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[PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

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  [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-03 05:10 +0100
    Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-04 01:50 +0100
      Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-04 07:20 +0100
        Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-05 02:50 +0100
          Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-05 05:20 +0100
            Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-07 02:00 +0100
              Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-07 09:00 +0100
                Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-07 23:20 +0100
                  Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-07 23:50 +0100
                    [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-08 01:20 +0100
                      Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable  for synchronization Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 08:00 +0100
                        Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-08 14:10 +0100
                          Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable  for synchronization Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 14:40 +0100
                            Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-08 14:50 +0100
                              Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable  for synchronization Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 15:00 +0100
                                Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-08 15:10 +0100
                                  Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable  for synchronization Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 16:00 +0100
                                    Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-08 17:20 +0100
                                      Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable  for synchronization Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 17:40 +0100
                    Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 07:50 +0100
                  Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 08:00 +0100
                    Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-08 13:50 +0100
                      Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-08 14:40 +0100
                        Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-08 14:40 +0100
    [PATCH V3 5/6] cpufreq: governor: replace per-cpu delayed work with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-04 07:20 +0100
      [PATCH V4 5/6] cpufreq: governor: replace per-cpu delayed work with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-09 03:10 +0100
        Re: [PATCH V4 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-09 22:40 +0100
          Re: [PATCH V4 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-10 03:40 +0100
            Re: [PATCH V4 5/6] cpufreq: governor: replace per-cpu delayed work with timers "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-12-10 22:50 +0100
              Re: [PATCH V4 5/6] cpufreq: governor: replace per-cpu delayed work  with timers Viresh Kumar <viresh.kumar@linaro.org> - 2015-12-11 02:50 +0100

Page 1 of 2  [1] 2  Next page →

#1282685 — [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

cpufreq governors evaluate load at sampling rate and based on that they
update frequency for a group of CPUs belonging to the same cpufreq
policy.

This is required to be done in a single thread for all policy->cpus, but
because we don't want to wakeup idle CPUs to do just that, we use
deferrable work for this. If we would have used a single delayed
deferrable work for the entire policy, there were chances that the CPU
required to run the handler can be in idle and we might end up not
changing the frequency for the entire group with load variations.

And so we were forced to keep per-cpu works, and only the one that
expires first need to do the real work and others are rescheduled for
next sampling time.

We have been using the more complex solution until now, where we used a
delayed deferrable work for this, which is a combination of a timer and
a work.

This could be made lightweight by keeping per-cpu deferred timers with a
single work item, which is scheduled by the first timer that expires.

This patch does just that and here are important changes:
- The timer handler will run in irq context and so we need to use a
  spin_lock instead of the timer_mutex. And so a separate timer_lock is
  created. This also makes the use of the mutex and lock quite clear, as
  we know what exactly they are protecting.
- A new field 'skip_work' is added to track when the timer handlers can
  queue a work. More comments present in code.

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
---
 drivers/cpufreq/cpufreq_governor.c | 139 +++++++++++++++++++++----------------
 drivers/cpufreq/cpufreq_governor.h |  20 ++++--
 drivers/cpufreq/cpufreq_ondemand.c |   8 +--
 3 files changed, 98 insertions(+), 69 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index 999e1f6addf9..a3f9bc9b98e9 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -158,47 +158,52 @@ void dbs_check_cpu(struct dbs_data *dbs_data, int cpu)
 }
 EXPORT_SYMBOL_GPL(dbs_check_cpu);
 
-static inline void __gov_queue_work(int cpu, struct dbs_data *dbs_data,
-		unsigned int delay)
+void gov_add_timers(struct cpufreq_policy *policy, unsigned int delay)
 {
-	struct cpu_dbs_info *cdbs = dbs_data->cdata->get_cpu_cdbs(cpu);
-
-	mod_delayed_work_on(cpu, system_wq, &cdbs->dwork, delay);
-}
-
-void gov_queue_work(struct dbs_data *dbs_data, struct cpufreq_policy *policy,
-		unsigned int delay, bool all_cpus)
-{
-	int i;
+	struct dbs_data *dbs_data = policy->governor_data;
+	struct cpu_dbs_info *cdbs;
+	int cpu;
 
-	if (!all_cpus) {
-		/*
-		 * Use raw_smp_processor_id() to avoid preemptible warnings.
-		 * We know that this is only called with all_cpus == false from
-		 * works that have been queued with *_work_on() functions and
-		 * those works are canceled during CPU_DOWN_PREPARE so they
-		 * can't possibly run on any other CPU.
-		 */
-		__gov_queue_work(raw_smp_processor_id(), dbs_data, delay);
-	} else {
-		for_each_cpu(i, policy->cpus)
-			__gov_queue_work(i, dbs_data, delay);
+	for_each_cpu(cpu, policy->cpus) {
+		cdbs = dbs_data->cdata->get_cpu_cdbs(cpu);
+		cdbs->timer.expires = jiffies + delay;
+		add_timer_on(&cdbs->timer, cpu);
 	}
 }
-EXPORT_SYMBOL_GPL(gov_queue_work);
+EXPORT_SYMBOL_GPL(gov_add_timers);
 
-static inline void gov_cancel_work(struct dbs_data *dbs_data,
-		struct cpufreq_policy *policy)
+static inline void gov_cancel_timers(struct cpufreq_policy *policy)
 {
+	struct dbs_data *dbs_data = policy->governor_data;
 	struct cpu_dbs_info *cdbs;
 	int i;
 
 	for_each_cpu(i, policy->cpus) {
 		cdbs = dbs_data->cdata->get_cpu_cdbs(i);
-		cancel_delayed_work_sync(&cdbs->dwork);
+		del_timer_sync(&cdbs->timer);
 	}
 }
 
+void gov_cancel_work(struct cpu_common_dbs_info *shared)
+{
+	unsigned long flags;
+
+	/*
+	 * No work will be queued from timer handlers after skip_work is
+	 * updated. And so we can safely cancel the work first and then the
+	 * timers.
+	 */
+	spin_lock_irqsave(&shared->timer_lock, flags);
+	shared->skip_work++;
+	spin_unlock_irqrestore(&shared->timer_lock, flags);
+
+	cancel_work_sync(&shared->work);
+
+	gov_cancel_timers(shared->policy);
+
+	shared->skip_work = 0;
+}
+
 /* Will return if we need to evaluate cpu load again or not */
 static bool need_load_eval(struct cpu_common_dbs_info *shared,
 			   unsigned int sampling_rate)
@@ -217,29 +222,21 @@ static bool need_load_eval(struct cpu_common_dbs_info *shared,
 	return true;
 }
 
