lkml.org 
[lkml]   [2015]   [Feb]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
SubjectRe: [rcu] [ INFO: suspicious RCU usage. ]
From
Date
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.

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
\
 
 \ /
  Last update: 2015-02-04 16:41    [W:0.073 / U:3.404 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site