Re: [PATCH v5 7/7] arm64/amu: Use capacity_ref_freq to set AMU ratio

[Will Deacon <will@kernel.org>]

On Tue, Nov 07, 2023 at 12:18:20PM +0100, Vincent Guittot wrote:
> On Tue, 7 Nov 2023 at 11:38, Will Deacon <will@kernel.org> wrote:
> >
> > On Sat, Nov 04, 2023 at 11:59:07AM +0100, Vincent Guittot wrote:
> > > Use the new capacity_ref_freq to set the ratio that is used by AMU for
> > > computing the arch_scale_freq_capacity().
> > > This helps to keep everything aligned using the same reference for
> > > computing CPUs capacity.
> > >
> > > The default value of the ratio (stored in per_cpu(arch_max_freq_scale))
> > > ensures that arch_scale_freq_capacity() returns max capacity until it is
> > > set to its correct value with the cpu capacity and capacity_ref_freq.
> > >
> > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> > > ---
> > >  arch/arm64/kernel/topology.c  | 26 ++++++++++++++------------
> > >  drivers/base/arch_topology.c  | 12 +++++++++++-
> > >  include/linux/arch_topology.h |  1 +
> > >  3 files changed, 26 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > > index 817d788cd866..615c1a20129f 100644
> > > --- a/arch/arm64/kernel/topology.c
> > > +++ b/arch/arm64/kernel/topology.c
> > > @@ -82,7 +82,12 @@ int __init parse_acpi_topology(void)
> > >  #undef pr_fmt
> > >  #define pr_fmt(fmt) "AMU: " fmt
> > >
> > > -static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale);
> > > +/*
> > > + * Ensure that amu_scale_freq_tick() will return SCHED_CAPACITY_SCALE until
> > > + * the CPU capacity and its associated frequency have been correctly
> > > + * initialized.
> > > + */
> > > +static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale) =  1UL << (2 * SCHED_CAPACITY_SHIFT);
> >
> > This doesn't seem to match the comment? SCHED_CAPACITY_SCALE doesn't have
> > the '2 *' multiplier.
> 
> The comment in freq_inv_set_max_ratio() says:
> 
> * We use a factor of 2 * SCHED_CAPACITY_SHIFT -> SCHED_CAPACITY_SCALE²
> * in order to ensure a good resolution for arch_max_freq_scale for
> * very low reference frequencies (down to the KHz range which should
> * be unlikely).
> 
> Then there is a  "  * arch_max_freq_scale) >> SCHED_CAPACITY_SHIFT"
> when computing the scale which brings the result back to
> SCHED_CAPACITY_SHIFT

Ah, I see; I'd not spotted that amu_scale_freq_tick() is doing some
arithmetic on the value (it doesn't return anything because it's 'void').
It's slightly confusing because the comment talks about SCHED_CAPACITY_SCALE
whereas all the code works on the shift, but I get it now.

> > >  static DEFINE_PER_CPU(u64, arch_const_cycles_prev);
> > >  static DEFINE_PER_CPU(u64, arch_core_cycles_prev);
> > >  static cpumask_var_t amu_fie_cpus;
> > > @@ -112,14 +117,14 @@ static inline bool freq_counters_valid(int cpu)
> > >       return true;
> > >  }
> > >
> > > -static int freq_inv_set_max_ratio(int cpu, u64 max_rate, u64 ref_rate)
> > > +void freq_inv_set_max_ratio(int cpu, u64 max_rate)
> > >  {
> > > -     u64 ratio;
> > > +     u64 ratio, ref_rate = arch_timer_get_rate();
> > >
> > >       if (unlikely(!max_rate || !ref_rate)) {
> > > -             pr_debug("CPU%d: invalid maximum or reference frequency.\n",
> > > +             WARN_ONCE(1, "CPU%d: invalid maximum or reference frequency.\n",
> > >                        cpu);
> > > -             return -EINVAL;
> > > +             return;
> > >       }
> > >
> > >       /*
> > > @@ -139,12 +144,12 @@ static int freq_inv_set_max_ratio(int cpu, u64 max_rate, u64 ref_rate)
> > >       ratio = div64_u64(ratio, max_rate);
> > >       if (!ratio) {
> > >               WARN_ONCE(1, "Reference frequency too low.\n");
> > > -             return -EINVAL;
> > > +             return;
> > >       }
> > >
> > > -     per_cpu(arch_max_freq_scale, cpu) = (unsigned long)ratio;
> > > +     WRITE_ONCE(per_cpu(arch_max_freq_scale, cpu), (unsigned long)ratio);
> >
> > Why is WRITE_ONCE() now needed?
> 
> the tick can already use it. We want to make sure to use either the
> old or the new one but not an intermediate value

Isn't that already the case without this patch? In other words, this should
be a separate change.

Will

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