-static void dbs_timer(struct work_struct *work)
+static void dbs_work_handler(struct work_struct *work)
 {
-	struct cpu_dbs_info *cdbs = container_of(work, struct cpu_dbs_info,
-						 dwork.work);
-	struct cpu_common_dbs_info *shared = cdbs->shared;
+	struct cpu_common_dbs_info *shared = container_of(work, struct
+					cpu_common_dbs_info, work);
 	struct cpufreq_policy *policy;
 	struct dbs_data *dbs_data;
 	unsigned int sampling_rate, delay;
-	bool modify_all = true;
-
-	mutex_lock(&shared->timer_mutex);
+	bool eval_load;
 
 	policy = shared->policy;
-
-	/*
-	 * Governor might already be disabled and there is no point continuing
-	 * with the work-handler.
-	 */
-	if (!policy)
-		goto unlock;
-
 	dbs_data = policy->governor_data;
 
+	/* Kill all timers */
+	gov_cancel_timers(policy);
+
 	if (dbs_data->cdata->governor == GOV_CONSERVATIVE) {
 		struct cs_dbs_tuners *cs_tuners = dbs_data->tuners;
 
@@ -250,14 +247,44 @@ static void dbs_timer(struct work_struct *work)
 		sampling_rate = od_tuners->sampling_rate;
 	}
 
-	if (!need_load_eval(cdbs->shared, sampling_rate))
-		modify_all = false;
+	eval_load = need_load_eval(shared, sampling_rate);
 
-	delay = dbs_data->cdata->gov_dbs_timer(policy, modify_all);
-	gov_queue_work(dbs_data, policy, delay, modify_all);
+	/*
+	 * Make sure cpufreq_governor_limits() isn't evaluating load in
+	 * parallel.
+	 */
+	mutex_lock(&shared->timer_mutex);
+	delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
+	mutex_unlock(&shared->timer_mutex);
+
+	shared->skip_work--;
+	gov_add_timers(policy, delay);
+}
+
+static void dbs_timer_handler(unsigned long data)
+{
+	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
+	struct cpu_common_dbs_info *shared = cdbs->shared;
+	struct cpufreq_policy *policy;
+	unsigned long flags;
+
+	spin_lock_irqsave(&shared->timer_lock, flags);
+	policy = shared->policy;
+
+	/*
+	 * Timer handler isn't allowed to queue work at the moment, because:
+	 * - Another timer handler has done that
+	 * - We are stopping the governor
+	 * - Or we are updating the sampling rate of ondemand governor
+	 */
+	if (shared->skip_work)
+		goto unlock;
+
+	shared->skip_work++;
+	queue_work(system_wq, &shared->work);
 
 unlock:
-	mutex_unlock(&shared->timer_mutex);
+	spin_unlock_irqrestore(&shared->timer_lock, flags);
 }
 
 static void set_sampling_rate(struct dbs_data *dbs_data,
@@ -288,6 +315,8 @@ static int alloc_common_dbs_info(struct cpufreq_policy *policy,
 		cdata->get_cpu_cdbs(j)->shared = shared;
 
 	mutex_init(&shared->timer_mutex);
+	spin_lock_init(&shared->timer_lock);
+	INIT_WORK(&shared->work, dbs_work_handler);
 	return 0;
 }
 
@@ -452,7 +481,9 @@ static int cpufreq_governor_start(struct cpufreq_policy *policy,
 		if (ignore_nice)
 			j_cdbs->prev_cpu_nice = kcpustat_cpu(j).cpustat[CPUTIME_NICE];
 
-		INIT_DEFERRABLE_WORK(&j_cdbs->dwork, dbs_timer);
+		__setup_timer(&j_cdbs->timer, dbs_timer_handler,
+			      (unsigned long)j_cdbs,
+			      TIMER_DEFERRABLE | TIMER_IRQSAFE);
 	}
 
 	if (cdata->governor == GOV_CONSERVATIVE) {
@@ -470,8 +501,7 @@ static int cpufreq_governor_start(struct cpufreq_policy *policy,
 		od_ops->powersave_bias_init_cpu(cpu);
 	}
 
-	gov_queue_work(dbs_data, policy, delay_for_sampling_rate(sampling_rate),
-		       true);
+	gov_add_timers(policy, delay_for_sampling_rate(sampling_rate));
 	return 0;
 }
 
@@ -485,16 +515,9 @@ static int cpufreq_governor_stop(struct cpufreq_policy *policy,
 	if (!shared || !shared->policy)
 		return -EBUSY;
 
-	/*
-	 * Work-handler must see this updated, as it should not proceed any
-	 * further after governor is disabled. And so timer_mutex is taken while
-	 * updating this value.
-	 */
-	mutex_lock(&shared->timer_mutex);
+	gov_cancel_work(shared);
 	shared->policy = NULL;
-	mutex_unlock(&shared->timer_mutex);
 
-	gov_cancel_work(dbs_data, policy);
 	return 0;
 }
 
diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
index 0c7589016b6c..76742902491e 100644
--- a/drivers/cpufreq/cpufreq_governor.h
+++ b/drivers/cpufreq/cpufreq_governor.h
@@ -132,12 +132,20 @@ static void *get_cpu_dbs_info_s(int cpu)				\
 struct cpu_common_dbs_info {
 	struct cpufreq_policy *policy;
 	/*
-	 * percpu mutex that serializes governor limit change with dbs_timer
-	 * invocation. We do not want dbs_timer to run when user is changing
-	 * the governor or limits.
+	 * Per policy mutex that serializes load evaluation from limit-change
+	 * and work-handler.
 	 */
 	struct mutex timer_mutex;
+
+	/*
+	 * Per policy lock that serializes access to queuing work from timer
+	 * handlers.
+	 */
+	spinlock_t timer_lock;
+
 	ktime_t time_stamp;
+	unsigned int skip_work;
+	struct work_struct work;
 };
 
 /* Per cpu structures */
@@ -152,7 +160,7 @@ struct cpu_dbs_info {
 	 * wake-up from idle.
 	 */
 	unsigned int prev_load;
-	struct delayed_work dwork;
+	struct timer_list timer;
 	struct cpu_common_dbs_info *shared;
 };
 
@@ -268,11 +276,11 @@ static ssize_t show_sampling_rate_min_gov_pol				\
 
 extern struct mutex cpufreq_governor_lock;
 
