netdev - Re: wait_for_unix_gc can cause CPU overload for well behaved programs

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Date: Fri, 20 Oct 2023 18:47:28 +0800
From: Hillf Danton <hdanton@...a.com>
To: Ivan Babrou <ivan@...udflare.com>
Cc: Linux Kernel Network Developers <netdev@...r.kernel.org>,
	kernel-team <kernel-team@...udflare.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: wait_for_unix_gc can cause CPU overload for well behaved programs

On Thu, 19 Oct 2023 15:35:01 -0700 Ivan Babrou <ivan@...udflare.com>
> 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.

See if the diff below works for you, which prevents concurrent spinning
of unix_gc_lock, a variant of spin_trylock().

Hillf
--- x/net/unix/garbage.c
+++ y/net/unix/garbage.c
@@ -211,15 +211,10 @@ void unix_gc(void)
 	struct list_head cursor;
 	LIST_HEAD(not_cycle_list);
 
+	if (test_and_set_bit(0, &gc_in_progress))
+		return;
 	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.
--