Re: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2015-04-23 08:11, Mike Galbraith wrote:
> On Mon, 2015-04-20 at 10:03 +0200, Mike Galbraith wrote:
>> On Thu, 2015-04-16 at 18:28 +0200, Jan Kiszka wrote:
>>> Instead of turning all irq_work requests into lazy ones on -rt, 
>>> just
>>> move their execution from hard into soft-irq context.
>>>
>>> This resolves deadlocks of ftrace which will queue work from 
>>> arbitrary
>>> contexts, including those that have locks held that are needed for
>>> raising a soft-irq.
>>
>> Yup, trace-cmd record -> dead-box fully repeatable, and now fixed.
> 
> Except you kinda forgot to run the raised list.  The reformatted 
> (which saved two whole lines;) patch below adds that to 
> irq_work_tick(), which fixes the livelock both powertop and perf top 
> otherwise meet.
> 
> Subject: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue
> Date:   Thu, 16 Apr 2015 18:28:16 +0200
> From:   Jan Kiszka <jan.kiszka@siemens.com>
> 
> Instead of turning all irq_work requests into lazy ones on -rt, just
> move their execution from hard into soft-irq context.
> 
> This resolves deadlocks of ftrace which will queue work from arbitrary
> contexts, including those that have locks held that are needed for
> raising a soft-irq.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
> 
> Second try, looks much better so far. And it also removes my concerns
> regarding other potential cases besides ftrace.
> 
>  kernel/irq_work.c |   84 ++++++++++++++++++++++++++----------------------------
>  1 file changed, 41 insertions(+), 43 deletions(-)
> 
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -80,17 +80,12 @@ bool irq_work_queue_on(struct irq_work *
>         if (!irq_work_claim(work))
>                 return false;
>  
> -#ifdef CONFIG_PREEMPT_RT_FULL
> -       if (work->flags & IRQ_WORK_HARD_IRQ)
> +       if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL) && (work->flags & IRQ_WORK_HARD_IRQ))
>                 raise_irqwork = llist_add(&work->llnode,
>                                           &per_cpu(hirq_work_list, cpu));
>         else
>                 raise_irqwork = llist_add(&work->llnode,
> -                                         &per_cpu(lazy_list, cpu));
> -#else
> -               raise_irqwork = llist_add(&work->llnode,
>                                           &per_cpu(raised_list, cpu));
> -#endif
>  
>         if (raise_irqwork)
>                 arch_send_call_function_single_ipi(cpu);
> @@ -103,6 +98,9 @@ EXPORT_SYMBOL_GPL(irq_work_queue_on);
>  /* Enqueue the irq work @work on the current CPU */
>  bool irq_work_queue(struct irq_work *work)
>  {
> +       bool realtime = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
> +       bool raise = false;
> +
>         /* Only queue if not already pending */
>         if (!irq_work_claim(work))
>                 return false;
> @@ -110,25 +108,22 @@ bool irq_work_queue(struct irq_work *wor
>         /* Queue the entry and raise the IPI if needed. */
>         preempt_disable();
>  
> -#ifdef CONFIG_PREEMPT_RT_FULL
> -       if (work->flags & IRQ_WORK_HARD_IRQ) {
> +       if (realtime && (work->flags & IRQ_WORK_HARD_IRQ)) {
>                 if (llist_add(&work->llnode, this_cpu_ptr(&hirq_work_list)))
> -                       arch_irq_work_raise();
> -       } else {
> -               if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
> -                   tick_nohz_tick_stopped())
> -                       raise_softirq(TIMER_SOFTIRQ);
> -       }
> -#else
> -       if (work->flags & IRQ_WORK_LAZY) {
> +                       raise = 1;
> +       } else if (work->flags & IRQ_WORK_LAZY) {
>                 if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
> -                   tick_nohz_tick_stopped())
> -                       arch_irq_work_raise();
> -       } else {
> -               if (llist_add(&work->llnode, this_cpu_ptr(&raised_list)))
> -                       arch_irq_work_raise();
> -       }
> -#endif
> +                   tick_nohz_tick_stopped()) {
> +                       if (realtime)
> +                               raise_softirq(TIMER_SOFTIRQ);
> +                       else
> +                               raise = true;
> +               }
> +       } else if (llist_add(&work->llnode, this_cpu_ptr(&raised_list)))
> +               raise = true;
> +
> +       if (raise)
> +               arch_irq_work_raise();
>  
>         preempt_enable();
>  
> @@ -143,12 +138,13 @@ bool irq_work_needs_cpu(void)
>         raised = this_cpu_ptr(&raised_list);
>         lazy = this_cpu_ptr(&lazy_list);
>  
> -       if (llist_empty(raised))
> -               if (llist_empty(lazy))
> -#ifdef CONFIG_PREEMPT_RT_FULL
> +       if (llist_empty(raised) && llist_empty(lazy)) {
> +               if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
>                         if (llist_empty(this_cpu_ptr(&hirq_work_list)))
> -#endif
>                                 return false;
> +               } else
> +                       return false;
> +       }
>  
>         /* All work should have been flushed before going offline */
>         WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
> @@ -162,9 +158,7 @@ static void irq_work_run_list(struct lli
>         struct irq_work *work;
>         struct llist_node *llnode;
>  
> -#ifndef CONFIG_PREEMPT_RT_FULL
> -       BUG_ON(!irqs_disabled());
> -#endif
> +       BUG_ON(!IS_ENABLED(CONFIG_PREEMPT_RT_FULL) && !irqs_disabled());
>  
>         if (llist_empty(list))
>                 return;
> @@ -200,26 +194,30 @@ static void irq_work_run_list(struct lli
>   */
>  void irq_work_run(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT_FULL
> -       irq_work_run_list(this_cpu_ptr(&hirq_work_list));
> -#else
> -       irq_work_run_list(this_cpu_ptr(&raised_list));
> -       irq_work_run_list(this_cpu_ptr(&lazy_list));
> -#endif
> +       if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
> +               irq_work_run_list(this_cpu_ptr(&hirq_work_list));
> +               /*
> +                * NOTE: we raise softirq via IPI for safety,
> +                * and execute in irq_work_tick() to move the
> +                * overhead from hard to soft irq context.
> +                */
> +               if (!llist_empty(this_cpu_ptr(&raised_list)))
> +                       raise_softirq(TIMER_SOFTIRQ);
> +       } else {
> +               irq_work_run_list(this_cpu_ptr(&raised_list));
> +               irq_work_run_list(this_cpu_ptr(&lazy_list));
> +       }
>  }
>  EXPORT_SYMBOL_GPL(irq_work_run);
>  
>  void irq_work_tick(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT_FULL
> -       irq_work_run_list(this_cpu_ptr(&lazy_list));
> -#else
> -       struct llist_head *raised = &__get_cpu_var(raised_list);
> +       struct llist_head *raised = this_cpu_ptr(&raised_list);
>  
> -       if (!llist_empty(raised) && !arch_irq_work_has_interrupt())
> +       if (!llist_empty(raised) && (!arch_irq_work_has_interrupt() ||
> +           IS_ENABLED(CONFIG_PREEMPT_RT_FULL)))

OK, that additional condition is addressing archs that don't have
irq_work support and fall back to the timer, right?

>                 irq_work_run_list(raised);
> -       irq_work_run_list(&__get_cpu_var(lazy_list));
> -#endif
> +       irq_work_run_list(this_cpu_ptr(&lazy_list));
>  }
>  
>  /*
> 

The patch is whitespace-damaged - could you resent? I'm trying to
visualize the full diff for me.

Thanks,
Jan

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