+void gov_add_timers(struct cpufreq_policy *policy, unsigned int delay);
+void gov_cancel_work(struct cpu_common_dbs_info *shared);
 void dbs_check_cpu(struct dbs_data *dbs_data, int cpu);
 int cpufreq_governor_dbs(struct cpufreq_policy *policy,
 		struct common_dbs_data *cdata, unsigned int event);
-void gov_queue_work(struct dbs_data *dbs_data, struct cpufreq_policy *policy,
-		unsigned int delay, bool all_cpus);
 void od_register_powersave_bias_handler(unsigned int (*f)
 		(struct cpufreq_policy *, unsigned int, unsigned int),
 		unsigned int powersave_bias);
diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
index fc0384b4d02d..f879012cf849 100644
--- a/drivers/cpufreq/cpufreq_ondemand.c
+++ b/drivers/cpufreq/cpufreq_ondemand.c
@@ -286,13 +286,11 @@ static void update_sampling_rate(struct dbs_data *dbs_data,
 			continue;
 
 		next_sampling = jiffies + usecs_to_jiffies(new_rate);
-		appointed_at = dbs_info->cdbs.dwork.timer.expires;
+		appointed_at = dbs_info->cdbs.timer.expires;
 
 		if (time_before(next_sampling, appointed_at)) {
-			cancel_delayed_work_sync(&dbs_info->cdbs.dwork);
-
-			gov_queue_work(dbs_data, policy,
-				       usecs_to_jiffies(new_rate), true);
+			gov_cancel_work(shared);
+			gov_add_timers(policy, usecs_to_jiffies(new_rate));
 
 		}
 	}
-- 
2.6.2.198.g614a2ac

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#1283486

On Thursday, December 03, 2015 09:37:53 AM Viresh Kumar wrote:
> cpufreq governors evaluate load at sampling rate and based on that they
> update frequency for a group of CPUs belonging to the same cpufreq
> policy.
> 
> This is required to be done in a single thread for all policy->cpus, but
> because we don't want to wakeup idle CPUs to do just that, we use
> deferrable work for this. If we would have used a single delayed
> deferrable work for the entire policy, there were chances that the CPU
> required to run the handler can be in idle and we might end up not
> changing the frequency for the entire group with load variations.
> 
> And so we were forced to keep per-cpu works, and only the one that
> expires first need to do the real work and others are rescheduled for
> next sampling time.
> 
> We have been using the more complex solution until now, where we used a
> delayed deferrable work for this, which is a combination of a timer and
> a work.
> 
> This could be made lightweight by keeping per-cpu deferred timers with a
> single work item, which is scheduled by the first timer that expires.
> 
> This patch does just that and here are important changes:
> - The timer handler will run in irq context and so we need to use a
>   spin_lock instead of the timer_mutex. And so a separate timer_lock is
>   created. This also makes the use of the mutex and lock quite clear, as
>   we know what exactly they are protecting.
> - A new field 'skip_work' is added to track when the timer handlers can
>   queue a work. More comments present in code.
> 
> Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> Reviewed-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>

I've tentatively queued this one up, but I still have a couple of questions.

> ---
>  drivers/cpufreq/cpufreq_governor.c | 139 +++++++++++++++++++++----------------
>  drivers/cpufreq/cpufreq_governor.h |  20 ++++--
>  drivers/cpufreq/cpufreq_ondemand.c |   8 +--
>  3 files changed, 98 insertions(+), 69 deletions(-)

[cut]

> @@ -250,14 +247,44 @@ static void dbs_timer(struct work_struct *work)
>  		sampling_rate = od_tuners->sampling_rate;
>  	}
>  
> -	if (!need_load_eval(cdbs->shared, sampling_rate))
> -		modify_all = false;
> +	eval_load = need_load_eval(shared, sampling_rate);
>  
> -	delay = dbs_data->cdata->gov_dbs_timer(policy, modify_all);
> -	gov_queue_work(dbs_data, policy, delay, modify_all);
> +	/*
> +	 * Make sure cpufreq_governor_limits() isn't evaluating load in
> +	 * parallel.
> +	 */
> +	mutex_lock(&shared->timer_mutex);
> +	delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
> +	mutex_unlock(&shared->timer_mutex);
> +
> +	shared->skip_work--;

Is there any reason for incrementing and decrementing this instead of setting
it to either 0 or 1 (or maybe either 'true' or 'false' for that matter)?

If my reading of the patch is correct, it can only be either 0 or 1 anyway, right?

> +	gov_add_timers(policy, delay);
> +}
> +
> +static void dbs_timer_handler(unsigned long data)
> +{
> +	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
> +	struct cpu_common_dbs_info *shared = cdbs->shared;
> +	struct cpufreq_policy *policy;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&shared->timer_lock, flags);
> +	policy = shared->policy;

Why do we need policy here?

> +
> +	/*
> +	 * Timer handler isn't allowed to queue work at the moment, because:
> +	 * - Another timer handler has done that
> +	 * - We are stopping the governor
> +	 * - Or we are updating the sampling rate of ondemand governor
> +	 */
> +	if (shared->skip_work)
> +		goto unlock;
> +
> +	shared->skip_work++;
> +	queue_work(system_wq, &shared->work);
>  
>  unlock:

What about writing the above as

	if (!shared->work_in_progress) {
		shared->work_in_progress = true;
		queue_work(system_wq, &shared->work);
	}

and then you won't need the unlock label.

> -	mutex_unlock(&shared->timer_mutex);
> +	spin_unlock_irqrestore(&shared->timer_lock, flags);
>  }
>  
>  static void set_sampling_rate(struct dbs_data *dbs_data,

Thanks,
Rafael

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#1283592 — Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

On 04-12-15, 02:18, Rafael J. Wysocki wrote:
> > +	shared->skip_work--;
> 
> Is there any reason for incrementing and decrementing this instead of setting
> it to either 0 or 1 (or maybe either 'true' or 'false' for that matter)?
> 
> If my reading of the patch is correct, it can only be either 0 or 1 anyway, right?

No. It can be 0, 1 or 2.

If the timer handler is running on any CPU, we increment skip_work, so
its value is 1. If at the same time, we try to stop the governor, we
increment it again and its value is 2 now.

Once timer-handler finishes, it decrements it and its value become 1.
Which guarantees that no other timer handler starts executing at this
point of time and we can safely do gov_cancel_timers(). And once we
are sure that we don't have any work/timer left, we make it 0 (as we
aren't sure of the current value, which can be 0 (if the timer handler
wasn't running when we stopped the governor) or 1 (if the timer
handler was running while stopping the governor)).

Hope this clarifies it.

> > +static void dbs_timer_handler(unsigned long data)
> > +{
> > +	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
> > +	struct cpu_common_dbs_info *shared = cdbs->shared;
> > +	struct cpufreq_policy *policy;
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&shared->timer_lock, flags);
> > +	policy = shared->policy;
> 
> Why do we need policy here?
> 
> > +
> > +	/*
> > +	 * Timer handler isn't allowed to queue work at the moment, because:
> > +	 * - Another timer handler has done that
> > +	 * - We are stopping the governor
> > +	 * - Or we are updating the sampling rate of ondemand governor
> > +	 */
> > +	if (shared->skip_work)
> > +		goto unlock;
> > +
> > +	shared->skip_work++;
> > +	queue_work(system_wq, &shared->work);
> >  
> >  unlock:
> 
> What about writing the above as
> 
> 	if (!shared->work_in_progress) {
> 		shared->work_in_progress = true;
> 		queue_work(system_wq, &shared->work);
> 	}
> 
> and then you won't need the unlock label.

