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

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Message-ID: <20130821144912.1b96c962@gandalf.local.home>
Date:	Wed, 21 Aug 2013 14:49:12 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jan Kara <jack@...e.cz>
Cc:	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 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));

>  
>  	/*
>  	 * 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().

-- Steve



> +}
> +
>  /*
>   * Run the irq_work entries on this cpu. Requires to be ran from hardirq
>   * context with local IRQs disabled.

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