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

From: Prarit Bhargava
Date: Thu Jul 31 2014 - 17:08:21 EST

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

I'll put something like

while [1];
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

and let it run unless you can think of something else. FWIW, manually trying
that yields no problems AFAICT. The sleeps are just me being paranoid and
keeping things in sequence.

> -Saravana
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