af_unix: Run GC on only one CPU.

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@amazon.com>
Link: https://lore.kernel.org/r/20240123170856.41348-5-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index d5ef46a75f1f40b1602b3e1f0170260664f5842e..3f599db59f9dc327a57b738624e7db034302961f 100644 (file)
--- 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.
@@ -322,8 +299,31 @@ 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);
+}