Re: [PATCH] cpufreq: Add scaling frequency range support

[Pan Xinhui]

hi, Rafael
	thanks for you reply.

On 2015å07æ29æ 08:18, Rafael J. Wysocki wrote:
> On Tuesday, July 28, 2015 12:53:33 PM Pan Xinhui wrote:
>> hi, Viresh
>> 	thanks for your reply :)
>> On 2015å07æ28æ 12:29, Viresh Kumar wrote:
>>> On 28-07-15, 11:32, Pan Xinhui wrote:
>>>> From: Pan Xinhui <xinhuix.pan@xxxxxxxxx>
>>>>
>>>> Userspace at most time do cpufreq tests very much inconveniently.
>>>> Currently they have to echo min and max cpu freq separately like below:
>>>> echo 480000  > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
>>>> echo 2240000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
>>>>
>>>> Add scaling_freq_range cpufreq attr to support userspace's demand.
>>>> Therefore it's easier for testers to write readable scripts like below: 
>>>> echo 480000-2240000 >
>>>> /sys/devices/system/cpu/cpu0/cpufreq/scaling_freq_range
>>>
>>> I don't think this brings any good change, we already have support for
>>> that with min/max freqs and I don't see how scripts can be less
>>> readable with that.
>>>
>> yes, min/max are supported, however it is inconvenient. sometime it's very easy to cause obscure bugs.
>> For example, some one might write a script like below.
>> echo 480000  > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
>> echo 960000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
>> .....//other works
>> echo 1120000  > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
>> echo 2240000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
>> ...//other works
>>
>> But it did not work when we echo 112000 to min-freq, as the current max freq is smaller than it.
>> It's hard to figure it out in a big script... we have many such scripts.
> 
> Fix them, then, pretty please.
> 
of course we will fix them. :)

> And adding this attribute is not going to magically fix them, is it?
> 
yes, this patch can not fix them without changing the script. BUT I have another patch which could magically fix them. :)

These two attribute files are very tricky. they are related with each other.
Not like some other attribute file in other part of kernel, for example, proc/sys/fs/file-max.
As the file-min is always zero. It's very reasonable to only support file-max attribute file.

The sequence we echoing value to min/max_freq is very important. Maybe we can also assume they have *state*.
Just like a developer writes a buf to a file. he should do in this way below.
fp = fopen(..)
 => fwrite(...)
  => fclose(...)

The script I mentioned above did not follow the right sequence. when script wants to set the min higher, we need set the max first to avoid min > max issue...
So max/min_freq have *state*. just like TCP Three-way handshake, SYN, ACK&SYN, ACK. the sequence(this is so-called state) is very important.

Now I want to offer a non-state attribute to user-space :)
This is a design/engineering problem. It's okay for kernel to not offer such attribute. But user-space will do more work.
For example, In the worst case, we need system call four times.
read min/max_freq (system call two times)
might set min or max freq first to avoid min > max issue (system call one time)
set min/max a new value (system call one time)

What if we offer *set freq range* attribute? just once. :)
set freq range (system call one time)

>From performance point, It's a good idea to offer such attribute.

There is another reason for why it's good to apply this patch.
If cpufreg range is 480000-960000, we call it powersave, 480000-2240000 is normal, 1920000-2240000 is performance.
Assume current cpufreq range is powersave, then user want to set it to performance because user wants to play a 3D game.
BUT user have to set it to normal first, then set it to performance because min(performance) > max(powersave).....
I don't know how people(end-user) would think about such behavior.... why we must be back to normal first, then performance?

As for the patch I mentioned above which could magically fix them.
The solution is: change store_scaling_max_freq and store_scaling_min_freq sysfs callback, let them have *state*.
Always keep the value from user-space.

patch like:
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 8772346..00e6965 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -615,6 +615,14 @@ static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
 static int cpufreq_set_policy(struct cpufreq_policy *policy,
                                struct cpufreq_policy *new_policy);
 
+static void
+cpufreq_get_user_policy_freq(struct cpufreq_real_policy *user_policy,
+                               struct cpufreq_policy *policy)
+{
+       policy->min = user_policy->min;
+       policy->max = user_policy->max;
+}
+
 /**
  * cpufreq_per_cpu_attr_write() / store_##file_name() - sysfs write access
  */
@@ -622,21 +630,20 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
 static ssize_t store_##file_name                                       \
 (struct cpufreq_policy *policy, const char *buf, size_t count)         \
 {                                                                      \
-       int ret, temp;                                                  \
+       int ret;                                                        \
        struct cpufreq_policy new_policy;                               \
                                                                        \
        ret = cpufreq_get_policy(&new_policy, policy->cpu);             \
        if (ret)                                                        \
                return -EINVAL;                                         \
                                                                        \
+       cpufreq_get_user_policy_freq(&policy->user_policy, &new_policy);\
        ret = sscanf(buf, "%u", &new_policy.object);                    \
        if (ret != 1)                                                   \
                return -EINVAL;                                         \
                                                                        \
-       temp = new_policy.object;                                       \
-       ret = cpufreq_set_policy(policy, &new_policy);          \
-       if (!ret)                                                       \
-               policy->user_policy.object = temp;                      \
+       policy->user_policy.object = policy->object;                    \
+       ret = cpufreq_set_policy(policy, &new_policy);                  \
                                                                        \
        return ret ? ret : count;                                       \
 }




Thanks
xinhui

> Thanks,
> Rafael
> 
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