaved programs.
> 
> This behavior is not new: 9915672d4127 ("af_unix: limit
> unix_tot_inflight") is from 2010.
> 
> I managed to come up with a repro for this behavior:
> 
> * https://gist.github.com/bobrik/82e5722261920c9f23d9402b88a0bb27
> 
> It also includes a flamegraph illustrating the issue. It's all in one
> program for convenience, but in reality the offender not picking up
> SCM_RIGHTS messages and the suffering program just minding its own
> business are separate.
> 
> It is also non-trivial to find the offender when this happens as it
> can be completely idle while wrecking havoc for the rest of the
> system.
> 
> I don't think it's fair to penalize every unix_stream_sendmsg like
> this. The 16k threshold also doesn't feel very flexible, surely
> computers are bigger these days and can handle more.

Probably we could do the gc async and enforce the penalty only on
the offender by checking user->unix_inflight.

compile test only:

---8<---
diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 824c258143a3..a119f37953cc 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -12,8 +12,9 @@ void unix_inflight(struct user_struct *user, struct file *fp);
 void unix_notinflight(struct user_struct *user, struct file *fp);
 void unix_destruct_scm(struct sk_buff *skb);
 void io_uring_destruct_scm(struct sk_buff *skb);
-void unix_gc(void);
 void wait_for_unix_gc(void);
+void unix_gc_start(void);
+void unix_gc_stop(void);
 struct sock *unix_get_socket(struct file *filp);
 struct sock *unix_peer_get(struct sock *sk);
 
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 3e8a04a13668..56db096b13f1 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -681,7 +681,7 @@ static void unix_release_sock(struct sock *sk, int embrion)
 	 */
 
 	if (READ_ONCE(unix_tot_inflight))
-		unix_gc();		/* Garbage collect fds */
+		unix_gc_start(); /* Garbage collect fds */
 }
 
 static void init_peercred(struct sock *sk)
@@ -3683,6 +3683,7 @@ static int __init af_unix_init(void)
 
 static void __exit af_unix_exit(void)
 {
+	unix_gc_stop();
 	sock_unregister(PF_UNIX);
 	proto_unregister(&unix_dgram_proto);
 	proto_unregister(&unix_stream_proto);
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 2405f0f9af31..fb24d62fe34a 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -185,24 +185,26 @@ static void inc_inflight_move_tail(struct unix_sock *u)
 		list_move_tail(&u->link, &gc_candidates);
 }
 
-static bool gc_in_progress;
 #define UNIX_INFLIGHT_TRIGGER_GC 16000
 
+static void unix_gc(struct work_struct *work);
+static DECLARE_WORK(unix_gc_work, unix_gc);
+
 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, gc_in_progress == false);
+	struct user_struct *user = get_uid(current_user());
+
+	if (READ_ONCE(unix_tot_inflight) > UNIX_INFLIGHT_TRIGGER_GC)
+		schedule_work(&unix_gc_work);
+
+	if (!READ_ONCE(user->unix_inflight))
+		return;
+
+	flush_work(&unix_gc_work);
 }
 
 /* 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;
@@ -213,13 +215,6 @@ 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.
@@ -325,11 +320,15 @@ void unix_gc(void)
 	/* All candidates should have been detached by now. */
 	BUG_ON(!list_empty(&gc_candidates));
 
-	/* Paired with READ_ONCE() in wait_for_unix_gc(). */
-	WRITE_ONCE(gc_in_progress, false);
+	spin_unlock(&unix_gc_lock);
+}
 
-	wake_up(&unix_gc_wait);
+void unix_gc_start(void)
+{
+	schedule_work(&unix_gc_work);
+}
 
- out:
-	spin_unlock(&unix_gc_lock);
+void __exit unix_gc_stop(void)
+{
+	flush_work(&unix_gc_work);
 }
---8<---