Here is a diff for that:

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index a3f9bc9b98e9..c9e420bd0eec 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -265,11 +265,9 @@ static void dbs_timer_handler(unsigned long data)
 {
        struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
        struct cpu_common_dbs_info *shared = cdbs->shared;
-       struct cpufreq_policy *policy;
        unsigned long flags;
 
        spin_lock_irqsave(&shared->timer_lock, flags);
-       policy = shared->policy;
 
        /*
         * Timer handler isn't allowed to queue work at the moment, because:
@@ -277,13 +275,11 @@ static void dbs_timer_handler(unsigned long data)
         * - We are stopping the governor
         * - Or we are updating the sampling rate of ondemand governor
         */
-       if (shared->skip_work)
-               goto unlock;
-
-       shared->skip_work++;
-       queue_work(system_wq, &shared->work);
+       if (!shared->skip_work) {
+               shared->skip_work++;
+               queue_work(system_wq, &shared->work);
+       }
 
-unlock:
        spin_unlock_irqrestore(&shared->timer_lock, flags);
 }

I will resend this patch now.

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#1284431

On Friday, December 04, 2015 11:41:01 AM Viresh Kumar wrote:
> On 04-12-15, 02:18, Rafael J. Wysocki wrote:
> > > +	shared->skip_work--;
> > 
> > Is there any reason for incrementing and decrementing this instead of setting
> > it to either 0 or 1 (or maybe either 'true' or 'false' for that matter)?
> > 
> > If my reading of the patch is correct, it can only be either 0 or 1 anyway, right?
> 
> No. It can be 0, 1 or 2.
> 
> If the timer handler is running on any CPU, we increment skip_work, so
> its value is 1. If at the same time, we try to stop the governor, we
> increment it again and its value is 2 now.
> 
> Once timer-handler finishes, it decrements it and its value become 1.
> Which guarantees that no other timer handler starts executing at this
> point of time and we can safely do gov_cancel_timers(). And once we
> are sure that we don't have any work/timer left, we make it 0 (as we
> aren't sure of the current value, which can be 0 (if the timer handler
> wasn't running when we stopped the governor) or 1 (if the timer
> handler was running while stopping the governor)).
> 
> Hope this clarifies it.

Well, almost, but not quite yet, because now the question is what prevents
gov_cancel_work() from racing with dbs_work_handler().

If you can guarantee that they'll never run in parallel with each other,
you probably don't need the whole counter dance.  Otherwise, dbs_work_handler()
should decrement the counter under the spinlock after all I suppose.

Thanks,
Rafael

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#1284449 — Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

On 05-12-15, 03:14, Rafael J. Wysocki wrote:
> Well, almost, but not quite yet, because now the question is what prevents
> gov_cancel_work() from racing with dbs_work_handler().
> 
> If you can guarantee that they'll never run in parallel with each other,

They can run in parallel and that's how we fix it now:
- raising skip_work to 2 makes sure that no new timer-handler can
  queue a new work.
- After raising the value of skip_work to 2, we do cancel_work_sync().
  Which will make sure that the work-handler has finished after
  cancel_work_sync() has returned.
- At this point of time we are sure that the works and their handlers
  are completely killed.
- All that is left is to kill all timer-handler (which might have
  gotten queued from the work handler, before it finished).
- And we do that with gov_cancel_timers().
- And then we are in safe state, where we are guaranteed that there
  are no leftovers.

> you probably don't need the whole counter dance.  Otherwise, dbs_work_handler()
> should decrement the counter under the spinlock after all I suppose.

Its not required because we don't have any race around that decrement
operation.

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#1284947

On Saturday, December 05, 2015 09:40:42 AM Viresh Kumar wrote:
> On 05-12-15, 03:14, Rafael J. Wysocki wrote:
> > Well, almost, but not quite yet, because now the question is what prevents
> > gov_cancel_work() from racing with dbs_work_handler().
> > 
> > If you can guarantee that they'll never run in parallel with each other,
> 
> They can run in parallel and that's how we fix it now:
> - raising skip_work to 2 makes sure that no new timer-handler can
>   queue a new work.

What about if that happens in parallel with the decrementation in
dbs_work_handler()?

Is there anything preventing that from happening?

> - After raising the value of skip_work to 2, we do cancel_work_sync().
>   Which will make sure that the work-handler has finished after
>   cancel_work_sync() has returned.
> - At this point of time we are sure that the works and their handlers
>   are completely killed.
> - All that is left is to kill all timer-handler (which might have
>   gotten queued from the work handler, before it finished).
> - And we do that with gov_cancel_timers().
> - And then we are in safe state, where we are guaranteed that there
>   are no leftovers.

Yes, that part will work.

> > you probably don't need the whole counter dance.  Otherwise, dbs_work_handler()
> > should decrement the counter under the spinlock after all I suppose.
> 
> Its not required because we don't have any race around that decrement
> operation.

As I said, if you can guarantee that the decrementation of the counter in
dbs_work_handler() cannot happen at the same time as the incrementation of
it in gov_cancel_work(), all is fine, but can you actually guarantee that?

That aside, I think you could avoid using the spinlock altogether if the
counter was atomic (and which would make the above irrelevant too).

Say, skip_work is atomic the the relevant code in dbs_timer_handler() is
written as

	atomic_inc(&shared->skip_work);
	smp_mb__after_atomic();
	if (atomic_read(&shared->skip_work) > 1)
		atomic_dec(&shared->skip_work);
	else
		queue_work(system_wq, &shared->work);

and the remaining incrementation and decrementation of skip_work are replaced
with the corresponding atomic operations, it still should work, no?

Thanks,
Rafael

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#1285063 — Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

On 07-12-15, 02:28, Rafael J. Wysocki wrote:
> What about if that happens in parallel with the decrementation in
> dbs_work_handler()?
> 
> Is there anything preventing that from happening?

Hmmm, you are right. Following is required for that.

