From 61aaead7e2904a2756ed72a71e36eeeb21591733 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Wed, 17 Sep 2025 03:10:28 -0400
Subject: kern/amd64: cpu: Add process load balancing

This commit introduces load balancing of processes between cores by
using a scalable "CPU arbiter" which decides how to fetch the next core
descriptor.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 src/sys/include/sys/cpuvar.h | 30 ++++++++++++++++++++++++++++++
 src/sys/os/os_sched.c        | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/src/sys/include/sys/cpuvar.h b/src/sys/include/sys/cpuvar.h
index a41131b..e4f3cd1 100644
--- a/src/sys/include/sys/cpuvar.h
+++ b/src/sys/include/sys/cpuvar.h
@@ -33,6 +33,7 @@
 #include <sys/types.h>
 #include <sys/cdefs.h>
 #include <sys/proc.h>
+#include <sys/param.h>
 #if defined(_KERNEL)
 #include <os/sched.h>
 #include <machine/mdcpu.h>
@@ -60,6 +61,35 @@ struct pcore {
 };
 
 #if defined(_KERNEL)
+
+typedef enum {
+    CORE_ARBITER_RR,    /* Round robin */
+} arbiter_type_t;
+
+/*
+ * The processor core arbiter assists in scheduling
+ * processor cores to be used for execution.
+ *
+ * @rr_id: Round robin ID; next processor to be scheduled
+ * @type: Arbitration policy (CORE_ARBITER_RR is default)
+ * @lock: Protects the ID
+ */
+struct core_arbiter {
+    size_t rr_id;
+    arbiter_type_t type;
+    struct spinlock lock;
+} __aligned(COHERENCY_UNIT);
+
+/*
+ * Return a pointer to the next processor that
+ * is ready for queues to be assigned to them
+ *
+ * [MI]
+ *
+ * Returns NULL on failure
+ */
+struct pcore *cpu_sched(void);
+
 /*
  * Configure a processor core on the system
  *
diff --git a/src/sys/os/os_sched.c b/src/sys/os/os_sched.c
index f1bcc8c..e7cc832 100644
--- a/src/sys/os/os_sched.c
+++ b/src/sys/os/os_sched.c
@@ -33,6 +33,7 @@
  */
 
 #include <sys/types.h>
+#include <sys/cdefs.h>
 #include <sys/errno.h>
 #include <sys/syslog.h>
 #include <sys/panic.h>
@@ -40,6 +41,41 @@
 #include <sys/cpuvar.h>
 #include <os/sched.h>
 
+__cacheline_aligned
+static struct core_arbiter arbiter = {
+    .rr_id = 0,
+    .type = CORE_ARBITER_RR
+};
+
+/*
+ * Schedule the next processor core
+ */
+struct pcore *
+cpu_sched(void)
+{
+    struct pcore *retval;
+
+    spinlock_acquire(&arbiter.lock);
+    switch (arbiter.type) {
+    case CORE_ARBITER_RR:
+        retval = cpu_get(arbiter.rr_id++);
+
+        /*
+         * If we made it at the end, wrap to the beginning.
+         * XXX: Us getting entry 0 would make the next be 1.
+         */
+        if (retval == NULL) {
+            arbiter.rr_id = 1;
+            retval = cpu_get(0);
+        }
+
+        break;
+    }
+
+    spinlock_release(&arbiter.lock);
+    return retval;
+}
+
 /*
  * Enqueue a process into a queue
  */
-- 
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