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 15:13:58 -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
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem
 to be held for duration of changing governors [v2]

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.

-Saravana

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