2013/11/7 Jan Kara <j...@suse.cz>:
> Provide new irq work flag - IRQ_WORK_UNBOUND - meaning that can be
> processed on any cpu. This flag implies IRQ_WORK_LAZY so that things are
> simple and we don't have to pick any particular cpu to do the work. We
> just do the work from a timer tick on whichever cpu it happens first.
> This is useful as a lightweight and simple code path without locking or
> other dependencies to offload work to other cpu if possible.
>
> We will use this type of irq work to make a guarantee of forward
> progress of printing to a (serial) console when printing on one cpu
> would cause interrupts to be disabled for too long.
>
> Signed-off-by: Jan Kara <j...@suse.cz>
> ---
> include/linux/irq_work.h | 2 ++
> kernel/irq_work.c | 41 +++++++++++++++++++++++++----------------
> 2 files changed, 27 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
> index 66017028dcb3..ca07a16355ed 100644
> --- a/include/linux/irq_work.h
> +++ b/include/linux/irq_work.h
> @@ -16,6 +16,8 @@
> #define IRQ_WORK_BUSY 2UL
> #define IRQ_WORK_FLAGS 3UL
> #define IRQ_WORK_LAZY 4UL /* Doesn't want IPI, wait for tick */
> +#define __IRQ_WORK_UNBOUND 8UL /* Use IRQ_WORK_UNBOUND instead! */
> +#define IRQ_WORK_UNBOUND (__IRQ_WORK_UNBOUND | IRQ_WORK_LAZY) /* Any
> cpu can process this work */
>
> struct irq_work {
> unsigned long flags;
> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index 55fcce6065cf..b06350b63c67 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, &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 +107,17 @@ bool irq_work_needs_cpu(void)
> return true;
> }
>
> -static void __irq_work_run(void)
> +static void process_irq_work_list(struct llist_head *llhead)
> {
> 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))
> + if (llist_empty(llhead))
> return;
>
> BUG_ON(!irqs_disabled());
> -
> - llnode = llist_del_all(this_list);
> + llnode = llist_del_all(llhead);
> while (llnode != NULL) {
> work = llist_entry(llnode, struct irq_work, llnode);
>
> @@ -146,6 +142,19 @@ static void __irq_work_run(void)
> }
> }
>
> +static void __irq_work_run(void)
> +{
> + /*
> + * 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();
> +
> + process_irq_work_list(&__get_cpu_var(irq_work_list));
> + process_irq_work_list(&unbound_irq_work_list);
> +}
> +
But then, who's going to process that work if every CPUs is idle?
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