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From:   Benjamin Segall <bsegall@google.com>
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [RFC PATCH] epoll: autoremove wakers even more aggressively
Date:   Wed, 15 Jun 2022 14:24:23 -0700
Message-ID: <xm26k09htybc.fsf@google.com>
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If a process is killed or otherwise exits while having active network
connections and many threads waiting on epoll_wait, the threads will all
be woken immediately, but not removed from ep->wq. Then when network
traffic scans ep->wq in wake_up, every wakeup attempt will fail, and
will not remove the entries from the list.

This means that the cost of the wakeup attempt is far higher than usual,
does not decrease, and this also competes with the dying threads trying
to actually make progress and remove themselves from the wq.

Handle this by removing visited epoll wq entries unconditionally, rather
than only when the wakeup succeeds - the structure of ep_poll means that
the only potential loss is the timed_out->eavail heuristic, which now
can race and result in a redundant ep_send_events attempt. (But only
when incoming data and a timeout actually race, not on every timeout)

Signed-off-by: Ben Segall <bsegall@google.com>
---
 fs/eventpoll.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index e2daa940ebce..8b56b94e2f56 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1745,10 +1745,25 @@ static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms)
 	ktime_get_ts64(&now);
 	*to = timespec64_add_safe(now, *to);
 	return to;
 }
 
+/*
+ * autoremove_wake_function, but remove even on failure to wake up, because we
+ * know that default_wake_function/ttwu will only fail if the thread is already
+ * woken, and in that case the ep_poll loop will remove the entry anyways, not
+ * try to reuse it.
+ */
+static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
+				       unsigned int mode, int sync, void *key)
+{
+	int ret = default_wake_function(wq_entry, mode, sync, key);
+
+	list_del_init(&wq_entry->entry);
+	return ret;
+}
+
 /**
  * ep_poll - Retrieves ready events, and delivers them to the caller-supplied
  *           event buffer.
  *
  * @ep: Pointer to the eventpoll context.
@@ -1826,12 +1841,19 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 		 * chance to harvest new event. Otherwise wakeup can be
 		 * lost. This is also good performance-wise, because on
 		 * normal wakeup path no need to call __remove_wait_queue()
 		 * explicitly, thus ep->lock is not taken, which halts the
 		 * event delivery.
+		 *
+		 * In fact, we now use an even more aggressive function that
+		 * unconditionally removes, because we don't reuse the wait
+		 * entry between loop iterations. This lets us also avoid the
+		 * performance issue if a process is killed, causing all of its
+		 * threads to wake up without being removed normally.
 		 */
 		init_wait(&wait);
+		wait.func = ep_autoremove_wake_function;
 
 		write_lock_irq(&ep->lock);
 		/*
 		 * Barrierless variant, waitqueue_active() is called under
 		 * the same lock on wakeup ep_poll_callback() side, so it
-- 
2.36.1.476.g0c4daa206d-goog