[ltt-dev] [RFC PATCH] Poll : add poll_wait_set_exclusive (fixing thundering herd problem in LTTng)

[Mathieu Desnoyers]

* KOSAKI Motohiro (kosaki.motohiro at jp.fujitsu.com) wrote:
> Hi
> 
> >  static struct poll_table_entry *poll_get_entry(poll_table *_p)
> >  {
> >  	struct poll_wqueues *p = container_of(_p, struct poll_wqueues, pt);
> > @@ -117,8 +135,10 @@ static struct poll_table_entry *poll_get
> >  }
> >  
> >  /* Add a new entry */
> > -static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
> > -				poll_table *p)
> > +static void __pollwait_common(struct file *filp,
> > +			      wait_queue_head_t *wait_address,
> > +			      poll_table *p,
> > +			      int exclusive)
> >  {
> >  	struct poll_table_entry *entry = poll_get_entry(p);
> >  	if (!entry)
> > @@ -127,7 +147,23 @@ static void __pollwait(struct file *filp
> >  	entry->filp = filp;
> >  	entry->wait_address = wait_address;
> >  	init_waitqueue_entry(&entry->wait, current);
> > -	add_wait_queue(wait_address, &entry->wait);
> > +	if (!exclusive)
> > +		add_wait_queue(wait_address, &entry->wait);
> > +	else
> > +		add_wait_queue_exclusive(wait_address, &entry->wait);
> > +}
> 
> 
> I fully agreed this feature is needed.
> Actually, I've made similar patch at one years ago.
> 
> http://marc.info/?l=linux-kernel&m=120257050719087&w=2
> 
> 
> but, I have one question.
> My version have epoll support, but yours donesn't have.
> Is it intensionally?
> 
> this is just dumb question, it doesn't mean any objection.

Hrm, actually, your patch seems cleaner than mine, but it adds a branch
in the standard hotpath, which mine does not do (but I am not sure it is
such an important optimization...). Is there any reason why your patch
did not get merged ?

The only reason I did not make a epoll version is simply because LTTng
currently does not support it. :)

Mathieu

-- 
Mathieu Desnoyers
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