linux-kernel - [RFC PATCH 8/9] irq_work: Handle queuing without IPI support in dyntick idle mode

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Date:	Mon, 29 Oct 2012 14:28:15 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: [RFC PATCH 8/9] irq_work: Handle queuing without IPI support in dyntick idle mode

If we enqueue a work while in dyntick idle mode and the arch doesn't
have self-IPI support, we may not find an opportunity to run the work
before a while.

In this case, exit the idle loop to re-evaluate irq_work_needs_cpu()
and restart the tick.

Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 kernel/irq_work.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index ce72b20..b500271 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -76,6 +76,17 @@ static void __irq_work_queue(struct irq_work *work)
 		 */
 		if (!arch_irq_work_has_ipi() || tick_nohz_tick_stopped())
 			arch_irq_work_raise();
+
+		/*
+		 * If we rely on the timer tick or some obscure way to run the work
+		 * while the CPU is in dyntick idle mode, we may not have an opportunity
+		 * to do so before a while. Let's just exit the idle loop and hope we
+		 * haven't yet reached the last need_resched() check before the CPU goes
+		 * to low power mode.
+		 */
+		if (!arch_irq_work_has_ipi() && tick_nohz_tick_stopped()
+		    && is_idle_task(current))
+			set_need_resched();
 	}
 out:
 	preempt_enable();
-- 
1.7.5.4

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