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


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