Re: [PATCH v5 13/23] PM: EM: Add performance field to struct em_perf_state

[Lukasz Luba <lukasz.luba@arm.com>]

On 12/17/23 18:00, Qais Yousef wrote:
> On 11/29/23 11:08, Lukasz Luba wrote:
>> The performance doesn't scale linearly with the frequency. Also, it may
>> be different in different workloads. Some CPUs are designed to be
>> particularly good at some applications e.g. images or video processing
>> and other CPUs in different. When those different types of CPUs are
>> combined in one SoC they should be properly modeled to get max of the HW
>> in Energy Aware Scheduler (EAS). The Energy Model (EM) provides the
>> power vs. performance curves to the EAS, but assumes the CPUs capacity
>> is fixed and scales linearly with the frequency. This patch allows to
>> adjust the curve on the 'performance' axis as well.
>>
>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
>> ---
>>   include/linux/energy_model.h | 11 ++++++-----
>>   kernel/power/energy_model.c  | 27 +++++++++++++++++++++++++++
>>   2 files changed, 33 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
>> index ae3ccc8b9f44..e30750500b10 100644
>> --- a/include/linux/energy_model.h
>> +++ b/include/linux/energy_model.h
>> @@ -13,6 +13,7 @@
>>   
>>   /**
>>    * struct em_perf_state - Performance state of a performance domain
>> + * @performance:	Non-linear CPU performance at a given frequency
>>    * @frequency:	The frequency in KHz, for consistency with CPUFreq
>>    * @power:	The power consumed at this level (by 1 CPU or by a registered
>>    *		device). It can be a total power: static and dynamic.
>> @@ -21,6 +22,7 @@
>>    * @flags:	see "em_perf_state flags" description below.
>>    */
>>   struct em_perf_state {
>> +	unsigned long performance;
>>   	unsigned long frequency;
>>   	unsigned long power;
>>   	unsigned long cost;
>> @@ -207,14 +209,14 @@ void em_free_table(struct em_perf_table __rcu *table);
>>    */
>>   static inline int
>>   em_pd_get_efficient_state(struct em_perf_state *table, int nr_perf_states,
>> -			  unsigned long freq, unsigned long pd_flags)
>> +			  unsigned long max_util, unsigned long pd_flags)
>>   {
>>   	struct em_perf_state *ps;
>>   	int i;
>>   
>>   	for (i = 0; i < nr_perf_states; i++) {
>>   		ps = &table[i];
>> -		if (ps->frequency >= freq) {
>> +		if (ps->performance >= max_util) {
>>   			if (pd_flags & EM_PERF_DOMAIN_SKIP_INEFFICIENCIES &&
>>   			    ps->flags & EM_PERF_STATE_INEFFICIENT)
>>   				continue;
>> @@ -246,8 +248,8 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
>>   				unsigned long allowed_cpu_cap)
>>   {
>>   	struct em_perf_table *runtime_table;
>> -	unsigned long freq, scale_cpu;
>>   	struct em_perf_state *ps;
>> +	unsigned long scale_cpu;
>>   	int cpu, i;
>>   
>>   	if (!sum_util)
>> @@ -274,14 +276,13 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
>>   
>>   	max_util = map_util_perf(max_util);
>>   	max_util = min(max_util, allowed_cpu_cap);
>> -	freq = map_util_freq(max_util, ps->frequency, scale_cpu);
>>   
>>   	/*
>>   	 * Find the lowest performance state of the Energy Model above the
>>   	 * requested frequency.
>>   	 */
>>   	i = em_pd_get_efficient_state(runtime_table->state, pd->nr_perf_states,
>> -				      freq, pd->flags);
>> +				      max_util, pd->flags);
>>   	ps = &runtime_table->state[i];
>>   
>>   	/*
>> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
>> index 614891fde8df..b5016afe6a19 100644
>> --- a/kernel/power/energy_model.c
>> +++ b/kernel/power/energy_model.c
>> @@ -46,6 +46,7 @@ static void em_debug_create_ps(struct em_perf_state *ps, struct dentry *pd)
>>   	debugfs_create_ulong("frequency", 0444, d, &ps->frequency);
>>   	debugfs_create_ulong("power", 0444, d, &ps->power);
>>   	debugfs_create_ulong("cost", 0444, d, &ps->cost);
>> +	debugfs_create_ulong("performance", 0444, d, &ps->performance);
>>   	debugfs_create_ulong("inefficient", 0444, d, &ps->flags);
>>   }
>>   
>> @@ -171,6 +172,30 @@ em_allocate_table(struct em_perf_domain *pd)
>>   	return table;
>>   }
>>   
>> +static void em_init_performance(struct device *dev, struct em_perf_domain *pd,
>> +				struct em_perf_state *table, int nr_states)
>> +{
>> +	u64 fmax, max_cap;
>> +	int i, cpu;
>> +
>> +	/* This is needed only for CPUs and EAS skip other devices */
>> +	if (!_is_cpu_device(dev))
>> +		return;
>> +
>> +	cpu = cpumask_first(em_span_cpus(pd));
>> +
>> +	/*
>> +	 * Calculate the performance value for each frequency with
>> +	 * linear relationship. The final CPU capacity might not be ready at
>> +	 * boot time, but the EM will be updated a bit later with correct one.
>> +	 */
>> +	fmax = (u64) table[nr_states - 1].frequency;
>> +	max_cap = (u64) arch_scale_cpu_capacity(cpu);
>> +	for (i = 0; i < nr_states; i++)
>> +		table[i].performance = div64_u64(max_cap * table[i].frequency,
>> +						 fmax);
> 
> Should we sanity check the returned performance value is correct in case we got
> passed a malformed table? Maybe the table is sanity checked and sorted before
> we get here; I didn't check to be honest.

The frequency values are checked if they have asc sorting order. It's
done in the em_create_perf_table(). There is even an error printed and
returned, so the EM registration will fail.

> 
> I think a warning that performance is always <= max_cap would be helpful in
> general as code evolved in the future.

I don't see that need. There are needed checks for frequency values and
this simple math formula is just linear. Nothing can happen when
frequencies are sorted asc. The whole EAS relies on that fact:

Frequencies are sorted ascending, thus
fmax = (u64) table[nr_states - 1].frequency
is always true.