Re: [PATCH 1/2] CPUFREQ: Remove unneeded dbs_mutexes from ondemand and conservative governors

Mathieu Desnoyers
2009-06-25T14:26:12Z

* Thomas Renninger (trenn@suse.de) wrote:
> Comment from Venkatesh:
> ...
> This mutex is just serializing the changes to those variables. I could't
> think of any functionality issues of not having the lock as such.
>
> -> rip it out.
>
> CC: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
> Signed-off-by: Thomas Renninger <trenn@suse.de>
> ---
> drivers/cpufreq/cpufreq_conservative.c | 61 +++-----------------------------
> drivers/cpufreq/cpufreq_ondemand.c | 48 +++----------------------
> 2 files changed, 10 insertions(+), 99 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq_conservative.c b/drivers/cpufreq/cpufreq_conservative.c
> index 7a74d17..6303379 100644
> --- a/drivers/cpufreq/cpufreq_conservative.c
> +++ b/drivers/cpufreq/cpufreq_conservative.c
> @@ -18,7 +18,6 @@
> #include <linux/cpu.h>
> #include <linux/jiffies.h>
> #include <linux/kernel_stat.h>
> -#include <linux/mutex.h>
> #include <linux/hrtimer.h>
> #include <linux/tick.h>
> #include <linux/ktime.h>
> @@ -84,19 +83,6 @@ static DEFINE_PER_CPU(struct cpu_dbs_info_s, cpu_dbs_info);
>
> static unsigned int dbs_enable; /* number of CPUs using this policy */
>
> -/*
> - * DEADLOCK ALERT! There is a ordering requirement between cpu_hotplug
> - * lock and dbs_mutex. cpu_hotplug lock should always be held before
> - * dbs_mutex. If any function that can potentially take cpu_hotplug lock
> - * (like __cpufreq_driver_target()) is being called with dbs_mutex taken, then
> - * cpu_hotplug lock should be taken before that. Note that cpu_hotplug lock
> - * is recursive for the same process. -Venki
> - * DEADLOCK ALERT! (2) : do_dbs_timer() must not take the dbs_mutex, because it
> - * would deadlock with cancel_delayed_work_sync(), which is needed for proper
> - * raceless workqueue teardown.
> - */
> -static DEFINE_MUTEX(dbs_mutex);
> -
> static struct workqueue_struct *kconservative_wq;
>
> static struct dbs_tuners {
> @@ -236,10 +222,7 @@ static ssize_t store_sampling_down_factor(struct cpufreq_policy *unused,
> if (ret != 1 || input > MAX_SAMPLING_DOWN_FACTOR || input < 1)
> return -EINVAL;
>
> - mutex_lock(&dbs_mutex);
> dbs_tuners_ins.sampling_down_factor = input;
> - mutex_unlock(&dbs_mutex);
> -
> return count;
> }
>
> @@ -253,10 +236,7 @@ static ssize_t store_sampling_rate(struct cpufreq_policy *unused,
> if (ret != 1)
> return -EINVAL;
>
> - mutex_lock(&dbs_mutex);
> dbs_tuners_ins.sampling_rate = max(input, minimum_sampling_rate());
> - mutex_unlock(&dbs_mutex);
> -
> return count;
> }
>
> @@ -267,16 +247,11 @@ static ssize_t store_up_threshold(struct cpufreq_policy *unused,
> int ret;
> ret = sscanf(buf, "%u", &input);
>
> - mutex_lock(&dbs_mutex);
> if (ret != 1 || input > 100 ||
> - input <= dbs_tuners_ins.down_threshold) {
> - mutex_unlock(&dbs_mutex);
> + input <= dbs_tuners_ins.down_threshold)
> return -EINVAL;
> - }
>
> dbs_tuners_ins.up_threshold = input;
> - mutex_unlock(&dbs_mutex);

Here, for instance, there might be a problem if down_threshold is
changed concurrently with a store_up_threshold() call. See that there is
a test before the modification, and we need the mutex there for it to be
consistent.

