linux-kernel - Re: [PATCH 2/4] irq_work: Provide a irq work that can be processed on any cpu

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Date:	Thu, 5 Sep 2013 17:56:57 +0200
From:	Jan Kara <jack@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, mhocko@...e.cz, hare@...e.de
Subject: Re: [PATCH 2/4] irq_work: Provide a irq work that can be processed
 on any cpu

On Wed 21-08-13 14:49:12, Steven Rostedt wrote:
> On Wed, 21 Aug 2013 10:08:30 +0200
> Jan Kara <jack@...e.cz> wrote:
> 
>   
> >  struct irq_work {
> >  	unsigned long flags;
> > diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> > index 55fcce6..446cd81 100644
> > --- a/kernel/irq_work.c
> > +++ b/kernel/irq_work.c
> > @@ -22,6 +22,9 @@
> >  static DEFINE_PER_CPU(struct llist_head, irq_work_list);
> >  static DEFINE_PER_CPU(int, irq_work_raised);
> >  
> > +/* List of irq-work any CPU can pick up */
> > +static LLIST_HEAD(unbound_irq_work_list);
> > +
> >  /*
> >   * Claim the entry so that no one else will poke at it.
> >   */
> > @@ -70,12 +73,16 @@ void irq_work_queue(struct irq_work *work)
> >  	/* Queue the entry and raise the IPI if needed. */
> >  	preempt_disable();
> >  
> > -	llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
> > +	if (!(work->flags & __IRQ_WORK_UNBOUND))
> > +		llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
> > +	else
> > +		llist_add(&work->llnode, &unbound_irq_work_list);
> 
> 
> Just for better readability I would have:
> 
> 	if (work->flags & __IRQ_WORK_UNBOUND)
> 		llist_add(&work->llnode, &unbound_irq_work_list);
> 	else
> 		llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
  OK, done.

> >  
> >  	/*
> >  	 * If the work is not "lazy" or the tick is stopped, raise the irq
> >  	 * work interrupt (if supported by the arch), otherwise, just wait
> > -	 * for the next tick.
> > +	 * for the next tick. We do this even for unbound work to make sure
> > +	 * *some* CPU will be doing the work.
> >  	 */
> >  	if (!(work->flags & IRQ_WORK_LAZY) || tick_nohz_tick_stopped()) {
> >  		if (!this_cpu_cmpxchg(irq_work_raised, 0, 1))
> > @@ -100,28 +107,11 @@ bool irq_work_needs_cpu(void)
> >  	return true;
> >  }
> >  
> > -static void __irq_work_run(void)
> > +static void process_irq_work_list(struct llist_node *llnode)
> >  {
> >  	unsigned long flags;
> >  	struct irq_work *work;
> > -	struct llist_head *this_list;
> > -	struct llist_node *llnode;
> > -
> > -
> > -	/*
> > -	 * Reset the "raised" state right before we check the list because
> > -	 * an NMI may enqueue after we find the list empty from the runner.
> > -	 */
> > -	__this_cpu_write(irq_work_raised, 0);
> > -	barrier();
> >  
> > -	this_list = &__get_cpu_var(irq_work_list);
> > -	if (llist_empty(this_list))
> > -		return;
> > -
> > -	BUG_ON(!irqs_disabled());
> > -
> > -	llnode = llist_del_all(this_list);
> >  	while (llnode != NULL) {
> >  		work = llist_entry(llnode, struct irq_work, llnode);
> >  
> > @@ -146,6 +136,27 @@ static void __irq_work_run(void)
> >  	}
> >  }
> >  
> > +static void __irq_work_run(void)
> > +{
> > +	struct llist_head *this_list;
> > +
> > +	/*
> > +	 * Reset the "raised" state right before we check the list because
> > +	 * an NMI may enqueue after we find the list empty from the runner.
> > +	 */
> > +	__this_cpu_write(irq_work_raised, 0);
> > +	barrier();
> > +
> > +	this_list = &__get_cpu_var(irq_work_list);
> > +	if (llist_empty(this_list) && llist_empty(&unbound_irq_work_list))
> > +		return;
> > +
> > +	BUG_ON(!irqs_disabled());
> > +
> > +	process_irq_work_list(llist_del_all(this_list));
> > +	process_irq_work_list(llist_del_all(&unbound_irq_work_list));
> 
> OK, I'm being a bit of an micro optimization freak here, but...
> 
> How about moving the list_empty() and BUG_ON into the
> process_irq_work_list() call.
> 
> That is,
> 
> 	if (list_empty(llnode))
> 		return;
> 
> 	BUG_ON(!irqs_disabled());
> 
> That way we avoid the double xchg() that is done with the two calls to
> llist_del_all().
  OK, done that.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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