linux-kernel - Re: NOHZ: WARNING: at arch/x86/kernel/smp.c:123 native_smp_send

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Date:	Mon, 20 May 2013 15:25:04 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Borislav Petkov <bp@...en8.de>, Tejun Heo <tj@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, rjw@...k.pl,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: NOHZ: WARNING: at arch/x86/kernel/smp.c:123 native_smp_send_reschedule,
 round 2

Hi, Viresh
On 05/20/2013 03:12 PM, Viresh Kumar wrote:
> Hi Michael,
> 
> I haven't followed this mail chain earlier and saw this mail only as I am
> added in cc now. I probably have answers to few questions here:

Thanks for your quick respond :)

> 
> On 20 May 2013 12:36, Michael Wang <wangyun@...ux.vnet.ibm.com> wrote:
>> On 05/20/2013 02:58 PM, Michael Wang wrote:
>>> On 05/20/2013 02:47 PM, Borislav Petkov wrote:
>>>> On Mon, May 20, 2013 at 02:23:37PM +0800, Michael Wang wrote:
>>>>> On 05/20/2013 12:50 PM, Borislav Petkov wrote:
>>>>> So there are two questions here:
>>>>> 1. Is gov_queue_work() want to queue the work on offline cpu?
> 
> No. We are only working with online cpus now in cpufreq core and governors.
> 
>>> Besides, the cpu gov_queue_work() is using 'policy->cpus' which seems to
>>> be updated during UP DOWN notify, I think they are supposed to be online.
>>>
>>> But we need expert in cpufreq to confirm all these...
> 
> I confirm this. policy->cpus only contains online cpus.. and
> policy->related_cpus
> always contain online+offline cpus.

Nice to be confirmed :)

> 
>> And I guess this may help to reduce the chance to trigger WARN:
>>
>> diff --git a/drivers/cpufreq/cpufreq_governor.c
>> b/drivers/cpufreq/cpufreq_governor.c
>> index 443442d..0f96013 100644
>> --- a/drivers/cpufreq/cpufreq_governor.c
>> +++ b/drivers/cpufreq/cpufreq_governor.c
>> @@ -180,7 +180,7 @@ void gov_queue_work(struct dbs_data *dbs_data,
>> struct cpufreq_policy *policy,
>>         if (!all_cpus) {
>>                 __gov_queue_work(smp_processor_id(), dbs_data, delay);
>>         } else {
>> -               for_each_cpu(i, policy->cpus)
>> +               for_each_cpu_and(i, policy->cpus, cpu_online_mask)
>>                         __gov_queue_work(i, dbs_data, delay);
>>         }
>>  }
> 
> Not required at all... policy->cpus is guaranteed to have only online cpus.

Yeah, that's right, I guess the issue is, although the policy->cpus is
correct at a given time, after get cpu from it, it's possible to be
changed, unless we disabled preempt or irq, or hotplug before we use it...

Like such issue cases:
				get x from policy->cpus
	DOWN notifier
	change policy->cpus
	do offline x
				send ipi to x

Will that happen?

Regards,
Michael Wang


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