Re: [PATCH] poll: Avoid extra wakeups in select/poll
From: Eric Dumazet
Date: Wed Apr 29 2009 - 04:30:15 EST
Eric Dumazet a écrit :
> Andrew Morton a écrit :
>> On Tue, 28 Apr 2009 17:06:11 +0200 Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
>>
>>> [PATCH] 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.
>>>
>> Seems that this is a virtuous patch even though Christoph is struggling
>> a bit to test it?
>>
>>> fs/select.c | 28 +++++++++++++++++++++++++---
>>> include/linux/poll.h | 3 +++
>>> 2 files changed, 28 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fs/select.c b/fs/select.c
>>> index 0fe0e14..2708187 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);
>>> @@ -418,8 +429,16 @@ 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)
>>> + if (f_op && f_op->poll) {
>>> + if (wait) {
>>> + wait->key = POLLEX_SET;
>>> + if (in & bit)
>>> + wait->key |= POLLIN_SET;
>>> + if (out & bit)
>>> + wait->key |= POLLOUT_SET;
>>> + }
>>> mask = (*f_op->poll)(file, retval ? NULL : wait);
>>> + }
>> <resizes xterm rather a lot>
>>
>> Can we (and should we) avoid all that manipulation of wait->key if
>> `retval' is zero?
>
> yes, we could set wait to NULL as soon as retval is incremented.
> and also do :
>
> mask = (*f_op->poll)(file, wait);
>
[PATCH] 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@xxxxxxxxxxxxx>
Acked-by: David S. Miller <davem@xxxxxxxxxxxxx>
Acked-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
fs/select.c | 33 +++++++++++++++++++++++++++++----
include/linux/poll.h | 3 +++
2 files changed, 32 insertions(+), 4 deletions(-)
diff --git a/fs/select.c b/fs/select.c
index 0fe0e14..71377fd 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);
@@ -418,20 +429,31 @@ 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) {
+ if (wait) {
+ wait->key = POLLEX_SET;
+ if (in & bit)
+ wait->key |= POLLIN_SET;
+ if (out & bit)
+ wait->key |= POLLOUT_SET;
+ }
+ 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 +707,11 @@ 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);
diff --git a/include/linux/poll.h b/include/linux/poll.h
index 8c24ef8..3327389 100644
--- a/include/linux/poll.h
+++ b/include/linux/poll.h
@@ -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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