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

From: Eric Dumazet <[email-masked]>
Date: Wed, 29 Apr 2009 09:39:40 +0200

Andrew Morton a écrit :
> On Tue, 28 Apr 2009 17:06:11 +0200 Eric Dumazet <[email-masked]> 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);

> 
>> --- 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 */
> 
> I kind of prefer to use plain old -1 for the all-ones pattern.  Because
> it always just works, and doesn't send the reviewer off to check if the
> type was really u64 or something.
> 
> It's a bit ugly though.
> 
>>  }
>>  
>>  struct poll_table_entry {
>>  	struct file *filp;
>> +	unsigned long key;
>>  	wait_queue_t wait;
>>  	wait_queue_head_t *wait_address;
>>  };
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to [email-masked]
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 
>