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Date: Thu, 23 Apr 2015 08:29:48 +0200
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Mike Galbraith <umgwanakikbuti@...il.com>
CC: Steven Rostedt <rostedt@...dmis.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
RT <linux-rt-users@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue
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@...mens.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@...mens.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
--
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
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