[PATCH 5/8] epoll: simplify and optimize busy loop logic

From: Soheil Hassas Yeganeh
Date: Fri Nov 06 2020 - 18:18:14 EST


From: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>

ep_events_available() is called multiple times around the busy loop
logic, even though the logic is generally not used.
ep_reset_busy_poll_napi_id() is similarly always called, even when
busy loop is not used.

Eliminate ep_reset_busy_poll_napi_id() and inline it inside
ep_busy_loop(). Make ep_busy_loop() return whether there are any
events available after the busy loop. This will eliminate unnecessary
loads and branches, and simplifies the loop.

Signed-off-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Reviewed-by: Willem de Bruijn <willemb@xxxxxxxxxx>
Reviewed-by: Khazhismel Kumykov <khazhy@xxxxxxxxxx>
---
fs/eventpoll.c | 42 +++++++++++++++++++-----------------------
1 file changed, 19 insertions(+), 23 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 5226b0cb1098..28c1d341d2e6 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -391,19 +391,26 @@ static bool ep_busy_loop_end(void *p, unsigned long start_time)
* busy loop will return if need_resched or ep_events_available.
*
* we must do our busy polling with irqs enabled
+ *
+ * Returns whether new events are available after a successful busy loop.
*/
-static void ep_busy_loop(struct eventpoll *ep, int nonblock)
+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) && net_busy_loop_on()) {
napi_busy_loop(napi_id, nonblock ? NULL : ep_busy_loop_end, ep);
-}
-
-static inline void ep_reset_busy_poll_napi_id(struct eventpoll *ep)
-{
- if (ep->napi_id)
+ if (ep_events_available(ep))
+ return true;
+ /*
+ * Busy poll timed out. Drop NAPI ID for now, we can add
+ * it back in when we have moved a socket with a valid NAPI
+ * ID onto the ready list.
+ */
ep->napi_id = 0;
+ return false;
+ }
+ return false;
}

/*
@@ -444,12 +451,9 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)

#else

-static inline void ep_busy_loop(struct eventpoll *ep, int nonblock)
-{
-}
-
-static inline void ep_reset_busy_poll_napi_id(struct eventpoll *ep)
+static inline bool ep_busy_loop(struct eventpoll *ep, int nonblock)
{
+ return false;
}

static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
@@ -1857,21 +1861,13 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
}

fetch_events:
-
- if (!ep_events_available(ep))
- ep_busy_loop(ep, timed_out);
-
eavail = ep_events_available(ep);
+ if (!eavail)
+ eavail = ep_busy_loop(ep, timed_out);
+
if (eavail)
goto send_events;

- /*
- * Busy poll timed out. Drop NAPI ID for now, we can add
- * it back in when we have moved a socket with a valid NAPI
- * ID onto the ready list.
- */
- ep_reset_busy_poll_napi_id(ep);
-
do {
if (signal_pending(current))
return -EINTR;
--
2.29.1.341.ge80a0c044ae-goog