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index c9e420bd0eec..d8a89e653933 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -230,6 +230,7 @@ static void dbs_work_handler(struct work_struct *work)
        struct dbs_data *dbs_data;
        unsigned int sampling_rate, delay;
        bool eval_load;
+       unsigned long flags;
 
        policy = shared->policy;
        dbs_data = policy->governor_data;
@@ -257,7 +258,10 @@ static void dbs_work_handler(struct work_struct *work)
        delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
        mutex_unlock(&shared->timer_mutex);
 
+       spin_lock_irqsave(&shared->timer_lock, flags);
        shared->skip_work--;
+       spin_unlock_irqrestore(&shared->timer_lock, flags);
+
        gov_add_timers(policy, delay);
 }

> That aside, I think you could avoid using the spinlock altogether if the
> counter was atomic (and which would make the above irrelevant too).
> 
> Say, skip_work is atomic the the relevant code in dbs_timer_handler() is
> written as
> 
> 	atomic_inc(&shared->skip_work);
> 	smp_mb__after_atomic();
> 	if (atomic_read(&shared->skip_work) > 1)
> 		atomic_dec(&shared->skip_work);
> 	else

At this point we might end up decrementing skip_work from
gov_cancel_work() and then cancel the work which we haven't queued
yet. And the end result will be that the work is still queued while
gov_cancel_work() has finished.

And we have to keep the atomic operation, as well as queue_work()
within the lock.

> 		queue_work(system_wq, &shared->work);
> 
> and the remaining incrementation and decrementation of skip_work are replaced
> with the corresponding atomic operations, it still should work, no?

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#1286010

On Monday, December 07, 2015 01:20:27 PM Viresh Kumar wrote:
> On 07-12-15, 02:28, Rafael J. Wysocki wrote:
> > What about if that happens in parallel with the decrementation in
> > dbs_work_handler()?
> > 
> > Is there anything preventing that from happening?
> 
> Hmmm, you are right. Following is required for that.
> 
> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> index c9e420bd0eec..d8a89e653933 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -230,6 +230,7 @@ static void dbs_work_handler(struct work_struct *work)
>         struct dbs_data *dbs_data;
>         unsigned int sampling_rate, delay;
>         bool eval_load;
> +       unsigned long flags;
>  
>         policy = shared->policy;
>         dbs_data = policy->governor_data;
> @@ -257,7 +258,10 @@ static void dbs_work_handler(struct work_struct *work)
>         delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
>         mutex_unlock(&shared->timer_mutex);
>  
> +       spin_lock_irqsave(&shared->timer_lock, flags);
>         shared->skip_work--;
> +       spin_unlock_irqrestore(&shared->timer_lock, flags);
> +
>         gov_add_timers(policy, delay);
>  }

OK, so can you please send an updated patch with the above change folded in?

> > That aside, I think you could avoid using the spinlock altogether if the
> > counter was atomic (and which would make the above irrelevant too).
> > 
> > Say, skip_work is atomic the the relevant code in dbs_timer_handler() is
> > written as
> > 
> > 	atomic_inc(&shared->skip_work);
> > 	smp_mb__after_atomic();
> > 	if (atomic_read(&shared->skip_work) > 1)
> > 		atomic_dec(&shared->skip_work);
> > 	else
> 
> At this point we might end up decrementing skip_work from
> gov_cancel_work() and then cancel the work which we haven't queued
> yet. And the end result will be that the work is still queued while
> gov_cancel_work() has finished.

I'm not quite sure how that can happen.

There is a bug in this code snippet, but it may cause us to fail to queue
the work at all, so the incrementation and the check need to be done
under the spinlock.

> And we have to keep the atomic operation, as well as queue_work()
> within the lock.

Putting queue_work() under the lock doesn't prevent any races from happening,
because only one of the CPUs can execute that part of the function anyway.

> > 		queue_work(system_wq, &shared->work);
> > 
> > and the remaining incrementation and decrementation of skip_work are replaced
> > with the corresponding atomic operations, it still should work, no?

Well, no, the above wouldn't work.

But what about something like this instead:

	if (atomic_inc_return(&shared->skip_work) > 1)
		atomic_dec(&shared->skip_work);
	else
		queue_work(system_wq, &shared->work);

(plus the changes requisite replacements in the other places)?

Only one CPU can see the result of the atomic_inc_return() as 1 and this is the
only one that will queue up the work item, unless I'm missing anything super
subtle.

Thanks,
Rafael

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#1286062

On Monday, December 07, 2015 11:43:50 PM Rafael J. Wysocki wrote:
> On Monday, December 07, 2015 01:20:27 PM Viresh Kumar wrote:
> > On 07-12-15, 02:28, Rafael J. Wysocki wrote:
> > > What about if that happens in parallel with the decrementation in
> > > dbs_work_handler()?
> > > 
> > > Is there anything preventing that from happening?
> > 
> > Hmmm, you are right. Following is required for that.
> > 
> > diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> > index c9e420bd0eec..d8a89e653933 100644
> > --- a/drivers/cpufreq/cpufreq_governor.c
> > +++ b/drivers/cpufreq/cpufreq_governor.c
> > @@ -230,6 +230,7 @@ static void dbs_work_handler(struct work_struct *work)
> >         struct dbs_data *dbs_data;
> >         unsigned int sampling_rate, delay;
> >         bool eval_load;
> > +       unsigned long flags;
> >  
> >         policy = shared->policy;
> >         dbs_data = policy->governor_data;
> > @@ -257,7 +258,10 @@ static void dbs_work_handler(struct work_struct *work)
> >         delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
> >         mutex_unlock(&shared->timer_mutex);
> >  
> > +       spin_lock_irqsave(&shared->timer_lock, flags);
> >         shared->skip_work--;
> > +       spin_unlock_irqrestore(&shared->timer_lock, flags);
> > +
> >         gov_add_timers(policy, delay);
> >  }
> 
> OK, so can you please send an updated patch with the above change folded in?

In fact, I've already folded the above changes into the $subject patch (but this
is an exception).

I'll send the "atomic" changes separately.

Thanks,
Rafael

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#1286117 — [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Use the observation that if skip_work in struct cpu_common_dbs_info
is an atomic_t variable, the code may be rearranged to avoid using
the timer_lock spinlock in which case that lock is not necessary any
more.

Make that change and drop timer_lock.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

This is on top of my current linux-next branch.  Completely experimental and
untested.

