Re: [PATCH 01/10] alpha: Add missing RCU idle APIs on idle loop

On Wed, Aug 22, 2012 at 10:19:30AM -0700, Paul E. McKenney wrote:

On Wed, Aug 22, 2012 at 06:23:39PM +0200, Frederic Weisbecker wrote:

quoted

In the old times, the whole idle task was considered
as an RCU quiescent state. But as RCU became more and
more successful overtime, some RCU read side critical
section have been added even in the code of some
architectures idle tasks, for tracing for example.

So nowadays, rcu_idle_enter() and rcu_idle_exit() must
be called by the architecture to tell RCU about the part
in the idle loop that doesn't make use of rcu read side
critical sections, typically the part that puts the CPU
in low power mode.

This is necessary for RCU to find the quiescent states in
idle in order to complete grace periods.

Add this missing pair of calls in the Alpha's idle loop.

Reported-by: Paul E. McKenney <redacted>
Signed-off-by: Frederic Weisbecker <redacted>
Cc: Richard Henderson <redacted>
Cc: Ivan Kokshaysky <redacted>
Cc: Matt Turner <mattst88@gmail.com>
Cc: alpha <redacted>
Cc: Paul E. McKenney <redacted>
Cc: 3.2.x.. <stable@kernel.org>
---
 arch/alpha/kernel/process.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/arch/alpha/kernel/process.c b/arch/alpha/kernel/process.c
index 153d3fc..2ebf7b5 100644
--- a/arch/alpha/kernel/process.c
+++ b/arch/alpha/kernel/process.c

@@ -28,6 +28,7 @@
 #include <linux/tty.h>
 #include <linux/console.h>
 #include <linux/slab.h>
+#include <linux/rcupdate.h>

 #include <asm/reg.h>
 #include <asm/uaccess.h>

@@ -50,13 +51,16 @@ cpu_idle(void)
 {
 	set_thread_flag(TIF_POLLING_NRFLAG);

+	preempt_disable();

I don't understand the above preempt_disable() not having a matching
preempt_enable() at exit, but the rest of the patches in this series
look good to me.

The current code is preemptable, at least it appears so because it calls
schedule() directly. And if I call rcu_idle_enter() in a preemptable section,
I'm in trouble because I'll schedule while in extended QS.

Thus I need to disable preemption here at least until I call rcu_idle_exit().

Now this is an endless loop so there is no need to re-enable
preemption after the loop. And schedule_preempt_disabled()
takes care of enabling preemption before schedule() and redisabling
it afterward.

							Thanx, Paul

quoted

 	while (1) {
 		/* FIXME -- EV6 and LCA45 know how to power down
 		   the CPU.  */

+		rcu_idle_enter();
 		while (!need_resched())
 			cpu_relax();
-		schedule();
+		rcu_idle_exit();
+		schedule_preempt_disabled();
 	}
 }

-- 
1.7.5.4