linux-kernel - Re: [PATCH v2 1/2] sched: fix init NOHZ

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Date:	Fri, 8 Feb 2013 16:35:38 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	peterz@...radead.org, mingo@...nel.org,
	Mike Galbraith <efault@....de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 1/2] sched: fix init NOHZ_IDLE flag

2013/2/4 Vincent Guittot <vincent.guittot@...aro.org>:
> On 1 February 2013 19:03, Frederic Weisbecker <fweisbec@...il.com> wrote:
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index 257002c..fd41924 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -5884,6 +5884,7 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd)
>>>
>>>         update_group_power(sd, cpu);
>>>         atomic_set(&sg->sgp->nr_busy_cpus, sg->group_weight);
>>> +       clear_bit(NOHZ_IDLE, nohz_flags(cpu));
>>
>> So that's a real issue indeed.  nr_busy_cpus was never correct.
>>
>> Now I'm still a bit worried with this solution. What if an idle task
>> started in smp_init() has not yet stopped its tick, but is about to do
>> so? The domains are not yet available to the task but the nohz flags
>> are. When it later restarts the tick, it's going to erroneously
>> increase nr_busy_cpus.
>
> My 1st idea was to clear NOHZ_IDLE flag and nr_busy_cpus in
> init_sched_groups_power instead of setting them as it is done now. If
> a CPU enters idle during the init sequence, the flag is already
> cleared, and nohz_flags and nr_busy_cpus will stay synced and cleared
> while a NULL sched_domain is attached to the CPU thanks to patch 2.
> This should solve all use cases ?

This may work on smp_init(). But the per cpu domain can be changed concurrently
anytime on cpu hotplug, with a new sched group power struct, right?

What if the following happen (inventing function names but you get the idea):

CPU 0                                           CPU 1

dom = new_domain(...) {
           nr_cpus_busy = 0;
           set_idle(CPU 1);                  old_dom =get_dom()
                                                     clear_idle(CPU 1)
}
rcu_assign_pointer(cpu1_dom, dom);


Can this scenario happen?


>>
>> It probably won't happen in practice. But then there is more: sched
>> domains can be concurrently rebuild anytime, right?  So what if we
>> call set_cpu_sd_state_idle() and decrease nr_busy_cpus while the
>> domain is switched concurrently. Are we having a new sched group along
>> the way? If so we have a bug here as well because we can have
>> NOHZ_IDLE set but nr_busy_cpus accounting the CPU.
>
> When the sched_domain are rebuilt, we set a null sched_domain during
> the rebuild sequence and a new sched_group_power is created as well

So at that time we may race with a CPU setting/clearing its NOHZ_IDLE flag
as in my above scenario?
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