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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>
This commit is contained in:
Eric Dumazet 2009-06-16 15:33:36 -07:00 committed by Linus Torvalds
parent 02d5341ae5
commit 4938d7e023
2 changed files with 39 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

40
fs/select.c
View File

@ -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);

3
include/linux/poll.h
View File

@ -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;
};