---
 drivers/cpufreq/cpufreq_governor.c |   29 +++++++----------------------
 drivers/cpufreq/cpufreq_governor.h |    9 ++-------
 2 files changed, 9 insertions(+), 29 deletions(-)

Index: linux-pm/drivers/cpufreq/cpufreq_governor.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.c
+++ linux-pm/drivers/cpufreq/cpufreq_governor.c
@@ -186,22 +186,15 @@ static inline void gov_cancel_timers(str
 
 void gov_cancel_work(struct cpu_common_dbs_info *shared)
 {
-	unsigned long flags;
-
 	/*
 	 * No work will be queued from timer handlers after skip_work is
 	 * updated. And so we can safely cancel the work first and then the
 	 * timers.
 	 */
-	spin_lock_irqsave(&shared->timer_lock, flags);
-	shared->skip_work++;
-	spin_unlock_irqrestore(&shared->timer_lock, flags);
-
+	atomic_inc(&shared->skip_work);
 	cancel_work_sync(&shared->work);
-
 	gov_cancel_timers(shared->policy);
-
-	shared->skip_work = 0;
+	atomic_set(&shared->skip_work, 0);
 }
 
 /* Will return if we need to evaluate cpu load again or not */
@@ -229,7 +222,6 @@ static void dbs_work_handler(struct work
 	struct cpufreq_policy *policy;
 	struct dbs_data *dbs_data;
 	unsigned int sampling_rate, delay;
-	unsigned long flags;
 	bool eval_load;
 
 	policy = shared->policy;
@@ -258,9 +250,7 @@ static void dbs_work_handler(struct work
 	delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
 	mutex_unlock(&shared->timer_mutex);
 
-	spin_lock_irqsave(&shared->timer_lock, flags);
-	shared->skip_work--;
-	spin_unlock_irqrestore(&shared->timer_lock, flags);
+	atomic_dec(&shared->skip_work);
 
 	gov_add_timers(policy, delay);
 }
@@ -269,9 +259,6 @@ static void dbs_timer_handler(unsigned l
 {
 	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
 	struct cpu_common_dbs_info *shared = cdbs->shared;
-	unsigned long flags;
-
-	spin_lock_irqsave(&shared->timer_lock, flags);
 
 	/*
 	 * Timer handler isn't allowed to queue work at the moment, because:
@@ -279,12 +266,10 @@ static void dbs_timer_handler(unsigned l
 	 * - We are stopping the governor
 	 * - Or we are updating the sampling rate of ondemand governor
 	 */
-	if (!shared->skip_work) {
-		shared->skip_work++;
+	if (atomic_inc_return(&shared->skip_work) > 1)
+		atomic_dec(&shared->skip_work);
+	else
 		queue_work(system_wq, &shared->work);
-	}
-
-	spin_unlock_irqrestore(&shared->timer_lock, flags);
 }
 
 static void set_sampling_rate(struct dbs_data *dbs_data,
@@ -315,7 +300,7 @@ static int alloc_common_dbs_info(struct
 		cdata->get_cpu_cdbs(j)->shared = shared;
 
 	mutex_init(&shared->timer_mutex);
-	spin_lock_init(&shared->timer_lock);
+	atomic_set(&shared->skip_work, 0);
 	INIT_WORK(&shared->work, dbs_work_handler);
 	return 0;
 }
Index: linux-pm/drivers/cpufreq/cpufreq_governor.h
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.h
+++ linux-pm/drivers/cpufreq/cpufreq_governor.h
@@ -17,6 +17,7 @@
 #ifndef _CPUFREQ_GOVERNOR_H
 #define _CPUFREQ_GOVERNOR_H
 
+#include <linux/atomic.h>
 #include <linux/cpufreq.h>
 #include <linux/kernel_stat.h>
 #include <linux/module.h>
@@ -137,14 +138,8 @@ struct cpu_common_dbs_info {
 	 */
 	struct mutex timer_mutex;
 
-	/*
-	 * Per policy lock that serializes access to queuing work from timer
-	 * handlers.
-	 */
-	spinlock_t timer_lock;
-
 	ktime_t time_stamp;
-	unsigned int skip_work;
+	atomic_t skip_work;
 	struct work_struct work;
 };
 

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#1286285 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On 08-12-15, 01:39, Rafael J. Wysocki wrote:
> @@ -269,9 +259,6 @@ static void dbs_timer_handler(unsigned l
>  {
>  	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
>  	struct cpu_common_dbs_info *shared = cdbs->shared;
> -	unsigned long flags;
> -
> -	spin_lock_irqsave(&shared->timer_lock, flags);
>  
>  	/*
>  	 * Timer handler isn't allowed to queue work at the moment, because:
> @@ -279,12 +266,10 @@ static void dbs_timer_handler(unsigned l
>  	 * - We are stopping the governor
>  	 * - Or we are updating the sampling rate of ondemand governor
>  	 */
> -	if (!shared->skip_work) {
> -		shared->skip_work++;
> +	if (atomic_inc_return(&shared->skip_work) > 1)
> +		atomic_dec(&shared->skip_work);
> +	else
>  		queue_work(system_wq, &shared->work);
> -	}

As explained in the other email, this is wrong..

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#1286454 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On Tuesday, December 08, 2015 12:29:05 PM Viresh Kumar wrote:
> On 08-12-15, 01:39, Rafael J. Wysocki wrote:
> > @@ -269,9 +259,6 @@ static void dbs_timer_handler(unsigned l
> >  {
> >  	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
> >  	struct cpu_common_dbs_info *shared = cdbs->shared;
> > -	unsigned long flags;
> > -
> > -	spin_lock_irqsave(&shared->timer_lock, flags);
> >  
> >  	/*
> >  	 * Timer handler isn't allowed to queue work at the moment, because:
> > @@ -279,12 +266,10 @@ static void dbs_timer_handler(unsigned l
> >  	 * - We are stopping the governor
> >  	 * - Or we are updating the sampling rate of ondemand governor
> >  	 */
> > -	if (!shared->skip_work) {
> > -		shared->skip_work++;
> > +	if (atomic_inc_return(&shared->skip_work) > 1)
> > +		atomic_dec(&shared->skip_work);
> > +	else
> >  		queue_work(system_wq, &shared->work);
> > -	}
> 
> As explained in the other email, this is wrong..

OK, but instead of relying on the spinlock to wait for the already running
dbs_timer_handler() in gov_cancel_work() (which is really easy to overlook
and should at least be mentioned in a comment) we can wait for it explicitly.

That is, if the relevant code in gov_cancel_work() is like this:


	atomic_inc(&shared->skip_work);
	gov_cancel_timers(shared->policy);
	cancel_work_sync(&shared->work);
	gov_cancel_timers(shared->policy);
	atomic_set(&shared->skip_work, 0);

then the work item should not be leaked behind the cancel_work_sync() any more
AFAICS.

Thanks,
Rafael

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#1286485 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On 08-12-15, 14:30, Rafael J. Wysocki wrote:
> OK, but instead of relying on the spinlock to wait for the already running

That's the purpose of the spinlock, not a side-effect.

> dbs_timer_handler() in gov_cancel_work() (which is really easy to overlook
> and should at least be mentioned in a comment) we can wait for it explicitly.

I agree, and I will add explicit comment about it.

> That is, if the relevant code in gov_cancel_work() is like this:
> 
> 
> 	atomic_inc(&shared->skip_work);
> 	gov_cancel_timers(shared->policy);
> 	cancel_work_sync(&shared->work);
> 	gov_cancel_timers(shared->policy);

Apart from it being *really* ugly (we should know exactly what should
be done, it rather looks like hit and try), it is still racy.

