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 <[email protected]> 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 <[email protected]> --- 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
