Re: [PATCH RESEND v10] sched/deadline: support dl task migration during cpu hotplug

[Wanpeng Li]

Hi Juri,
On Tue, Mar 24, 2015 at 09:27:09AM +0000, Juri Lelli wrote:
>Hi,
>
>On 23/03/2015 08:55, Peter Zijlstra wrote:
>> On Mon, Mar 23, 2015 at 08:25:04AM +0100, Ingo Molnar wrote:
>>>>>>>>>> +			if (cpu >= nr_cpu_ids) {
>>>>>>>>>> +				if (dl_bandwidth_enabled()) {
>>>>>>>>>> +					/*
>>>>>>>>>> +					 * Fail to find any suitable cpu.
>>>>>>>>>> +					 * The task will never come back!
>>>>>>>>>> +					 */
>>>>>>>>>> +					WARN_ON(1);
>>>>>>>>>
>>>>>>>>> Can this condition happen to users with a non-buggy kernel?
>> 
>>> I still haven't seen a satisfactory answer to this question. Please 
>>> don't resend patches without clearing questions raised during review.
>> 
>> So I had a look on Friday, it _should_ not happen, but it does due to a
>> second bug Juri is currently chasing down.
>> 
>
>Right, it should not happen. It happens because hotplug operations are
>destructive w.r.t. cpusets. Peter, how about we move the check you put
>in sched_cpu_inactive() to cpuset_cpu_inactive()? This way, if we fail,
>we don't need to destroy/rebuild the domains.

I remember you mentioned that there is a bug through IRC last week, if this 
patch solve it?

Regards,
Wanpeng Li 

>
>Thanks,
>
>- Juri
>
>>From 65e8033e05f8b70116747062d00d5a5c266699fb Mon Sep 17 00:00:00 2001
>From: Juri Lelli <juri.lelli@xxxxxxxxx>
>Date: Tue, 24 Mar 2015 07:47:03 +0000
>Subject: [PATCH] sched/core: check for available -dl bandwidth in
> cpuset_cpu_inactive
>
>Signed-off-by: Juri Lelli <juri.lelli@xxxxxxx>
>---
> kernel/sched/core.c | 56 ++++++++++++++++++++++++++---------------------------
> 1 file changed, 28 insertions(+), 28 deletions(-)
>
>diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>index 50927eb..3723ad0 100644
>--- a/kernel/sched/core.c
>+++ b/kernel/sched/core.c
>@@ -5318,36 +5318,13 @@ static int sched_cpu_active(struct notifier_block *nfb,
> static int sched_cpu_inactive(struct notifier_block *nfb,
> 					unsigned long action, void *hcpu)
> {
>-	unsigned long flags;
>-	long cpu = (long)hcpu;
>-	struct dl_bw *dl_b;
>-
> 	switch (action & ~CPU_TASKS_FROZEN) {
> 	case CPU_DOWN_PREPARE:
>-		set_cpu_active(cpu, false);
>-
>-		/* explicitly allow suspend */
>-		if (!(action & CPU_TASKS_FROZEN)) {
>-			bool overflow;
>-			int cpus;
>-
>-			rcu_read_lock_sched();
>-			dl_b = dl_bw_of(cpu);
>-
>-			raw_spin_lock_irqsave(&dl_b->lock, flags);
>-			cpus = dl_bw_cpus(cpu);
>-			overflow = __dl_overflow(dl_b, cpus, 0, 0);
>-			raw_spin_unlock_irqrestore(&dl_b->lock, flags);
>-
>-			rcu_read_unlock_sched();
>-
>-			if (overflow)
>-				return notifier_from_errno(-EBUSY);
>-		}
>+		set_cpu_active((long)hcpu, false);
> 		return NOTIFY_OK;
>+	default:
>+		return NOTIFY_DONE;
> 	}
>-
>-	return NOTIFY_DONE;
> }
> 
> static int __init migration_init(void)
>@@ -7001,7 +6978,6 @@ static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action,
> 		 */
> 
> 	case CPU_ONLINE:
>-	case CPU_DOWN_FAILED:
> 		cpuset_update_active_cpus(true);
> 		break;
> 	default:
>@@ -7013,8 +6989,32 @@ static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action,
> static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action,
> 			       void *hcpu)
> {
>-	switch (action) {
>+	unsigned long flags;
>+	long cpu = (long)hcpu;
>+	struct dl_bw *dl_b;
>+
>+	switch (action & ~CPU_TASKS_FROZEN) {
> 	case CPU_DOWN_PREPARE:
>+		/* explicitly allow suspend */
>+		if (!(action & CPU_TASKS_FROZEN)) {
>+			bool overflow;
>+			int cpus;
>+
>+			rcu_read_lock_sched();
>+			dl_b = dl_bw_of(cpu);
>+
>+			raw_spin_lock_irqsave(&dl_b->lock, flags);
>+			cpus = dl_bw_cpus(cpu);
>+			overflow = __dl_overflow(dl_b, cpus, 0, 0);
>+			raw_spin_unlock_irqrestore(&dl_b->lock, flags);
>+
>+			rcu_read_unlock_sched();
>+
>+			if (overflow) {
>+				trace_printk("hotplug failed for cpu %lu", cpu);
>+				return notifier_from_errno(-EBUSY);
>+			}
>+		}
> 		cpuset_update_active_cpus(false);
> 		break;
> 	case CPU_DOWN_PREPARE_FROZEN:
>-- 
>2.3.0
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