> 	atomic_set(&shared->skip_work, 0);
> 
> then the work item should not be leaked behind the cancel_work_sync() any more
> AFAICS.

Suppose queue_work() isn't done within the spin lock.

CPU0                                            CPU1

cpufreq_governor_stop()                         dbs_timer_handler()
-> gov_cancel_work()                            -> lock
                                                -> shared->skip_work++, as skip_work was 0. //skip_work=1
                                                -> unlock
   -> lock
   -> shared->skip_work++; //skip_work=2
   -> unlock
                                                -> queue_work();
   -> gov_cancel_timers(shared->policy);
                                                dbs_work_handler();
                                                -> queue-timers again (as we aren't checking skip_work here)
   -> cancel_work_sync(&shared->work);
                                                dbs_timer_handler()
                                                -> lock
                                                -> shared->skip_work++, as skip_work was 0. //skip_work=1
                                                -> unlock
                                                ->queue_work()
   -> gov_cancel_timers(shared->policy);
   -> shared->skip_work = 0;


And we have the same situation again. I have thought of all this
before I wrote the initial patch, and really tried the ugly double
timer-cancel thing. But the current approach is really the right thing
to do.

I will send a patch adding the comment.

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#1286492 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On Tuesday, December 08, 2015 07:06:33 PM Viresh Kumar wrote:
> On 08-12-15, 14:30, Rafael J. Wysocki wrote:
> > OK, but instead of relying on the spinlock to wait for the already running
> 
> That's the purpose of the spinlock, not a side-effect.
> 
> > dbs_timer_handler() in gov_cancel_work() (which is really easy to overlook
> > and should at least be mentioned in a comment) we can wait for it explicitly.
> 
> I agree, and I will add explicit comment about it.
> 
> > That is, if the relevant code in gov_cancel_work() is like this:
> > 
> > 
> > 	atomic_inc(&shared->skip_work);
> > 	gov_cancel_timers(shared->policy);
> > 	cancel_work_sync(&shared->work);
> > 	gov_cancel_timers(shared->policy);
> 
> Apart from it being *really* ugly (we should know exactly what should
> be done, it rather looks like hit and try),

We know what should be done.  We need to wait for the timer function to
complete, then cancel the work item spawned by it (if any) and then
cancel the timers set by that work item.

> it is still racy.
> 
> > 	atomic_set(&shared->skip_work, 0);
> > 
> > then the work item should not be leaked behind the cancel_work_sync() any more
> > AFAICS.
> 
> Suppose queue_work() isn't done within the spin lock.
> 
> CPU0                                            CPU1
> 
> cpufreq_governor_stop()                         dbs_timer_handler()
> -> gov_cancel_work()                            -> lock
>                                                 -> shared->skip_work++, as skip_work was 0. //skip_work=1
>                                                 -> unlock
>    -> lock
>    -> shared->skip_work++; //skip_work=2

(*)

>    -> unlock
>                                                 -> queue_work();
>    -> gov_cancel_timers(shared->policy);
>                                                 dbs_work_handler();
>                                                 -> queue-timers again (as we aren't checking skip_work here)

skip_work = 1 (because dbs_work_handler() decrements it).

>    -> cancel_work_sync(&shared->work);
>                                                 dbs_timer_handler()
>                                                 -> lock
>                                                 -> shared->skip_work++, as skip_work was 0.

No, it wasn't 0, it was 1, because (*) incremented it
and it has only been decremented once by dbs_work_handler().

> //skip_work=1
>                                                 -> unlock

And the below won't happen.

>                                                 ->queue_work()
>
>    -> gov_cancel_timers(shared->policy);
>    -> shared->skip_work = 0;
> 
> 
> And we have the same situation again.

I don't really think so.

Thanks,
Rafael

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#1286502 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On 08-12-15, 15:19, Rafael J. Wysocki wrote:
> We know what should be done.  We need to wait for the timer function to
> complete, then cancel the work item spawned by it (if any) and then
> cancel the timers set by that work item.

Yeah, there is no race, but it looks ugly to me. I have written it
earlier, and then the spinlock thing looked better to me. :)

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#1286514 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On Tuesday, December 08, 2015 07:25:18 PM Viresh Kumar wrote:
> On 08-12-15, 15:19, Rafael J. Wysocki wrote:
> > We know what should be done.  We need to wait for the timer function to
> > complete, then cancel the work item spawned by it (if any) and then
> > cancel the timers set by that work item.
> 
> Yeah, there is no race, but it looks ugly to me. I have written it
> earlier, and then the spinlock thing looked better to me. :)

It doesn't look nice, but then having a lockless timer function is worth
it in my view.

The code in gov_cancel_work() runs relatively rarely, but the timer
function can run very often, so avoiding the lock in there is a priority
to me.

Plus we can avoid disabling interrupts in two places this way.

Thanks,
Rafael

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#1286561 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On 08-12-15, 15:30, Rafael J. Wysocki wrote:
> It doesn't look nice, but then having a lockless timer function is worth
> it in my view.
> 
> The code in gov_cancel_work() runs relatively rarely, but the timer
> function can run very often, so avoiding the lock in there is a priority
> to me.
> 
> Plus we can avoid disabling interrupts in two places this way.

Okay, that's good enough then. I hope you will be sending these
patches now, right? And ofcourse, we need documentation in this case
as well.

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#1286644 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On Tuesday, December 08, 2015 08:26:58 PM Viresh Kumar wrote:
> On 08-12-15, 15:30, Rafael J. Wysocki wrote:
> > It doesn't look nice, but then having a lockless timer function is worth
> > it in my view.
> > 
> > The code in gov_cancel_work() runs relatively rarely, but the timer
> > function can run very often, so avoiding the lock in there is a priority
> > to me.
> > 
> > Plus we can avoid disabling interrupts in two places this way.
> 
> Okay, that's good enough then. I hope you will be sending these
> patches now, right? And ofcourse, we need documentation in this case
> as well.

Your series is in my linux-next branch now, so that's just one patch on top
of it.  The current version of it is appended.  Unfortunately, I can't test
it here, but I'll do that later today.

I have updated the comments too, so please let me know if they are clear enough.

Thanks,
Rafael


---
From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Subject: [PATCH] cpufreq: governor: Use lockless timer function

It is possible to get rid of the timer_lock spinlock used by the
governor timer function for synchronization, but a couple of races
need to be avoided.

The first race is between multiple dbs_timer_handler() instances
that may be running in parallel with each other on different
CPUs.  Namely, one of them has to queue up the work item, but it
cannot be queued up more than once.  To achieve that,
atomic_inc_return() can be used on the skip_work field of
struct cpu_common_dbs_info.

