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

From: Ingo Molnar <[email-masked]>
Date: Wed, 29 Apr 2009 12:27:34 +0200

* Eric Dumazet <[email-masked]> wrote:

> Ingo Molnar a écrit :
> > * Eric Dumazet <[email-masked]> 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 <[email-masked]>
> Acked-by: David S. Miller <[email-masked]>
> Acked-by: Andi Kleen <[email-masked]>
> Acked-by: Ingo Molnar <[email-masked]>

> ---
>  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?

> +
>  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;
>  					}
>  				}
>  			}

Looks much nicer now!  [ I'd still suggest to factor out the guts of 
do_select() as its nesting is excessive that hurts its reviewability 
quite a bit - but now your patch does not make the situation any 
worse. ]

Even-More-Acked-by: Ingo Molnar <[email-masked]>

	Ingo