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Date: Mon, 10 Dec 2012 13:12:13 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: rjw@...k.pl
Cc: linaro-dev@...ts.linaro.org, nicolas.pitre@...aro.org,
amit.kucheria@...aro.org, mathieu.poirier@...aro.org,
vishal.bhoj@...aro.org, linux-kernel@...r.kernel.org,
cpufreq@...r.kernel.org, Viresh Kumar <viresh.kumar@...aro.org>
Subject: [RFC] cpufreq: Fix issues with hot-[un]plugging of cpus
Hi Rafael and other cpufreq masters,
I am really not sure if this patch is a HACK or a valid solution :)
Please have a look.
Setup:
-----
- ARM big LITTLE Platform with 5 cpus. cpus 2, 3 & 4 share clock and hence are
part of policy->cpus. cpus 0 & 1 share clock but they aren't discussed here.
- Interactive governor: I know its not mainlined and will never be. Problem
should be same with all the governors we have. So, problem is not really
governor dependent. I have added few prints at start/stop cases of governor,
to see what policy->cpus is set to.
Problem Statement:
-----------------
There are many issues :(
1) I couldn't understand what we really want the value of policy->cpus be? Only
the online cpus sharing clock? or all the possible cpus sharing clock?
When we offline a cpu, we remove it from cpus field: cpumask_clear_cpu(cpu, data->cpus);
Which shows that it is mask of online cpus only. But there are many places
checking if a cpu is online or not (cpu in policy->cpus) in cpufreq.c. Which
shows something else.
2) Kernel crashes when hot-unplugging cpus
When we do: for i in 2 3 4; do echo 0 > /sys/devices/system/cpu/cpu$i/online; done
We get:
cpufreq_governor_interactive: CPUFREQ_GOV_STOP cpu 2 policy->cpus 2-4
cpufreq_vexpress: CPUFreq for CPU 3 initialized
cpufreq_governor_interactive: CPUFREQ_GOV_START cpu 3 policy->cpus 2-4
CPU2: shutdown
cpufreq_governor_interactive: CPUFREQ_GOV_STOP cpu 3 policy->cpus 3-4
cpufreq_vexpress: CPUFreq for CPU 4 initialized
cpufreq_governor_interactive: CPUFREQ_GOV_START cpu 4 policy->cpus 2-4
And then crash from interactive governor.
Note the following lines:
------------------------
cpufreq_governor_interactive: CPUFREQ_GOV_STOP cpu 2 policy->cpus 2-4
cpufreq_governor_interactive: CPUFREQ_GOV_START cpu 3 policy->cpus 2-4
cpufreq_governor_interactive: CPUFREQ_GOV_STOP cpu 3 policy->cpus 3-4
cpufreq_governor_interactive: CPUFREQ_GOV_START cpu 4 policy->cpus 2-4
So the governor is always started for CPUs 2 through 4 despite the fact
that CPU 2 is removed. This makes the governor unstable due to wrong info
passed.
FIX: keep only online cpus in policy->cpus
---
3) With above issue solved, i tried to run the same stuff again and look what i
got:
cpufreq_governor_interactive: CPUFREQ_GOV_STOP cpu 2 policy->cpus 2-4
cpufreq_vexpress: CPUFreq for CPU 3 initialized
cpufreq_governor_interactive: CPUFREQ_GOV_START cpu 3 policy->cpus 2-4
CPU2: shutdown
a) Because cpu 2 is still not completely hot unplugged and is still present in
cpu_online_mask.
b) If we try to remove cpu 3, governor wouldn't be notified of it, because cpu 3
wasn't policy->cpu.
FIX: Stop/Start governor on every non policy->cpu removal
---
4) Hot unplug is fine now, but not hotplug :(
for i in 2 3 4; do echo 1 > /sys/devices/system/cpu/cpu$i/online; done
CPU2: Booted secondary processor
cpufreq_vexpress: CPUFreq for CPU 2 initialized
cpufreq_governor_interactive: CPUFREQ_GOV_START cpu 2 policy->cpus 2
CPU3: Booted secondary processor
cpufreq_vexpress: CPUFreq for CPU 3 initialized
CPU4: Booted secondary processor
cpufreq_vexpress: CPUFreq for CPU 4 initialized
Hmmm, Governor is not informed about 3 and 4. :(
FIX: Stop/Start governor on every managed_policy cpu, i.e. cpu for which
--- policy struct exists
Please share your comments, i know there would be many :)
Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
drivers/cpufreq/cpufreq.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 0c3e29a..69bd739 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -749,11 +749,16 @@ static int cpufreq_add_dev_policy(unsigned int cpu,
return -EBUSY;
}
+ __cpufreq_governor(managed_policy, CPUFREQ_GOV_STOP);
+
spin_lock_irqsave(&cpufreq_driver_lock, flags);
cpumask_copy(managed_policy->cpus, policy->cpus);
per_cpu(cpufreq_cpu_data, cpu) = managed_policy;
spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
+ __cpufreq_governor(managed_policy, CPUFREQ_GOV_START);
+ __cpufreq_governor(managed_policy, CPUFREQ_GOV_LIMITS);
+
pr_debug("CPU already managed, adding link\n");
ret = sysfs_create_link(&dev->kobj,
&managed_policy->kobj,
@@ -968,6 +973,13 @@ static int cpufreq_add_dev(struct device *dev, struct subsys_interface *sif)
pr_debug("initialization failed\n");
goto err_unlock_policy;
}
+
+ /*
+ * affected cpus must always be the one, which are online. We aren't
+ * managing offline cpus here.
+ */
+ cpumask_and(policy->cpus, policy->cpus, cpu_online_mask);
+
policy->user_policy.min = policy->min;
policy->user_policy.max = policy->max;
@@ -1057,8 +1069,13 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
*/
if (unlikely(cpu != data->cpu)) {
pr_debug("removing link\n");
+ __cpufreq_governor(data, CPUFREQ_GOV_STOP);
cpumask_clear_cpu(cpu, data->cpus);
spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
+
+ __cpufreq_governor(data, CPUFREQ_GOV_START);
+ __cpufreq_governor(data, CPUFREQ_GOV_LIMITS);
+
kobj = &dev->kobj;
cpufreq_cpu_put(data);
unlock_policy_rwsem_write(cpu);
--
1.7.12.rc2.18.g61b472e
--
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