powerpc/smp: Avoid asym packing within thread_group of a core

JIRA: https://issues.redhat.com/browse/RHEL-118964

commit 0e93f1c
Author: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Date:   Thu Dec 14 23:37:14 2023 +0530

    powerpc/smp: Avoid asym packing within thread_group of a core

    PowerVM Hypervisor will schedule at a core granularity. However each
    core can have more than one thread_groups. For better utilization in
    case of a shared processor, its preferable for the scheduler to pack to
    the lowest core. However there is no benefit of moving a thread between
    two thread groups of the same core.

    Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://msgid.link/20231214180720.310852-5-srikar@linux.vnet.ibm.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

arch/powerpc/kernel/smp.c

@@ -1779,6 +1779,19 @@ void __init smp_cpus_done(unsigned int max_cpus)
 	set_sched_topology(powerpc_topology);
 }
 
+/*
+ * For asym packing, by default lower numbered CPU has higher priority.
+ * On shared processors, pack to lower numbered core. However avoid moving
+ * between thread_groups within the same core.
+ */
+int arch_asym_cpu_priority(int cpu)
+{
+	if (static_branch_unlikely(&splpar_asym_pack))
+		return -cpu / threads_per_core;
+
+	return -cpu;
+}
+
 #ifdef CONFIG_HOTPLUG_CPU
 int __cpu_disable(void)
 {