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From: Joe Damato <jdamato@fastly.com>
To: linux-kernel@vger.kernel.org,
	netdev@vger.kernel.org
Cc: chuck.lever@oracle.com,
	jlayton@kernel.org,
	linux-api@vger.kernel.org,
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	kuba@kernel.org,
	willemdebruijn.kernel@gmail.com,
	weiwan@google.com,
	David.Laight@ACULAB.COM,
	arnd@arndb.de,
	Joe Damato <jdamato@fastly.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jan Kara <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org (open list:FILESYSTEMS (VFS and infrastructure))
Subject: [PATCH net-next v4 1/3] eventpoll: support busy poll per epoll instance
Date: Wed, 31 Jan 2024 01:47:31 +0000
Message-Id: <20240131014738.469858-2-jdamato@fastly.com>
In-Reply-To: <20240131014738.469858-1-jdamato@fastly.com>
References: <20240131014738.469858-1-jdamato@fastly.com>
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Allow busy polling on a per-epoll context basis. The per-epoll context
usec timeout value is preferred, but the pre-existing system wide sysctl
value is still supported if it specified.

Note that this change uses an xor: either per epoll instance busy polling
is enabled on the epoll instance or system wide epoll is enabled. Enabling
both is disallowed.

Signed-off-by: Joe Damato <jdamato@fastly.com>
---
 fs/eventpoll.c | 49 +++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 45 insertions(+), 4 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 3534d36a1474..ce75189d46df 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -227,6 +227,8 @@ struct eventpoll {
 #ifdef CONFIG_NET_RX_BUSY_POLL
 	/* used to track busy poll napi_id */
 	unsigned int napi_id;
+	/* busy poll timeout */
+	u64 busy_poll_usecs;
 #endif
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -386,12 +388,44 @@ static inline int ep_events_available(struct eventpoll *ep)
 		READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
 }
 
+/**
+ * busy_loop_ep_timeout - check if busy poll has timed out. The timeout value
+ * from the epoll instance ep is preferred, but if it is not set fallback to
+ * the system-wide global via busy_loop_timeout.
+ *
+ * @start_time: The start time used to compute the remaining time until timeout.
+ * @ep: Pointer to the eventpoll context.
+ *
+ * Return: true if the timeout has expired, false otherwise.
+ */
+static inline bool busy_loop_ep_timeout(unsigned long start_time, struct eventpoll *ep)
+{
+#ifdef CONFIG_NET_RX_BUSY_POLL
+	unsigned long bp_usec = READ_ONCE(ep->busy_poll_usecs);
+
+	if (bp_usec) {
+		unsigned long end_time = start_time + bp_usec;
+		unsigned long now = busy_loop_current_time();
+
+		return time_after(now, end_time);
+	} else {
+		return busy_loop_timeout(start_time);
+	}
+#endif
+	return true;
+}
+
 #ifdef CONFIG_NET_RX_BUSY_POLL
+static bool ep_busy_loop_on(struct eventpoll *ep)
+{
+	return !!ep->busy_poll_usecs || net_busy_loop_on();
+}
+
 static bool ep_busy_loop_end(void *p, unsigned long start_time)
 {
 	struct eventpoll *ep = p;
 
-	return ep_events_available(ep) || busy_loop_timeout(start_time);
+	return ep_events_available(ep) || busy_loop_ep_timeout(start_time, ep);
 }
 
 /*
@@ -404,7 +438,7 @@ static bool ep_busy_loop(struct eventpoll *ep, int nonblock)
 {
 	unsigned int napi_id = READ_ONCE(ep->napi_id);
 
-	if ((napi_id >= MIN_NAPI_ID) && net_busy_loop_on()) {
+	if ((napi_id >= MIN_NAPI_ID) && ep_busy_loop_on(ep)) {
 		napi_busy_loop(napi_id, nonblock ? NULL : ep_busy_loop_end, ep, false,
 			       BUSY_POLL_BUDGET);
 		if (ep_events_available(ep))
@@ -430,7 +464,8 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
 	struct socket *sock;
 	struct sock *sk;
 
-	if (!net_busy_loop_on())
+	ep = epi->ep;
+	if (!ep_busy_loop_on(ep))
 		return;
 
 	sock = sock_from_file(epi->ffd.file);
@@ -442,7 +477,6 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
 		return;
 
 	napi_id = READ_ONCE(sk->sk_napi_id);
-	ep = epi->ep;
 
 	/* Non-NAPI IDs can be rejected
 	 *	or
@@ -466,6 +500,10 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
 {
 }
 
+static inline bool ep_busy_loop_on(struct eventpoll *ep)
+{
+	return false;
+}
 #endif /* CONFIG_NET_RX_BUSY_POLL */
 
 /*
@@ -2058,6 +2096,9 @@ static int do_epoll_create(int flags)
 		error = PTR_ERR(file);
 		goto out_free_fd;
 	}
+#ifdef CONFIG_NET_RX_BUSY_POLL
+	ep->busy_poll_usecs = 0;
+#endif
 	ep->file = file;
 	fd_install(fd, file);
 	return fd;
-- 
2.25.1