On Tue, 7 Nov 2023 at 11:38, Will Deacon [off-list ref] wrote:

On Sat, Nov 04, 2023 at 11:59:07AM +0100, Vincent Guittot wrote:

quoted

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

quoted

 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

quoted

-     return 0;
+     return;

Pointless 'return;' statement.

fair enough

Will

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