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    Subject[PATCH] poll: Avoid extra wakeups in select/poll
    [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 <dada1@cosmosbay.com>
    Acked-by: David S. Miller <davem@davemloft.net>
    Acked-by: Andi Kleen <ak@linux.intel.com>
    ---
    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);
    + }
    fput_light(file, fput_needed);
    if ((mask & POLLIN_SET) && (in & bit)) {
    res_in |= bit;
    @@ -685,8 +704,11 @@ static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
    mask = POLLNVAL;
    if (file != NULL) {
    mask = DEFAULT_POLLMASK;
    - if (file->f_op && file->f_op->poll)
    + if (file->f_op && file->f_op->poll) {
    + if (pwait)
    + pwait->key = pollfd->events | POLLERR | POLLHUP;
    mask = file->f_op->poll(file, pwait);
    + }
    /* Mask out unneeded events. */
    mask &= pollfd->events | POLLERR | POLLHUP;
    fput_light(file, fput_needed);
    diff --git a/include/linux/poll.h b/include/linux/poll.h
    index 8c24ef8..3327389 100644
    --- 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 */
    }

    struct poll_table_entry {
    struct file *filp;
    + unsigned long key;
    wait_queue_t wait;
    wait_queue_head_t *wait_address;
    };


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