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Subject: Re: [rcu] [ INFO: suspicious RCU usage. ]
From: Krzysztof Kozlowski <k.kozlowski@samsung.com>
To: paulmck@linux.vnet.ibm.com
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Fengguang Wu <fengguang.wu@intel.com>, LKP <lkp@01.org>,
        linux-kernel@vger.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        linux-arm-kernel@lists.infradead.org, Arnd Bergmann <arnd@arndb.de>,
        MarkRutland <mark.rutland@arm.com>
Date: Wed, 04 Feb 2015 16:22:28 +0100
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On śro, 2015-02-04 at 07:10 -0800, Paul E. McKenney wrote:
> On Wed, Feb 04, 2015 at 03:16:27PM +0100, Krzysztof Kozlowski wrote:
> > On śro, 2015-02-04 at 05:14 -0800, Paul E. McKenney wrote:
> > > On Wed, Feb 04, 2015 at 01:00:18PM +0000, Russell King - ARM Linux wrote:
> > > > On Wed, Feb 04, 2015 at 12:39:07PM +0100, Krzysztof Kozlowski wrote:
> > > > > +Cc some ARM people
> > > > 
> > > > I wish that people would CC this list with problems seen on ARM.  I'm
> > > > minded to just ignore this message because of this in the hope that by
> > > > doing so, people will learn something...
> > > > 
> > > > > > Another thing I could do would be to have an arch-specific Kconfig
> > > > > > variable that made ARM responsible for informing RCU that the CPU
> > > > > > was departing, which would allow a call to as follows to be placed
> > > > > > immediately after the complete():
> > > > > > 
> > > > > > rcu_cpu_notify(NULL, CPU_DYING_IDLE, (void *)(long)smp_processor_id());
> > > > > > 
> > > > > > Note:  This absolutely requires that the rcu_cpu_notify() -always-
> > > > > > be allowed to execute!!!  This will not work if there is -any- possibility
> > > > > > of __cpu_die() powering off the outgoing CPU before the call to
> > > > > > rcu_cpu_notify() returns.
> > > > 
> > > > Exactly, so that's not going to be possible.  The completion at that
> > > > point marks the point at which power _could_ be removed from the CPU
> > > > going down.
> > > 
> > > OK, sounds like a polling loop is required.
> > 
> > I thought about using wait_on_bit() in __cpu_die() (the waiting thread)
> > and clearing the bit on CPU being powered down. What do you think about
> > such idea?
> 
> Hmmm...  It looks to me that wait_on_bit() calls out_of_line_wait_on_bit(),
> which in turn calls __wait_on_bit(), which calls prepare_to_wait() and
> finish_wait().  These are in the scheduler, but this is being called from
> the CPU that remains online, so that should be OK.
> 
> But what do you invoke on the outgoing CPU?  Can you get away with
> simply clearing the bit, or do you also have to do a wakeup?  It looks
> to me like a wakeup is required, which would be illegal on the outgoing
> CPU, which is at a point where it cannot legally invoke the scheduler.
> Or am I missing something?

Actually the timeout versions but I think that doesn't matter.
The wait_on_bit will busy-loop with testing for the bit. Inside the loop
it calls the 'action' which in my case will be bit_wait_io_timeout().
This calls schedule_timeout().

See proof of concept in attachment. One observed issue: hot unplug from
commandline takes a lot more time. About 7 seconds instead of ~0.5.
Probably I did something wrong.

> 
> You know, this situation is giving me a bad case of nostalgia for the
> old Sequent Symmetry and NUMA-Q hardware.  On those platforms, the
> outgoing CPU could turn itself off, and thus didn't need to tell some
> other CPU when it was ready to be turned off.  Seems to me that this
> self-turn-off capability would be a great feature for future systems!

There are a lot more issues with hotplug on ARM...

Patch/RFC attached.


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>From feaad18a483871747170fa797f80b49592489ad1 Mon Sep 17 00:00:00 2001
From: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Date: Wed, 4 Feb 2015 16:14:41 +0100
Subject: [RFC] ARM: Don't use complete() during __cpu_die

The complete() should not be used on offlined CPU.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
---
 arch/arm/kernel/smp.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 86ef244c5a24..f3a5ad80a253 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -26,6 +26,7 @@
 #include <linux/completion.h>
 #include <linux/cpufreq.h>
 #include <linux/irq_work.h>
+#include <linux/wait.h>
 
 #include <linux/atomic.h>
 #include <asm/smp.h>
@@ -76,6 +77,9 @@ enum ipi_msg_type {
 
 static DECLARE_COMPLETION(cpu_running);
 
+#define CPU_DIE_WAIT_BIT		0
+static unsigned long wait_cpu_die;
+
 static struct smp_operations smp_ops;
 
 void __init smp_set_ops(struct smp_operations *ops)
@@ -133,7 +137,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
 		pr_err("CPU%u: failed to boot: %d\n", cpu, ret);
 	}
 
-
+	set_bit(CPU_DIE_WAIT_BIT, &wait_cpu_die);
 	memset(&secondary_data, 0, sizeof(secondary_data));
 	return ret;
 }
@@ -213,7 +217,17 @@ int __cpu_disable(void)
 	return 0;
 }
 
-static DECLARE_COMPLETION(cpu_died);
+static int wait_for_cpu_die(void)
+{
+	might_sleep();
+
+	if (!test_bit(CPU_DIE_WAIT_BIT, &wait_cpu_die))
+		return 0;
+
+	return out_of_line_wait_on_bit_timeout(&wait_cpu_die, CPU_DIE_WAIT_BIT,
+					bit_wait_timeout, TASK_UNINTERRUPTIBLE,
+					msecs_to_jiffies(5000));
+}
 
 /*
  * called on the thread which is asking for a CPU to be shutdown -
@@ -221,7 +235,7 @@ static DECLARE_COMPLETION(cpu_died);
  */
 void __cpu_die(unsigned int cpu)
 {
-	if (!wait_for_completion_timeout(&cpu_died, msecs_to_jiffies(5000))) {
+	if (wait_for_cpu_die()) {
 		pr_err("CPU%u: cpu didn't die\n", cpu);
 		return;
 	}
@@ -267,7 +281,7 @@ void __ref cpu_die(void)
 	 * this returns, power and/or clocks can be removed at any point
 	 * from this CPU and its cache by platform_cpu_kill().
 	 */
-	complete(&cpu_died);
+	clear_bit(CPU_DIE_WAIT_BIT, &wait_cpu_die);
 
 	/*
 	 * Ensure that the cache lines associated with that completion are
-- 
1.9.1


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