Hi, Cpufreq Guys
We meet one race condition for cpufreq governor start/stop during
hotplug and manually governor change.
we have 4 cpus and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
the normal sequence is as below:
1) Current governor is userspace, one application tries to set
governor to ondemand. it will call __cpufreq_set_policy in which it
will stop userspace governor and then start ondemand governor.
2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
call cpufreq_add_policy_cpu. on which it first stop governor
userspace, and then start governor userspace.
Now if the sequence of abover two cases interleaves, it becames below sequence:
1) application stops userspace governor
2)
hotplug stops userspace governor
3) application starts ondemand governor
4)
hotplug starts a governor
in step 4, hotplug is supposed to start userspace governor, but now
the governor has been changed by application to ondemand, so hotplug
starts ondemand governor again !!!!
there are some consequences for this issue:
ondemand governor will be started for multi-times which causes its
timer be inialized multitimes, if we use object debug to trace this,
we get a waring that the timer object is alive but it is initialized
again.
The issue happens because we use one same cpufreq policy for all cpus,
once the policy is changed, all cpus will be affected. I tried to add
a mutex lock before governor stop and after governor start. but it
cannot solve the problem completely as there maybe some other places
that changes the cpufreq policy, if it doesn't use the same mutex
lock, policy may still be changed during stop/start sequence. the
final governor may still not the expected one.
Do we have some discussion for this issue before ? any suggestions for this?
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