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

[Thomas Renninger <trenn@suse.de>]

On Thursday 25 June 2009 04:25:52 pm Mathieu Desnoyers wrote:
> * 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.
Thanks, I was rather quick with the conservative changes..., but
it should still be ok.

It should be assured that if userspace is doing:
echo x > down_threshold
echo y > up_threshold
that the first one will be served/finished first?

If userspace is writing different values for each core to the global 
conservative/ondemand tunables, or you have concurent userspace tools
trying to configure ondemand/conservative, it's a userspace bug.
It's confusing that ondemand/conservative allows per core reads/writes to
global variables and I hope to be able to provide something to change that in 
some days, maybe weeks.

If you still can think of a possible issue, a userspace scenario would
help.

> > -
> >  	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...
This can be solved by moving this_dbs_info->enable incremenation
after sysfs_create_group.
But yes, I forgot that in my patch, thanks!

> I'm not convinced this mutex is not needed.
I am. Maybe it still takes some more thinking or step by step rework.
Finding an unintrusive, riskless short term solution for .30 is a challenge, 
though.

    Thomas

> 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