On Wed, May 14, 2014 at 12:25:54AM +0200, Frederic Weisbecker wrote:
> irq work currently only supports local callbacks. However its code
> is mostly ready to run remote callbacks and we have some potential user.
> 
> The full nohz subsystem currently open codes its own remote irq work
> on top of the scheduler ipi when it wants a CPU to reevaluate its next
> tick. However this ad hoc solution bloats the scheduler IPI.
> 
> Lets just extend the irq work subsystem to support remote queuing on top
> of the generic SMP IPI to handle this kind of user. This shouldn't add
> noticeable overhead.
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Kevin Hilman <khilman@linaro.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> ---
>  include/linux/irq_work.h |  2 ++
>  kernel/irq_work.c        | 19 ++++++++++++++++++-
>  kernel/smp.c             |  4 ++++
>  3 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
> index 19ae05d..ae44aa2 100644
> --- a/include/linux/irq_work.h
> +++ b/include/linux/irq_work.h
> @@ -33,6 +33,8 @@ void init_irq_work(struct irq_work *work, void (*func)(struct irq_work *))
>  #define DEFINE_IRQ_WORK(name, _f) struct irq_work name = { .func = (_f), }
>  
>  bool irq_work_queue(struct irq_work *work);
> +bool irq_work_queue_on(struct irq_work *work, int cpu);
> +
>  void irq_work_run(void);
>  void irq_work_sync(struct irq_work *work);
>  
> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index a82170e..9f9be55 100644
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -56,11 +56,28 @@ void __weak arch_irq_work_raise(void)
>  }
>  
>  /*
> - * Enqueue the irq_work @entry unless it's already pending
> + * Enqueue the irq_work @work on @cpu unless it's already pending
>   * somewhere.
>   *
>   * Can be re-enqueued while the callback is still in progress.
>   */
> +bool irq_work_queue_on(struct irq_work *work, int cpu)
> +{
> +	/* Only queue if not already pending */
> +	if (!irq_work_claim(work))
> +		return false;
> +
> +	/* All work should have been flushed before going offline */
> +	WARN_ON_ONCE(cpu_is_offline(cpu));

	WARN_ON_ONCE(in_nmi());

> +
> +	llist_add(&work->llnode, &per_cpu(irq_work_list, cpu));
> +	native_send_call_func_single_ipi(cpu);

At the very leastestest make that:

	if (llist_add(&work->llnode, &per_cpu(irq_work_list, cpu)))
		native_send_call_func_single_ipi(cpu);

But ideally, also test the IRQ_WORK_LAZY support, its weird to have that
only be supported for the other queue.

Hmm, why do we need that LAZY crap, that completely wrecks a perfectly
simple thing.

The changelog (bc6679aef673f), not the printk() usage make much sense,
printk() can't cause an IPI storm... printk() isn't fast enough to storm
anything.

> +
> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(irq_work_queue_on);