Commit 8b90a9f8 authored by Kuniyuki Iwashima's avatar Kuniyuki Iwashima Committed by Jakub Kicinski
Browse files

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: default avatarKuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240123170856.41348-5-kuniyu@amazon.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 5b17307b

net/unix/garbage.c

+27 −27
Original line number Diff line number Diff line
@@ -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);

}