Re: [PATCH 12/15] hwmon, fam15h_power: introduce a cpu accumulated power reporting algorithm

Guenter Roeck
2015-08-28T14:05:21Z

On 08/28/2015 03:45 AM, Huang Rui wrote:
> On Thu, Aug 27, 2015 at 10:30:43AM -0700, Guenter Roeck wrote:
>> On Thu, Aug 27, 2015 at 04:07:43PM +0800, Huang Rui wrote:
>>> This patch introduces an algorithm that computes the average power by
>>> reading a delta value of “core power accumulator” register during
>>> measurement interval, and then dividing delta value by the length of
>>> the time interval.
>>>
>>> User is able to use power1_acc entry to measure the processor power
>>> consumption and power1_acc just needs to be read twice with an needed
>>> interval in-between.
>>>
>>> A simple example:
>>>
>>> $ cat /sys/bus/pci/devices/0000\:00\:18.4/hwmon/hwmon0/power1_acc
>>> $ sleep 10000s
>>> $ cat /sys/bus/pci/devices/0000\:00\:18.4/hwmon/hwmon0/power1_acc
>>>
>>> The result is current average processor power consumption in 10000
>>> seconds. The unit of the result is uWatt.
>>>
>>> Signed-off-by: Huang Rui <ray.huang@amd.com>
>>> ---
>>> drivers/hwmon/fam15h_power.c | 62 ++++++++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 62 insertions(+)
>>>
>>> diff --git a/drivers/hwmon/fam15h_power.c b/drivers/hwmon/fam15h_power.c
>>> index d529e4b..3bab797 100644
>>> --- a/drivers/hwmon/fam15h_power.c
>>> +++ b/drivers/hwmon/fam15h_power.c
>>> @@ -60,6 +60,7 @@ struct fam15h_power_data {
>>> u64 cu_acc_power[MAX_CUS];
>>> /* performance timestamp counter */
>>> u64 cpu_sw_pwr_ptsc[MAX_CUS];
>>> + struct mutex acc_pwr_mutex;
>>> };
>>>
>>> static ssize_t show_power(struct device *dev,
>>> @@ -121,17 +122,74 @@ static DEVICE_ATTR(power1_crit, S_IRUGO, show_power_crit, NULL);
>>> static struct attribute_group fam15h_power_group;
>>> __ATTRIBUTE_GROUPS(fam15h_power);
>>>
>>> +static ssize_t show_power_acc(struct device *dev,
>>> + struct device_attribute *attr, char *buf)
>>> +{
>>> + int cpu, cu, cu_num, cores_per_cu;
>>> + u64 curr_cu_acc_power[MAX_CUS],
>>> + curr_ptsc[MAX_CUS], jdelta[MAX_CUS];
>>> + u64 tdelta, avg_acc;
>>> + struct fam15h_power_data *data = dev_get_drvdata(dev);
>>> +
>>> + cores_per_cu = amd_get_cores_per_cu();
>>> + cu_num = boot_cpu_data.x86_max_cores / cores_per_cu;
>>> +
>>> + for (cpu = 0, avg_acc = 0; cpu < cu_num * cores_per_cu; cpu += cores_per_cu) {
>>> + cu = cpu / cores_per_cu;
>>> + if (rdmsrl_safe_on_cpu(cpu, MSR_F15H_PTSC, &curr_ptsc[cu])) {
>>> + pr_err("Failed to read PTSC counter MSR on core%d\n",
>>> + cpu);
>>> + return 0;
>>> + }
>>> +
>>> + if (rdmsrl_safe_on_cpu(cpu, MSR_F15H_CU_PWR_ACCUMULATOR,
>>> + &curr_cu_acc_power[cu])) {
>>> + pr_err("Failed to read compute unit power accumulator MSR on core%d\n",
>>> + cpu);
>>> + return 0;
>>> + }
>>> +
>>> + if (curr_cu_acc_power[cu] < data->cu_acc_power[cu]) {
>>> + jdelta[cu] = data->max_cu_acc_power + curr_cu_acc_power[cu];
>>> + jdelta[cu] -= data->cu_acc_power[cu];
>>> + } else {
>>> + jdelta[cu] = curr_cu_acc_power[cu] - data->cu_acc_power[cu];
>>> + }
>>> + tdelta = curr_ptsc[cu] - data->cpu_sw_pwr_ptsc[cu];
>>> + jdelta[cu] *= data->cpu_pwr_sample_ratio * 1000;
>>> + do_div(jdelta[cu], tdelta);
>>> +
>>> + mutex_lock(&data->acc_pwr_mutex);
>>> + data->cu_acc_power[cu] = curr_cu_acc_power[cu];
>>> + data->cpu_sw_pwr_ptsc[cu] = curr_ptsc[cu];
>>> + mutex_unlock(&data->acc_pwr_mutex);
>>> +
>>> + /* the unit is microWatt */
>>> + avg_acc += jdelta[cu];
>>> + }
>>> +
>>> + return sprintf(buf, "%u\n", (unsigned int) avg_acc);
>>> +}
>>> +static DEVICE_ATTR(power1_acc, S_IRUGO, show_power_acc, NULL);
>>
>> I am not really a friend of introducing a non-standard attribute.
>> Does the energy attribute not work here ?
>>
>
> You're right. Non-standard attribute might not be good. Could you
> please give me some hints if I use "energy" instead?
>
1 Joule = 1 Watt-second.

Something else, though - did you make sure that your code doesn't overflow ?
Even though you calculate the average in an u64, you display it as unsigned.

100w * 10,000s = 1,000,000ws = 1,000,000,000,000 micro-watt-seconds, which is
a bit large for an unsigned.

Also, the values should not be reset after reading, but accumulate.

Also, I think your code may be vulnerable to overflows on the CPU register side.
How long does it take before the CPU counters overflow ?

Thanks,
Guenter