> -
> return count;
> }
>
> @@ -287,17 +262,12 @@ static ssize_t store_down_threshold(struct cpufreq_policy *unused,
> int ret;
> ret = sscanf(buf, "%u", &input);
>
> - mutex_lock(&dbs_mutex);
> /* cannot be lower than 11 otherwise freq will not fall */
> if (ret != 1 || input < 11 || input > 100 ||
> - input >= dbs_tuners_ins.up_threshold) {
> - mutex_unlock(&dbs_mutex);
> + input >= dbs_tuners_ins.up_threshold)
> return -EINVAL;
> - }
>
> dbs_tuners_ins.down_threshold = input;
> - mutex_unlock(&dbs_mutex);
> -
> return count;
> }
>
> @@ -316,11 +286,9 @@ static ssize_t store_ignore_nice_load(struct cpufreq_policy *policy,
> if (input > 1)
> input = 1;
>
> - mutex_lock(&dbs_mutex);
> - if (input == dbs_tuners_ins.ignore_nice) { /* nothing to do */
> - mutex_unlock(&dbs_mutex);
> + if (input == dbs_tuners_ins.ignore_nice) /* nothing to do */
> return count;
> - }
> +
> dbs_tuners_ins.ignore_nice = input;
>
> /* we need to re-evaluate prev_cpu_idle */
> @@ -332,8 +300,6 @@ static ssize_t store_ignore_nice_load(struct cpufreq_policy *policy,
> if (dbs_tuners_ins.ignore_nice)
> dbs_info->prev_cpu_nice = kstat_cpu(j).cpustat.nice;
> }
> - mutex_unlock(&dbs_mutex);
> -
> return count;
> }
>
> @@ -352,10 +318,7 @@ static ssize_t store_freq_step(struct cpufreq_policy *policy,
>
> /* no need to test here if freq_step is zero as the user might actually
> * want this, they would be crazy though :) */
> - mutex_lock(&dbs_mutex);
> dbs_tuners_ins.freq_step = input;
> - mutex_unlock(&dbs_mutex);
> -
> return count;
> }
>
> @@ -566,13 +529,9 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,

Hrm, this is where we want the mutexes removed, but I fear this is
creating a race between sysfs_create_group (sysfs file creation) and
policy initialization...

I'm not convinced this mutex is not needed.

