Commit 4938d7e0 authored by Eric Dumazet's avatar Eric Dumazet Committed by Linus Torvalds
Browse files

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: default avatarEric Dumazet <dada1@cosmosbay.com>
Acked-by: default avatarDavid S. Miller <davem@davemloft.net>
Acked-by: default avatarAndi Kleen <ak@linux.intel.com>
Acked-by: default avatarIngo Molnar <mingo@elte.hu>
Acked-by: default avatarDavide Libenzi <davidel@xmailserver.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 02d5341a

fs/select.c

+36 −4
Original line number Diff line number Diff line
@@ -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);

include/linux/poll.h

+3 −0
Original line number Diff line number Diff line
@@ -32,6 +32,7 @@ typedef void (*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_ 


typedef struct poll_table_struct {

	poll_queue_proc qproc;

	unsigned long key;

} poll_table;



static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
@@ -43,10 +44,12 @@ static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_addres 
static inline void init_poll_funcptr(poll_table *pt, poll_queue_proc qproc)

{

	pt->qproc = qproc;

	pt->key   = ~0UL; /* all events enabled */

}



struct poll_table_entry {

	struct file *filp;

	unsigned long key;

	wait_queue_t wait;

	wait_queue_head_t *wait_address;

};

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