linux-kernel - [DEBUG PATCH 5/5] powerpc: Use manual hint for cpu parking

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Message-ID: <20250523181448.3777233-6-sshegde@linux.ibm.com>
Date: Fri, 23 May 2025 23:44:48 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, tglx@...utronix.de, yury.norov@...il.com,
        maddy@...ux.ibm.com
Cc: sshegde@...ux.ibm.com, vschneid@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, jstultz@...gle.com, kprateek.nayak@....com,
        huschle@...ux.ibm.com, srikar@...ux.ibm.com,
        linux-kernel@...r.kernel.org, linux@...musvillemoes.dk
Subject: [DEBUG PATCH 5/5] powerpc: Use manual hint for cpu parking

Use sysfs to provide hint. Depending on the system configuration one
needs to decide the number. vp - virtual processor or vCPU. 

For example, when 40 > vp_manual_hint means scheduler is supposed to use
only 0-39 vCPUs. By default, vp_manual_hint is set to all possible CPUs 
and it has be at least 1. 

This is for illustration only. Not meant to be merged. One can modify as
per their arch.  

Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 arch/powerpc/kernel/smp.c | 45 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 5ac7084eebc0..37eb6aa71613 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -64,6 +64,7 @@
 #include <asm/systemcfg.h>
 
 #include <trace/events/ipi.h>
+#include <linux/debugfs.h>
 
 #ifdef DEBUG
 #include <asm/udbg.h>
@@ -82,6 +83,7 @@ bool has_big_cores __ro_after_init;
 bool coregroup_enabled __ro_after_init;
 bool thread_group_shares_l2 __ro_after_init;
 bool thread_group_shares_l3 __ro_after_init;
+static int vp_manual_hint = NR_CPUS;
 
 DEFINE_PER_CPU(cpumask_var_t, cpu_sibling_map);
 DEFINE_PER_CPU(cpumask_var_t, cpu_smallcore_map);
@@ -1727,6 +1729,7 @@ static void __init build_sched_topology(void)
 	BUG_ON(i >= ARRAY_SIZE(powerpc_topology) - 1);
 
 	set_sched_topology(powerpc_topology);
+	vp_manual_hint = num_present_cpus();
 }
 
 void __init smp_cpus_done(unsigned int max_cpus)
@@ -1807,4 +1810,46 @@ void __noreturn arch_cpu_idle_dead(void)
 	start_secondary_resume();
 }
 
+/*
+ * sysfs hint to mark CPUs as parked. This will help in restricting
+ * the workload to specified number of CPUs.
+ * For example 40 > vp_manual_hint means, workload will run on
+ * 0-39 CPUs.
+ */
+
+static int pv_vp_manual_hint_set(void *data, u64 val)
+{
+	int cpu;
+
+	if (val == 0 || vp_manual_hint > num_present_cpus())
+		vp_manual_hint = num_present_cpus();
+
+	if (val != vp_manual_hint)
+		vp_manual_hint = val;
+
+	for_each_present_cpu(cpu) {
+		if (cpu >= vp_manual_hint)
+			set_cpu_parked(cpu, true);
+		else
+			set_cpu_parked(cpu, false);
+	}
+	return 0;
+}
+
+static int pv_vp_manual_hint_get(void *data, u64 *val)
+{
+	*val = vp_manual_hint;
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_pv_vp_manual_hint, pv_vp_manual_hint_get, pv_vp_manual_hint_set, "%llu\n");
+
+static __init int paravirt_debugfs_init(void)
+{
+	if (is_shared_processor())
+		debugfs_create_file("vp_manual_hint", 0600, arch_debugfs_dir, NULL, &fops_pv_vp_manual_hint);
+	return 0;
+}
+
+device_initcall(paravirt_debugfs_init)
 #endif
-- 
2.39.3