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

Qais Yousef
2023-12-28T17:45:34Z

On 12/20/23 08:21, Lukasz Luba wrote:
>
>
> 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.

I saw that but wasn't sure if this is always guaranteed. It seems it is from
you're saying, then yes no issues here then.