[PATCH] cpufreq: Don't track pressure if policy->max equals top of freq_table

[Giovanni Gherdovich]

The acpi-cpufreq driver stores non-boost frequencies in
policy->freq_table, but policy->cpuinfo.max_freq contains the max
boost frequency.

policy->max is constrained to be, at most, the largest value in
freq_table. This means that under nominal conditions, policy->max is
substantially lower than policy->cpuinfo.max_freq, resulting in large
values of cpufreq_pressure, on all CPUs, for no reason.
The result is large performance regressions on compute/memory intensive
benchmarks that use a limited number of CPUs; tasks migrate more and
lose locality. My tests are with the "NAS Parallel Benchmarks" suite,
limiting to 1/4 of available CPUs.

This change sets cpufreq_pressure to zero if policy->max is equal
(or above) the maximum frequency in freq_table.

Here a concrete example from a 1st generation AMD EPYC (Naples) using
acpi-cpufreq. Under nominal conditions (no capping, no throttling):

freq_table               : 1200 MHz, 1700 MHz, 2200 MHz
policy->max              : 2200 MHz (maximum from freq_table)
policy->cpuinfo.max_freq : 3200 MHz (max boost)

Before this change, in the example above we'd get a cpufreq_pressure
value of 320 (32% pressure); with the change, the pressure is zero as
it's expected to be.

Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
---
Clarifying the recipients list, beyond the cpufreq maintainers:

Mario Limonciello, Huang Rui: the patch isn't for amd-pstate, but
   AMD EPYC up to Genoa (Zen4) defaults to acpi-cpufreq, so they may come
   across this if they haven't already.
Vincent Guittot: for the cpufreq_pressure and scheduler load balancer
   implications.
Ricardo Neri: he's been looking at cpufreq_pressure on x86, albeit
   this patch isn't for intel_pstate.
Pierre Gondois: he looked at policy->max in freq_table.c earlier this year.

 drivers/cpufreq/cpufreq.c    | 3 +++
 drivers/cpufreq/freq_table.c | 1 +
 include/linux/cpufreq.h      | 1 +
 3 files changed, 5 insertions(+)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 54dde8419bdc..aa652750f711 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -2601,6 +2601,9 @@ static void cpufreq_update_pressure(struct cpufreq_policy *policy)
 	 */
 	if (max_freq <= capped_freq) {
 		pressure = 0;
+	} else if (policy->freq_table && policy->max_table_freq &&
+		   policy->max_table_freq <= capped_freq) {
+		pressure = 0;
 	} else {
 		max_capacity = arch_scale_cpu_capacity(cpu);
 		pressure = max_capacity -
diff --git a/drivers/cpufreq/freq_table.c b/drivers/cpufreq/freq_table.c
index ea994647abc8..55c3293685c2 100644
--- a/drivers/cpufreq/freq_table.c
+++ b/drivers/cpufreq/freq_table.c
@@ -50,6 +50,7 @@ int cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy)
 	}
 
 	policy->cpuinfo.min_freq = min_freq;
+	policy->max_table_freq = max_freq;
 	/*
 	 * If the driver has set its own cpuinfo.max_freq above max_freq, leave
 	 * it as is.
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index d3d0d9d02aa4..4a3316e47f4e 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -67,6 +67,7 @@ struct cpufreq_policy {
 	unsigned int		max;    /* in kHz */
 	unsigned int		cur;    /* in kHz, only needed if cpufreq
 					 * governors are used */
+	unsigned int		max_table_freq; /* max freq in the table */
 	unsigned int		suspend_freq; /* freq to set during suspend */
 
 	unsigned int		policy; /* see above */
-- 
2.43.0