linux-kernel - Re: [PATCH] cpufreq: Initialize the governor again while restoring policy

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Date:	Thu, 9 Jul 2015 10:40:24 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	Pi-Cheng Chen <pi-cheng.chen@...aro.org>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	open list <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] cpufreq: Initialize the governor again while restoring
 policy

On 09-07-15, 02:33, Rafael J. Wysocki wrote:
> > We also missed marking policy->governor as NULL while restoring the
> > policy. Because of that, we call __cpufreq_governor(CPUFREQ_GOV_LIMITS)
> 
> How exactly does that happen?

Should have mentioned that in detail, sorry for being lazy. Hopefully
this will look better:

---------------------------8<---------------------------

Message-Id: <5f17361741c009a7f0d8488f7f94bab80d9317fd.1436418101.git.viresh.kumar@...aro.org>
From: Viresh Kumar <viresh.kumar@...aro.org>
Date: Wed, 8 Jul 2015 10:45:53 +0530
Subject: [PATCH V2] cpufreq: Initialize the governor again while restoring policy

When all CPUs of a policy are hot-unplugged, we EXIT the governor but
don't mark policy->governor as NULL. This was done in order to keep last
used governor's information intact in sysfs, while the CPUs are offline.

But we also marking policy->governor as NULL while restoring the policy.

Because policy->governor still points to the last governor while policy
is restored, following sequence of event happens:
- cpufreq_init_policy() called while restoring policy
- find_governor() matches last_governor string for present governors and
  returns last used governor's pointer, say ondemand. policy->governor
  already has the same address, unless the governor was removed in
  between.
- cpufreq_set_policy() is called with both old/new policies governor set
  as ondemand.
- Because governors matched, we skip governor initialization and return
  after calling __cpufreq_governor(CPUFREQ_GOV_LIMITS). Because the
  governor wasn't initialized for this policy, it returned -EBUSY.
- cpufreq_init_policy() exits the policy on this error, but doesn't
  destroy it properly (should be fixed separately).
- And so we enter a scenario where the policy isn't completely
  initialized but used.

Fix this by setting policy->governor to NULL while restoring the policy.

Reported-and-tested-by: Pi-Cheng Chen <pi-cheng.chen@...aro.org>
Reported-and-tested-by: "Jon Medhurst (Tixy)" <tixy@...aro.org>
Tested-by: Steven Rostedt <rostedt@...dmis.org>
Fixes: 18bf3a124ef8 ("cpufreq: Mark policy->governor = NULL for inactive policies")
Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
V2: Detailed changelog

 drivers/cpufreq/cpufreq.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index b612411655f9..2c22e3902e72 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1132,6 +1132,7 @@ static struct cpufreq_policy *cpufreq_policy_restore(unsigned int cpu)
 
 		down_write(&policy->rwsem);
 		policy->cpu = cpu;
+		policy->governor = NULL;
 		up_write(&policy->rwsem);
 	}
--
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