UBUNTU: SAUCE: sched/smt: Introduce sched_smt_{active,present}

Introduce the scheduler's 'sched_smt_present' static key and provide
the query function 'sched_smt_active()' to be used in subsequent x86
speculation code.

Loosely based on the following upstream commits:
  - 321a874a ("sched/smt: Expose sched_smt_present static key")
  - c5511d03 ("sched/smt: Make sched_smt_present track topology")
  - ba2591a5 ("sched/smt: Update sched_smt_present at runtime")
  - 1b568f0a ("sched/core: Optimize SCHED_SMT")

CVE-2018-12126
CVE-2018-12127
CVE-2018-12130
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

include/linux/sched/smt.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_SCHED_SMT_H
+#define _LINUX_SCHED_SMT_H
+
+#include <linux/static_key.h>
+
+#ifdef CONFIG_SCHED_SMT
+extern struct static_key_false sched_smt_present;
+
+static __always_inline bool sched_smt_active(void)
+{
+	return static_branch_likely(&sched_smt_present);
+}
+#else
+static inline bool sched_smt_active(void) { return false; }
+#endif
+
+#endif

kernel/sched/core.c

@@ -5621,6 +5621,14 @@ static int sched_cpu_active(struct notifier_block *nfb,
 		return NOTIFY_OK;
 
 	case CPU_ONLINE:
+#ifdef CONFIG_SCHED_SMT
+		/*
+		 * When going up, increment the number of cores with SMT
+		 * present.
+		 */
+		if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
+			static_branch_inc(&sched_smt_present);
+#endif
 		/*
 		 * At this point a starting CPU has marked itself as online via
 		 * set_cpu_online(). But it might not yet have marked itself
@@ -5631,6 +5639,14 @@ static int sched_cpu_active(struct notifier_block *nfb,
 		return NOTIFY_OK;
 
 	case CPU_DOWN_FAILED:
+#ifdef CONFIG_SCHED_SMT
+		/*
+		 * Downing the CPU failed but we already decremented the number
+		 * of cores with SMT present so need to increment it again.
+		 */
+		if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
+			static_branch_inc(&sched_smt_present);
+#endif
 		set_cpu_active(cpu, true);
 		return NOTIFY_OK;
 
@@ -5642,9 +5658,19 @@ static int sched_cpu_active(struct notifier_block *nfb,
 static int sched_cpu_inactive(struct notifier_block *nfb,
 					unsigned long action, void *hcpu)
 {
+	int cpu = (long)hcpu;
+
 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_DOWN_PREPARE:
-		set_cpu_active((long)hcpu, false);
+#ifdef CONFIG_SCHED_SMT
+		/*
+		 * When going down, decrement the number of cores with SMT
+		 * present.
+		 */
+		if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
+			static_branch_dec(&sched_smt_present);
+#endif
+		set_cpu_active(cpu, false);
 		return NOTIFY_OK;
 	default:
 		return NOTIFY_DONE;

kernel/sched/fair.c

@@ -4922,6 +4922,10 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
 	return shallowest_idle_cpu != -1 ? shallowest_idle_cpu : least_loaded_cpu;
 }
 
+#ifdef CONFIG_SCHED_SMT
+DEFINE_STATIC_KEY_FALSE(sched_smt_present);
+#endif
+
 /*
  * Try and locate an idle CPU in the sched_domain.
  */

kernel/sched/sched.h

 
 #include <linux/sched.h>
+#include <linux/sched/smt.h>
 #include <linux/sched/sysctl.h>
 #include <linux/sched/rt.h>
 #include <linux/sched/deadline.h>