Messages in this thread | | | Date | Wed, 29 Apr 2009 00:20:49 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] poll: Avoid extra wakeups in select/poll |
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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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