Re: [PATCH] poll: Avoid extra wakeups in select/poll

Eric Dumazet
2009-04-29T12:31:22Z

Ingo Molnar a écrit :
> * Eric Dumazet <dada1@cosmosbay.com> wrote:
>
>> Ingo Molnar a écrit :
>>> * Eric Dumazet <dada1@cosmosbay.com> wrote:
>>>
>>>> @@ -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);
>>>> + }
>>>> fput_light(file, fput_needed);
>>>> if ((mask & POLLIN_SET) && (in & bit)) {
>>>> res_in |= bit;
>>> Please factor this whole 'if (file)' branch out into a helper.
>>> Typical indentation levels go from 1 to 3 tabs - 4 should be avoided
>>> if possible and 5 is pretty excessive already. This goes to eight.
>>>
>> Thanks Ingo,
>>
>> Here is v3 of patch, with your Acked-by included :)
>>
>> This is IMHO clearer since helper immediatly follows POLLIN_SET / POLLOUT_SET /
>> POLLEX_SET defines.
>>
>> [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@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>
>
>> ---
>> fs/select.c | 40 ++++++++++++++++++++++++++++++++++++----
>> include/linux/poll.h | 3 +++
>> 2 files changed, 39 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/select.c b/fs/select.c
>> index 0fe0e14..ba068ad 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 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;
>> + }
>> +}
>
> should be inline perhaps?

Well, I thought current practice was not using inline for such trivial functions,
as gcc already inlines them anyway.

Quoting Documentation/CodingStyle :
Often people argue that adding inline to functions that are static and used
only once is always a win since there is no space tradeoff. While this is
technically correct, gcc is capable of inlining these automatically without
help, and the maintenance issue of removing the inline when a second user
appears outweighs the potential value of the hint that tells gcc to do
something it would have done anyway.

Anyway :

[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@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>
---
fs/select.c | 40 ++++++++++++++++++++++++++++++++++++----
include/linux/poll.h | 3 +++
2 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 0fe0e14..ba068ad 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);
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;
};