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

[Prarit Bhargava <prarit@redhat.com>]

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.

P.