cpufreq: intel_pstate: Make it possible to avoid enabling CAS

Capacity-aware scheduling (CAS) is enabled by default by intel_pstate on
hybrid systems without SMT, but in some usage scenarios it may be more
attractive to place tasks for maximum CPU performance regardless of the
extra cost in terms of energy, which is the case on such systems when
CAS is not enabled, so introduce a command line option to forbid
intel_pstate to enable CAS.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by:Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://patch.msgid.link/2781262.mvXUDI8C0e@rjwysocki.net

Author: rafaeljw

Documentation/admin-guide/kernel-parameters.txt

@@ -2316,6 +2316,9 @@
 			per_cpu_perf_limits
 			  Allow per-logical-CPU P-State performance control limits using
 			  cpufreq sysfs interface
+			no_cas
+			  Do not enable capacity-aware scheduling (CAS) on
+			  hybrid systems
 
 	intremap=	[X86-64,Intel-IOMMU,EARLY]
 			on	enable Interrupt Remapping (default)

Documentation/admin-guide/pm/intel_pstate.rst

@@ -696,6 +696,9 @@ of them have to be prepended with the ``intel_pstate=`` prefix.
 	Use per-logical-CPU P-State limits (see `Coordination of P-state
 	Limits`_ for details).
 
+``no_cas``
+	Do not enable capacity-aware scheduling (CAS) which is enabled by
+	default on hybrid systems.
 
 Diagnostics and Tuning
 ======================

drivers/cpufreq/intel_pstate.c

@@ -936,6 +936,8 @@ static struct freq_attr *hwp_cpufreq_attrs[] = {
 	NULL,
 };
 
+static bool no_cas __ro_after_init;
+
 static struct cpudata *hybrid_max_perf_cpu __read_mostly;
 /*
  * Protects hybrid_max_perf_cpu, the capacity_perf fields in struct cpudata,
@@ -1041,6 +1043,10 @@ static void hybrid_refresh_cpu_capacity_scaling(void)
 
 static void hybrid_init_cpu_capacity_scaling(bool refresh)
 {
+	/* Bail out if enabling capacity-aware scheduling is prohibited. */
+	if (no_cas)
+		return;
+
 	/*
 	 * If hybrid_max_perf_cpu is set at this point, the hybrid CPU capacity
 	 * scaling has been enabled already and the driver is just changing the
@@ -3835,6 +3841,9 @@ static int __init intel_pstate_setup(char *str)
 	if (!strcmp(str, "no_hwp"))
 		no_hwp = 1;
 
+	if (!strcmp(str, "no_cas"))
+		no_cas = true;
+
 	if (!strcmp(str, "force"))
 		force_load = 1;
 	if (!strcmp(str, "hwp_only"))