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

From: Saravana Kannan
Date: Thu Jul 31 2014 - 18:14:08 EST

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:


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

The actual race can only happen with 2 threads. I'm just trying to trigger a lockdep warning here.


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