Mathieu

> if (this_dbs_info->enable) /* Already enabled */
> break;
>
> - mutex_lock(&dbs_mutex);
> -
> rc = sysfs_create_group(&policy->kobj, &dbs_attr_group);
> - if (rc) {
> - mutex_unlock(&dbs_mutex);
> + if (rc)
> return rc;
> - }
>
> for_each_cpu(j, policy->cpus) {
> struct cpu_dbs_info_s *j_dbs_info;
> @@ -612,13 +571,9 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> CPUFREQ_TRANSITION_NOTIFIER);
> }
> dbs_timer_init(this_dbs_info);
> -
> - mutex_unlock(&dbs_mutex);
> -
> break;
>
> case CPUFREQ_GOV_STOP:
> - mutex_lock(&dbs_mutex);
> dbs_timer_exit(this_dbs_info);
> sysfs_remove_group(&policy->kobj, &dbs_attr_group);
> dbs_enable--;
> @@ -631,13 +586,9 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> cpufreq_unregister_notifier(
> &dbs_cpufreq_notifier_block,
> CPUFREQ_TRANSITION_NOTIFIER);
> -
> - mutex_unlock(&dbs_mutex);
> -
> break;
>
> case CPUFREQ_GOV_LIMITS:
> - mutex_lock(&dbs_mutex);
> if (policy->max < this_dbs_info->cur_policy->cur)
> __cpufreq_driver_target(
> this_dbs_info->cur_policy,
> @@ -646,8 +597,6 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> __cpufreq_driver_target(
> this_dbs_info->cur_policy,
> policy->min, CPUFREQ_RELATION_L);
> - mutex_unlock(&dbs_mutex);
> -
> break;
> }
> return 0;
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index e741c33..d080a48 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -17,7 +17,6 @@
> #include <linux/cpu.h>
> #include <linux/jiffies.h>
> #include <linux/kernel_stat.h>
> -#include <linux/mutex.h>
> #include <linux/hrtimer.h>
> #include <linux/tick.h>
> #include <linux/ktime.h>
> @@ -91,19 +90,6 @@ static DEFINE_PER_CPU(struct cpu_dbs_info_s, cpu_dbs_info);
>
> static unsigned int dbs_enable; /* number of CPUs using this policy */
>
> -/*
> - * DEADLOCK ALERT! There is a ordering requirement between cpu_hotplug
> - * lock and dbs_mutex. cpu_hotplug lock should always be held before
> - * dbs_mutex. If any function that can potentially take cpu_hotplug lock
> - * (like __cpufreq_driver_target()) is being called with dbs_mutex taken, then
> - * cpu_hotplug lock should be taken before that. Note that cpu_hotplug lock
> - * is recursive for the same process. -Venki
> - * DEADLOCK ALERT! (2) : do_dbs_timer() must not take the dbs_mutex, because it
> - * would deadlock with cancel_delayed_work_sync(), which is needed for proper
> - * raceless workqueue teardown.
> - */
> -static DEFINE_MUTEX(dbs_mutex);
> -
> static struct workqueue_struct *kondemand_wq;
>
> static struct dbs_tuners {
> @@ -269,14 +255,10 @@ static ssize_t store_sampling_rate(struct cpufreq_policy *unused,
> int ret;
> ret = sscanf(buf, "%u", &input);
>
> - mutex_lock(&dbs_mutex);
> - if (ret != 1) {
> - mutex_unlock(&dbs_mutex);
> + if (ret != 1)
> return -EINVAL;
> - }
> - dbs_tuners_ins.sampling_rate = max(input, minimum_sampling_rate());
> - mutex_unlock(&dbs_mutex);
>
> + dbs_tuners_ins.sampling_rate = max(input, minimum_sampling_rate());
> return count;
> }
>
> @@ -287,16 +269,11 @@ static ssize_t store_up_threshold(struct cpufreq_policy *unused,
> int ret;
> ret = sscanf(buf, "%u", &input);
>
> - mutex_lock(&dbs_mutex);
> if (ret != 1 || input > MAX_FREQUENCY_UP_THRESHOLD ||
> - input < MIN_FREQUENCY_UP_THRESHOLD) {
> - mutex_unlock(&dbs_mutex);
> + input < MIN_FREQUENCY_UP_THRESHOLD)
> return -EINVAL;
> - }
>
> dbs_tuners_ins.up_threshold = input;
> - mutex_unlock(&dbs_mutex);
> -
> return count;
> }
>
> @@ -315,11 +292,9 @@ static ssize_t store_ignore_nice_load(struct cpufreq_policy *policy,
> if (input > 1)
> input = 1;
>
> - mutex_lock(&dbs_mutex);
> - if (input == dbs_tuners_ins.ignore_nice) { /* nothing to do */
> - mutex_unlock(&dbs_mutex);
> + if (input == dbs_tuners_ins.ignore_nice) /* nothing to do */
> return count;
> - }
> +
> dbs_tuners_ins.ignore_nice = input;
>
> /* we need to re-evaluate prev_cpu_idle */
> @@ -332,8 +307,6 @@ static ssize_t store_ignore_nice_load(struct cpufreq_policy *policy,
> dbs_info->prev_cpu_nice = kstat_cpu(j).cpustat.nice;
>
> }
> - mutex_unlock(&dbs_mutex);
> -
> return count;
> }
>
> @@ -350,10 +323,8 @@ static ssize_t store_powersave_bias(struct cpufreq_policy *unused,
> if (input > 1000)
> input = 1000;
>
> - mutex_lock(&dbs_mutex);
> dbs_tuners_ins.powersave_bias = input;
> ondemand_powersave_bias_init();
> - mutex_unlock(&dbs_mutex);
>
> return count;
> }
> @@ -586,13 +557,11 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> if (this_dbs_info->enable) /* Already enabled */
> break;
>
> - mutex_lock(&dbs_mutex);
> dbs_enable++;
>
> rc = sysfs_create_group(&policy->kobj, &dbs_attr_group);
> if (rc) {
> dbs_enable--;
> - mutex_unlock(&dbs_mutex);
> return rc;
> }
>
> @@ -627,28 +596,21 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> dbs_tuners_ins.sampling_rate = def_sampling_rate;
> }
> dbs_timer_init(this_dbs_info);
> -
> - mutex_unlock(&dbs_mutex);
> break;
>
> case CPUFREQ_GOV_STOP:
> - mutex_lock(&dbs_mutex);
> dbs_timer_exit(this_dbs_info);
> sysfs_remove_group(&policy->kobj, &dbs_attr_group);
> dbs_enable--;
> - mutex_unlock(&dbs_mutex);
> -
> break;
>
> case CPUFREQ_GOV_LIMITS:
> - mutex_lock(&dbs_mutex);
> if (policy->max < this_dbs_info->cur_policy->cur)
> __cpufreq_driver_target(this_dbs_info->cur_policy,
> policy->max, CPUFREQ_RELATION_H);
> else if (policy->min > this_dbs_info->cur_policy->cur)
> __cpufreq_driver_target(this_dbs_info->cur_policy,
> policy->min, CPUFREQ_RELATION_L);
> - mutex_unlock(&dbs_mutex);
> break;
> }
> return 0;
> --
> 1.6.0.2
>

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68