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

Andrew Morton
2009-04-29T07:29:00Z

On Tue, 28 Apr 2009 17:06:11 +0200 Eric Dumazet <dada1@cosmosbay.com> 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;
> };