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Date:	Fri, 21 Mar 2014 13:16:44 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	rjw@...ysocki.net, linaro-kernel@...ts.linaro.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"ego@...ux.vnet.ibm.com" <ego@...ux.vnet.ibm.com>
Subject: Re: [PATCH V4 1/3] cpufreq: Make sure frequency transitions are serialized

On 03/21/2014 11:04 AM, Viresh Kumar wrote:
> From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
> 
> Whenever we change the frequency of a CPU, we call the PRECHANGE and POSTCHANGE
> notifiers. They must be serialized, i.e. PRECHANGE and POSTCHANGE notifiers
> should strictly alternate, thereby preventing two different sets of PRECHANGE or
> POSTCHANGE notifiers from interleaving arbitrarily.
> 
> The following examples illustrate why this is important:
> 
> Scenario 1:
> -----------
> A thread reading the value of cpuinfo_cur_freq, will call
> __cpufreq_cpu_get()->cpufreq_out_of_sync()->cpufreq_notify_transition()
> 
> The ondemand governor can decide to change the frequency of the CPU at the same
> time and hence it can end up sending the notifications via ->target().
> 
> If the notifiers are not serialized, the following sequence can occur:
> - PRECHANGE Notification for freq A (from cpuinfo_cur_freq)
> - PRECHANGE Notification for freq B (from target())
> - Freq changed by target() to B
> - POSTCHANGE Notification for freq B
> - POSTCHANGE Notification for freq A
> 
> We can see from the above that the last POSTCHANGE Notification happens for freq
> A but the hardware is set to run at freq B.
> 
> Where would we break then?: adjust_jiffies() in cpufreq.c & cpufreq_callback()
> in arch/arm/kernel/smp.c (which also adjusts the jiffies). All the
> loops_per_jiffy calculations will get messed up.
> 
> Scenario 2:
> -----------
> The governor calls __cpufreq_driver_target() to change the frequency. At the
> same time, if we change scaling_{min|max}_freq from sysfs, it will end up
> calling the governor's CPUFREQ_GOV_LIMITS notification, which will also call
> __cpufreq_driver_target(). And hence we end up issuing concurrent calls to
> ->target().
> 
> Typically, platforms have the following logic in their ->target() routines:
> (Eg: cpufreq-cpu0, omap, exynos, etc)
> 
> A. If new freq is more than old: Increase voltage
> B. Change freq
> C. If new freq is less than old: decrease voltage
> 
> Now, if the two concurrent calls to ->target() are X and Y, where X is trying to
> increase the freq and Y is trying to decrease it, we get the following race
> condition:
> 
> X.A: voltage gets increased for larger freq
> Y.A: nothing happens
> Y.B: freq gets decreased
> Y.C: voltage gets decreased
> X.B: freq gets increased
> X.C: nothing happens
> 
> Thus we can end up setting a freq which is not supported by the voltage we have
> set. That will probably make the clock to the CPU unstable and the system might
> not work properly anymore.
> 
> This patch introduces a set of synchronization primitives to serialize frequency
> transitions, which are to be used as shown below:
> 
> cpufreq_freq_transition_begin();
> 
> //Perform the frequency change
> 
> cpufreq_freq_transition_end();
> 
> The _begin() call sends the PRECHANGE notification whereas the _end() call sends
> the POSTCHANGE notification. Also, all the necessary synchronization is handled
> within these calls. In particular, even drivers which set the ASYNC_NOTIFICATION
> flag can also use these APIs for performing frequency transitions (ie., you can
> call _begin() from one task, and call the corresponding _end() from a different
> task).
> 
> The actual synchronization underneath is not that complicated:
> 
> The key challenge is to allow drivers to begin the transition from one thread
> and end it in a completely different thread (this is to enable drivers that do
> asynchronous POSTCHANGE notification from bottom-halves, to also use the same
> interface).
> 
> To achieve this, a 'transition_ongoing' flag, a 'transition_lock' spinlock and a
> wait-queue are added per-policy. The flag and the wait-queue are used in
> conjunction to create an "uninterrupted flow" from _begin() to _end(). The
> spinlock is used to ensure that only one such "flow" is in flight at any given
> time. Put together, this provides us all the necessary synchronization.
> 
> Based-on-patch-by: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> 
> I have kept your Authorship for this patch as is and did few modifications:
> -  removed 'state' parameter from begin/end routines.
> - added 'trasition_failed' parameter to end routine.
> - changed mutex with spinlock as discussed earlier.
> - Added WARN_ON() as discussed.
> - Exported these new routines.
> - Removed locks from end.
>

Wonderful! I was going to do this today, but thanks a lot for taking
care of this for me!