The second race is between an already running dbs_timer_handler()
and gov_cancel_work().  In that case the dbs_timer_handler() might
not notice the skip_work incrementation in gov_cancel_work() and
it might queue up its work item after gov_cancel_work() had
returned (and that work item would corrupt skip_work going
forward).  To prevent that from happening, gov_cancel_work()
can be made wait for the timer function to complete (on all CPUs)
right after skip_work has been incremented.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/cpufreq/cpufreq_governor.c |   49 ++++++++++++++++---------------------
 drivers/cpufreq/cpufreq_governor.h |    9 +-----
 2 files changed, 24 insertions(+), 34 deletions(-)

Index: linux-pm/drivers/cpufreq/cpufreq_governor.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.c
+++ linux-pm/drivers/cpufreq/cpufreq_governor.c
@@ -186,22 +186,24 @@ static inline void gov_cancel_timers(str
 
 void gov_cancel_work(struct cpu_common_dbs_info *shared)
 {
-	unsigned long flags;
-
+	/* Tell dbs_timer_handler() to skip queuing up work items. */
+	atomic_inc(&shared->skip_work);
 	/*
-	 * No work will be queued from timer handlers after skip_work is
-	 * updated. And so we can safely cancel the work first and then the
-	 * timers.
+	 * If dbs_timer_handler() is already running, it may not notice the
+	 * incremented skip_work, so wait for it to complete to prevent its work
+	 * item from being queued up after the cancel_work_sync() below.
+	 */
+	gov_cancel_timers(shared->policy);
+	/*
+	 * In case dbs_timer_handler() managed to run and spawn a work item
+	 * before the timers have been canceled, wait for that work item to
+	 * complete and then cancel all of the timers set up by it.  If
+	 * dbs_timer_handler() runs again at that point, it will see the
+	 * positive value of skip_work and won't spawn any more work items.
 	 */
-	spin_lock_irqsave(&shared->timer_lock, flags);
-	shared->skip_work++;
-	spin_unlock_irqrestore(&shared->timer_lock, flags);
-
 	cancel_work_sync(&shared->work);
-
 	gov_cancel_timers(shared->policy);
-
-	shared->skip_work = 0;
+	atomic_set(&shared->skip_work, 0);
 }
 
 /* Will return if we need to evaluate cpu load again or not */
@@ -229,7 +231,6 @@ static void dbs_work_handler(struct work
 	struct cpufreq_policy *policy;
 	struct dbs_data *dbs_data;
 	unsigned int sampling_rate, delay;
-	unsigned long flags;
 	bool eval_load;
 
 	policy = shared->policy;
@@ -258,9 +259,7 @@ static void dbs_work_handler(struct work
 	delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
 	mutex_unlock(&shared->timer_mutex);
 
-	spin_lock_irqsave(&shared->timer_lock, flags);
-	shared->skip_work--;
-	spin_unlock_irqrestore(&shared->timer_lock, flags);
+	atomic_dec(&shared->skip_work);
 
 	gov_add_timers(policy, delay);
 }
@@ -269,22 +268,18 @@ static void dbs_timer_handler(unsigned l
 {
 	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
 	struct cpu_common_dbs_info *shared = cdbs->shared;
-	unsigned long flags;
-
-	spin_lock_irqsave(&shared->timer_lock, flags);
 
 	/*
-	 * Timer handler isn't allowed to queue work at the moment, because:
+	 * Timer handler may not be allowed to queue the work at the moment,
+	 * because:
 	 * - Another timer handler has done that
 	 * - We are stopping the governor
-	 * - Or we are updating the sampling rate of ondemand governor
+	 * - Or we are updating the sampling rate of the ondemand governor
 	 */
-	if (!shared->skip_work) {
-		shared->skip_work++;
+	if (atomic_inc_return(&shared->skip_work) > 1)
+		atomic_dec(&shared->skip_work);
+	else
 		queue_work(system_wq, &shared->work);
-	}
-
-	spin_unlock_irqrestore(&shared->timer_lock, flags);
 }
 
 static void set_sampling_rate(struct dbs_data *dbs_data,
@@ -315,7 +310,7 @@ static int alloc_common_dbs_info(struct
 		cdata->get_cpu_cdbs(j)->shared = shared;
 
 	mutex_init(&shared->timer_mutex);
-	spin_lock_init(&shared->timer_lock);
+	atomic_set(&shared->skip_work, 0);
 	INIT_WORK(&shared->work, dbs_work_handler);
 	return 0;
 }
Index: linux-pm/drivers/cpufreq/cpufreq_governor.h
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.h
+++ linux-pm/drivers/cpufreq/cpufreq_governor.h
@@ -17,6 +17,7 @@
 #ifndef _CPUFREQ_GOVERNOR_H
 #define _CPUFREQ_GOVERNOR_H
 
+#include <linux/atomic.h>
 #include <linux/cpufreq.h>
 #include <linux/kernel_stat.h>
 #include <linux/module.h>
@@ -137,14 +138,8 @@ struct cpu_common_dbs_info {
 	 */
 	struct mutex timer_mutex;
 
-	/*
-	 * Per policy lock that serializes access to queuing work from timer
-	 * handlers.
-	 */
-	spinlock_t timer_lock;
-
 	ktime_t time_stamp;
-	unsigned int skip_work;
+	atomic_t skip_work;
 	struct work_struct work;
 };
 

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#1286664 — Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

On 08-12-15, 17:42, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Subject: [PATCH] cpufreq: governor: Use lockless timer function
> 
> It is possible to get rid of the timer_lock spinlock used by the
> governor timer function for synchronization, but a couple of races
> need to be avoided.
> 
> The first race is between multiple dbs_timer_handler() instances
> that may be running in parallel with each other on different
> CPUs.  Namely, one of them has to queue up the work item, but it
> cannot be queued up more than once.  To achieve that,
> atomic_inc_return() can be used on the skip_work field of
> struct cpu_common_dbs_info.
> 
> The second race is between an already running dbs_timer_handler()
> and gov_cancel_work().  In that case the dbs_timer_handler() might
> not notice the skip_work incrementation in gov_cancel_work() and
> it might queue up its work item after gov_cancel_work() had
> returned (and that work item would corrupt skip_work going
> forward).  To prevent that from happening, gov_cancel_work()
> can be made wait for the timer function to complete (on all CPUs)
> right after skip_work has been incremented.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>  drivers/cpufreq/cpufreq_governor.c |   49 ++++++++++++++++---------------------
>  drivers/cpufreq/cpufreq_governor.h |    9 +-----
>  2 files changed, 24 insertions(+), 34 deletions(-)

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

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#1286275 — Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

On 08-12-15, 00:17, Rafael J. Wysocki wrote:
> In fact, I've already folded the above changes into the $subject patch (but this
> is an exception).

Yeah I know, I would have sent a patch this morning though.

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