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

From: Andrew Morton <[email-masked]>
Date: Wed, 29 Apr 2009 00:20:49 -0700

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?

> --- 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;
>  };