epoll: simplify and optimize busy loop logic

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.

Link: https://lkml.kernel.org/r/20201106231635.3528496-6-soheil.kdev@gmail.com
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
Cc: Guantao Liu <guantaol@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index d8eef21..03d0ac0 100644 (file)
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -389,19 +389,24 @@ static bool ep_busy_loop_end(void *p, unsigned long start_time)
  *
  * we must do our busy polling with irqs enabled
  */
-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, false,
                               BUSY_POLL_BUDGET);
-}
-
-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;
 }
 
 /*
@@ -441,12 +446,9 @@ 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)
@@ -1772,21 +1774,13 @@ 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;