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

    * Eric Dumazet <dada1@cosmosbay.com> wrote:

    > Ingo Molnar a écrit :
    > > * Eric Dumazet <dada1@cosmosbay.com> 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 <dada1@cosmosbay.com>
    > Acked-by: David S. Miller <davem@davemloft.net>
    > Acked-by: Andi Kleen <ak@linux.intel.com>
    > Acked-by: Ingo Molnar <mingo@elte.hu>

    > ---
    > 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 <mingo@elte.hu>

    Ingo
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