linux-kernel - Re: [PATCH RFC idle] Make arm, sh, and x86 stop using RCU when idle

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Date:	Thu, 2 Feb 2012 11:29:59 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org
Subject: Re: [PATCH RFC idle] Make arm, sh, and x86 stop using RCU when idle

On Wed, Feb 01, 2012 at 04:42:53PM -0800, Paul E. McKenney wrote:
> Hello!
> 
> RCU's shiny new diagnostics (thank you, Frederic!) for using RCU when idle
> located a few problems in arm, sh, and x86.  This patch series contains
> alleged fixes for these problems.  And they are real problems -- if RCU
> believes that the CPU is idle, it is ignoring it.  Which means that the
> idle CPU can say "rcu_read_lock()" all it like, but there will be no
> useful effect.
> 
> I was tempted to break these up, but doing so is bad for bisectability.
> 
Presumably the same changes will also need to be reflected in cpuidle?

If so, here's a start:

Signed-off-by: Paul Mundt <lethal@...ux-sh.org>

---

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 59f4261..97adcd4 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -18,6 +18,7 @@
 #include <linux/ktime.h>
 #include <linux/hrtimer.h>
 #include <linux/module.h>
+#include <linux/rcupdate.h>
 #include <trace/events/power.h>
 
 #include "cpuidle.h"
@@ -89,6 +90,8 @@ int cpuidle_idle_call(void)
 	next_state = cpuidle_curr_governor->select(drv, dev);
 	if (need_resched()) {
 		local_irq_enable();
+		rcu_idle_enter();
+		rcu_idle_exit();
 		return 0;
 	}
 
@@ -96,9 +99,11 @@ int cpuidle_idle_call(void)
 
 	trace_power_start(POWER_CSTATE, next_state, dev->cpu);
 	trace_cpu_idle(next_state, dev->cpu);
+	rcu_idle_enter();
 
 	entered_state = target_state->enter(dev, drv, next_state);
 
+	rcu_idle_exit();
 	trace_power_end(dev->cpu);
 	trace_cpu_idle(PWR_EVENT_EXIT, dev->cpu);
 
@@ -173,8 +178,10 @@ static int poll_idle(struct cpuidle_device *dev,
 
 	t1 = ktime_get();
 	local_irq_enable();
+	rcu_idle_enter();
 	while (!need_resched())
 		cpu_relax();
+	rcu_idle_exit();
 
 	t2 = ktime_get();
 	diff = ktime_to_us(ktime_sub(t2, t1));
--
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