Re: [PATCH v3 4/5] cpufreq: qcom-cpufreq-hw: Use new thermal pressure update function

Lukasz Luba
2021-11-08T14:13:53Z

Hi Thara,

+CC Steev, who discovered this issue with boost
frequency

On 11/5/21 7:12 PM, Thara Gopinath wrote:
> Hi Lukasz,
>
>
> On 11/3/21 12:10 PM, Lukasz Luba wrote:
>> Thermal pressure provides a new API, which allows to use CPU frequency
>> as an argument. That removes the need of local conversion to capacity.
>> Use this new API and remove old local conversion code.
>>
>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
>> ---
>>   drivers/cpufreq/qcom-cpufreq-hw.c | 15 +++++----------
>>   1 file changed, 5 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/cpufreq/qcom-cpufreq-hw.c
>> b/drivers/cpufreq/qcom-cpufreq-hw.c
>> index 0138b2ec406d..425f351450ad 100644
>> --- a/drivers/cpufreq/qcom-cpufreq-hw.c
>> +++ b/drivers/cpufreq/qcom-cpufreq-hw.c
>> @@ -275,10 +275,10 @@ static unsigned int
>> qcom_lmh_get_throttle_freq(struct qcom_cpufreq_data *data)
>>   static void qcom_lmh_dcvs_notify(struct qcom_cpufreq_data *data)
>>   {
>> -    unsigned long max_capacity, capacity, freq_hz, throttled_freq;
>>       struct cpufreq_policy *policy = data->policy;
>>       int cpu = cpumask_first(policy->cpus);
>>       struct device *dev = get_cpu_device(cpu);
>> +    unsigned long freq_hz, throttled_freq;
>>       struct dev_pm_opp *opp;
>>       unsigned int freq;
>> @@ -295,17 +295,12 @@ static void qcom_lmh_dcvs_notify(struct
>> qcom_cpufreq_data *data)
>>       throttled_freq = freq_hz / HZ_PER_KHZ;
>> -    /* Update thermal pressure */
>> -
>> -    max_capacity = arch_scale_cpu_capacity(cpu);
>> -    capacity = mult_frac(max_capacity, throttled_freq,
>> policy->cpuinfo.max_freq);
>> -
>>       /* Don't pass boost capacity to scheduler */
>> -    if (capacity > max_capacity)
>> -        capacity = max_capacity;
>
> So, I think this should go into the common
> topology_update_thermal_pressure in lieu of
>
> +    if (WARN_ON(max_freq < capped_freq))
> +        return;
>
> This will fix the issue Steev Klimaszewski has been reporting
> https://lore.kernel.org/linux-arm-kernel/3cba148a-7077-7b6b-f131-dc65045aa348@arm.com/
>
>
>

Well, I think the issue is broader. Look at the code which
calculate this 'capacity'. It's just a multiplication & division:

max_capacity = arch_scale_cpu_capacity(cpu); // =1024 in our case
capacity = mult_frac(max_capacity, throttled_freq,
policy->cpuinfo.max_freq);

In the reported by Steev output from sysfs cpufreq we know
that the value of 'policy->cpuinfo.max_freq' is:
/sys/devices/system/cpu/cpu5/cpufreq/cpuinfo_max_freq:2956800

so when we put the values to the equation we get:
capacity = 1024 * 2956800 / 2956800; // =1024
The 'capacity' will be always <= 1024 and this check won't
be triggered:

/* Don't pass boost capacity to scheduler */
if (capacity > max_capacity)
capacity = max_capacity;

IIUC you original code, you don't want to have this boost
frequency to be treated as 1024 capacity. The reason is because
the whole capacity machinery in arch_topology.c is calculated based
on max freq value = 2841600,
so the max capacity 1024 would be pinned to that frequency
(according to Steeve's log:
[ 22.552273] THERMAL_PRESSURE: max_freq(2841) < capped_freq(2956) for
CPUs [4-7] )

Having all this in mind, the multiplication and division in your
original code should be done:

capacity = 1024 * 2956800 / 2841600; // = 1065

then clamped to 1024 value.

My change just unveiled this division issue.

With that in mind, I tend to agree that I should have not
rely on passed boost freq value and try to apply your suggestion check.
Let me experiment with that...

Regards,
Lukasz