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    SubjectRe: [PATCH] poll: Avoid extra wakeups in select/poll
    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;
    > };



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