linux-kernel - Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

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Date:	Thu, 31 Jul 2014 17:55:46 -0700
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Prarit Bhargava <prarit@...hat.com>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Lenny Szubowicz <lszubowi@...hat.com>,
	linux-pm@...r.kernel.org, Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem
 to be held for duration of changing governors [v2]

On 07/31/2014 03:58 PM, Prarit Bhargava wrote:
>
>
> On 07/31/2014 06:13 PM, Saravana Kannan wrote:
>> On 07/31/2014 02:08 PM, Prarit Bhargava wrote:
>>>
>>>
>>> On 07/31/2014 04:38 PM, Saravana Kannan wrote:
>>>> On 07/31/2014 01:30 PM, Prarit Bhargava wrote:
>>>>>
>>>>>
>>>>> On 07/31/2014 04:24 PM, Saravana Kannan wrote:
>>>>>>
>>>>>> Prarit,
>>>>>>
>>>>>> I'm not an expert on sysfs locking, but I would think the specific sysfs lock
>>>>>> would depend on the file/attribute group. So, can you please try to hotplug a
>>>>>> core in/out (to trigger the POLICY_EXIT) and then read a sysfs file
>>>>>> exported by
>>>>>> the governor? scaling_governor doesn't cut it since that file is not
>>>>>> removed on
>>>>>> policy exit event to governor. If it's ondemand, try reading/write it's
>>>>>> sampling
>>>>>> rate file.
>>>>>
>>>>> Thanks Saravana -- will do.  I will get back to you shortly on this.
>>>>>
>>>>
>>>> Thanks. Btw, in case you weren't already aware of it. You'll have to hoplug out
>>>> all the CPUs in a cluster to trigger a POLICY_EXIT for that cluster/policy.
>>>
>>> Yep -- the affected_cpus file should show all the cpus in the policy IIRC.  One
>>> of the systems I have has 1 cpu/policy and has 48 threads so the POLICY_EXIT is
>>> called.
>>>
>>> I'll put something like
>>>
>>> while [1];
>>> do
>>> echo ondemand > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor
>>> cat /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate
>>> echo 20000 > /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate
>>> cat /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate
>>> echo 0 > /sys/devices/system/cpu/cpu1/online
>>> sleep 1
>>> echo 1 > /sys/devices/system/cpu/cpu1/online
>>> sleep 1
>>> done
>>>
>>
>> The actual race can only happen with 2 threads. I'm just trying to trigger a
>> lockdep warning here.
>
> I ran the above in two separate terminals with cpuset -c 0 and cpuset -c 1 to
> multi-thread it all.  No deadlock or LOCKDEP trace after about 1/2 hour, so I
> think we're in the clear on that concern.
>

I wasn't convinced. So, I took some help from Stephen to test it.

It's been a while, so I didn't remember the original issue clearly when 
I gave you some test suggestions. Now that I looked at the code more 
closely, I have a proper way to reproduce the original issue.

Nack for this patch for 2 reasons:
1. You seem to have accidentally removed a GOV_STOP in your patch. We 
definitely can't do that. This broke changing governors and that's why 
your patch didn't cause any issues. Because all your governor echos were 
failing.
2. When we fixed that and actually tried a proper test (not the one I 
gave you), we reproduced the original issue.

To reproduce original issue:
Preconditions:
* lockdep is enabled
* governor per policy is enabled

Steps:
1. Set governor to ondemand.
2. Cat one of the ondemand sysfs files.
3. Change governor to conservative.

When you do that, there's an AB, BA dead lock issue with one thread 
trying to cat a governor sysfs file and another thread trying to change 
governors.

-Saravana

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