poll: avoid extra wakeups in select/poll

After introduction of keyed wakeups Davide Libenzi did on epoll, we are
able to avoid spurious wakeups in poll()/select() code too.

For example, typical use of poll()/select() is to wait for incoming
network frames on many sockets.  But TX completion for UDP/TCP frames call
sock_wfree() which in turn schedules thread.

When scheduled, thread does a full scan of all polled fds and can sleep
again, because nothing is really available.  If number of fds is large,
this cause significant load.

This patch makes select()/poll() aware of keyed wakeups and useless
wakeups are avoided.  This reduces number of context switches by about 50%
on some setups, and work performed by sofirq handlers.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Davide Libenzi <davidel@xmailserver.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 36 insertions, 4 deletions

diff --git a/fs/select.c b/fs/select.c
index 0fe0e1469df3..d870237e42c7 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -168,7 +168,7 @@ static struct poll_table_entry *poll_get_entry(struct poll_wqueues *p)
         return table->entry++;
 }
 
-static int pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
+static int __pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
 {
         struct poll_wqueues *pwq = wait->private;
         DECLARE_WAITQUEUE(dummy_wait, pwq->polling_task);
@@ -194,6 +194,16 @@ static int pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
         return default_wake_function(&dummy_wait, mode, sync, key);
 }
 
+static int pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+        struct poll_table_entry *entry;
+
+        entry = container_of(wait, struct poll_table_entry, wait);
+        if (key && !((unsigned long)key & entry->key))
+                return 0;
+        return __pollwake(wait, mode, sync, key);
+}
+
 /* Add a new entry */
 static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
                                 poll_table *p)
@@ -205,6 +215,7 @@ static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
         get_file(filp);
         entry->filp = filp;
         entry->wait_address = wait_address;
+        entry->key = p->key;
         init_waitqueue_func_entry(&entry->wait, pollwake);
         entry->wait.private = pwq;
         add_wait_queue(wait_address, &entry->wait);
@@ -362,6 +373,18 @@ get_max:
 #define POLLOUT_SET (POLLWRBAND | POLLWRNORM | POLLOUT | POLLERR)
 #define POLLEX_SET (POLLPRI)
 
+static inline void wait_key_set(poll_table *wait, unsigned long in,
+                                unsigned long out, unsigned long bit)
+{
+        if (wait) {
+                wait->key = POLLEX_SET;
+                if (in & bit)
+                        wait->key |= POLLIN_SET;
+                if (out & bit)
+                        wait->key |= POLLOUT_SET;
+        }
+}
+
 int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
 {
         ktime_t expire, *to = NULL;
@@ -418,20 +441,25 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
                                 if (file) {
                                         f_op = file->f_op;
                                         mask = DEFAULT_POLLMASK;
-                                        if (f_op && f_op->poll)
-                                                mask = (*f_op->poll)(file, retval ? NULL : wait);
+                                        if (f_op && f_op->poll) {
+                                                wait_key_set(wait, in, out, bit);
+                                                mask = (*f_op->poll)(file, wait);
+                                        }
                                         fput_light(file, fput_needed);
                                         if ((mask & POLLIN_SET) && (in & bit)) {
                                                 res_in |= bit;
                                                 retval++;
+                                                wait = NULL;
                                         }
                                         if ((mask & POLLOUT_SET) && (out & bit)) {
                                                 res_out |= bit;
                                                 retval++;
+                                                wait = NULL;
                                         }
                                         if ((mask & POLLEX_SET) && (ex & bit)) {
                                                 res_ex |= bit;
                                                 retval++;
+                                                wait = NULL;
                                         }
                                 }
                         }
@@ -685,8 +713,12 @@ static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
                 mask = POLLNVAL;
                 if (file != NULL) {
                         mask = DEFAULT_POLLMASK;
-                        if (file->f_op && file->f_op->poll)
+                        if (file->f_op && file->f_op->poll) {
+                                if (pwait)
+                                        pwait->key = pollfd->events |
+                                                        POLLERR | POLLHUP;
                                 mask = file->f_op->poll(file, pwait);
+                        }
                         /* Mask out unneeded events. */
                         mask &= pollfd->events | POLLERR | POLLHUP;
                         fput_light(file, fput_needed);