The patch looks good, but I have one comment below.

>  drivers/cpufreq/cpufreq.c | 37 +++++++++++++++++++++++++++++++++++++
>  include/linux/cpufreq.h   | 10 ++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index b349406..4279cc9 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -353,6 +353,41 @@ void cpufreq_notify_post_transition(struct cpufreq_policy *policy,
>  }
>  EXPORT_SYMBOL_GPL(cpufreq_notify_post_transition);
> 
> +void cpufreq_freq_transition_begin(struct cpufreq_policy *policy,
> +		struct cpufreq_freqs *freqs)
> +{
> +wait:
> +	wait_event(policy->transition_wait, !policy->transition_ongoing);
> +
> +	spin_lock(&policy->transition_lock);
> +
> +	if (unlikely(policy->transition_ongoing)) {
> +		spin_unlock(&policy->transition_lock);
> +		goto wait;
> +	}
> +
> +	policy->transition_ongoing = true;
> +
> +	spin_unlock(&policy->transition_lock);
> +
> +	cpufreq_notify_transition(policy, freqs, CPUFREQ_PRECHANGE);
> +}
> +EXPORT_SYMBOL_GPL(cpufreq_freq_transition_begin);
> +
> +void cpufreq_freq_transition_end(struct cpufreq_policy *policy,
> +		struct cpufreq_freqs *freqs, int transition_failed)
> +{
> +	if (unlikely(WARN_ON(!policy->transition_ongoing)))
> +		return;
> +
> +	cpufreq_notify_post_transition(policy, freqs, transition_failed);
> +
> +	policy->transition_ongoing = false;

We need this assignment to happen exactly at this point, that is, *after*
the call to post_transition() completes and before calling wake_up().

If the compiler or the CPU reorders the instructions and moves this
assignment to some other place, then we will be in trouble!

We might need memory barriers to ensure this doesn't get reordered.
Alternatively, we can simply hold the spin-lock around this assignment,
since locks automatically imply memory barriers. As an added advantage,
the code will then look more intuitive and easier to understand as well.

Thoughts?

Regards,
Srivatsa S. Bhat

> +
> +	wake_up(&policy->transition_wait);
> +}
> +EXPORT_SYMBOL_GPL(cpufreq_freq_transition_end);
> +
> 
>  /*********************************************************************
>   *                          SYSFS INTERFACE                          *
> @@ -982,6 +1017,8 @@ static struct cpufreq_policy *cpufreq_policy_alloc(void)
> 
>  	INIT_LIST_HEAD(&policy->policy_list);
>  	init_rwsem(&policy->rwsem);
> +	spin_lock_init(&policy->transition_lock);
> +	init_waitqueue_head(&policy->transition_wait);
> 
>  	return policy;
> 
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 70929bc..263173d 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -16,6 +16,7 @@
>  #include <linux/completion.h>
>  #include <linux/kobject.h>
>  #include <linux/notifier.h>
> +#include <linux/spinlock.h>
>  #include <linux/sysfs.h>
> 
>  /*********************************************************************
> @@ -104,6 +105,11 @@ struct cpufreq_policy {
>  	 *     __cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT);
>  	 */
>  	struct rw_semaphore	rwsem;
> +
> +	/* Synchronization for frequency transitions */
> +	bool			transition_ongoing; /* Tracks transition status */
> +	spinlock_t		transition_lock;
> +	wait_queue_head_t	transition_wait;
>  };
> 
>  /* Only for ACPI */
> @@ -336,6 +342,10 @@ void cpufreq_notify_transition(struct cpufreq_policy *policy,
>  		struct cpufreq_freqs *freqs, unsigned int state);
>  void cpufreq_notify_post_transition(struct cpufreq_policy *policy,
>  		struct cpufreq_freqs *freqs, int transition_failed);
> +void cpufreq_freq_transition_begin(struct cpufreq_policy *policy,
> +		struct cpufreq_freqs *freqs);
> +void cpufreq_freq_transition_end(struct cpufreq_policy *policy,
> +		struct cpufreq_freqs *freqs, int transition_failed);
> 
>  #else /* CONFIG_CPU_FREQ */
>  static inline int cpufreq_register_notifier(struct notifier_block *nb,
> 


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