linux-kernel - full no_hz: how works arch_irq_work_raise() with tick off on generic implementation

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Message-ID: <20150309140917.GB13768@linutronix.de>
Date:	Mon, 9 Mar 2015 15:09:17 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: full no_hz: how works arch_irq_work_raise() with tick off on generic
 implementation

I just stumbled over this in -RT. The code:

 87 bool irq_work_queue(struct irq_work *work)
 88 {
…
 96         /* If the work is "lazy", handle it from next tick if any */
 97         if (work->flags & IRQ_WORK_LAZY) {
 98                 if (llist_add(&work->llnode, this_cpu_ptr(&lazy_list)) &&
 99                     tick_nohz_tick_stopped())
100                         arch_irq_work_raise();
101         } else {
102                 if (llist_add(&work->llnode, this_cpu_ptr(&raised_list)))
103                         arch_irq_work_raise();
104         }
…
109 }

so what I asked myself: What happens if the CPU is in NO_HZ mode with
the tick off and you invoke arch_irq_work_raise() on an architecture
without a special IRQ_WORK interrupt?
And why you don't this tick_nohz_tick_stopped() check for the
raised_list?
Wouldn't it be a easier for your full NO_HZ code to simply run in the
hardirq exit path instead of using irq_work?

Sebastian
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