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

Eric Dumazet
2009-04-29T07:42:25Z

Andrew Morton a écrit :
> 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?

yes, we could set wait to NULL as soon as retval is incremented.
and also do :

mask = (*f_op->poll)(file, wait);

>
>> --- 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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