[RFC 1/2] x86/bugs: Disable coresched on hardware that does not need it

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

Some hardware such as certain AMD variants don't have cross-HT MDS/L1TF
issues. Detect this and don't enable core scheduling as it can
needlessly slow the device done.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 arch/x86/kernel/cpu/bugs.c | 8 ++++++++
 kernel/sched/core.c        | 7 +++++++
 kernel/sched/sched.h       | 5 +++++
 3 files changed, 20 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index dece79e4d1e9..0e6e61e49b23 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -152,6 +152,14 @@ void __init check_bugs(void)
 #endif
 }
 
+/*
+ * Do not need core scheduling if CPU does not have MDS/L1TF vulnerability.
+ */
+int arch_allow_core_sched(void)
+{
+	return boot_cpu_has_bug(X86_BUG_MDS) || boot_cpu_has_bug(X86_BUG_L1TF);
+}
+
 void
 x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
 {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 64c559192634..c6158b4959fe 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -319,6 +319,13 @@ static void __sched_core_enable(void)
 	for_each_online_cpu(cpu)
 		BUG_ON(!sched_core_empty(cpu_rq(cpu)));
 
+	/*
+	 * Some architectures may not want coresched. (ex, AMD does not have
+	 * MDS/L1TF issues so it wants SMT completely on).
+	 */
+	if (!arch_allow_core_sched())
+		return;
+
 	static_branch_enable(&__sched_core_enabled);
 	stop_machine(__sched_core_stopper, (void *)true, NULL);
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3cf08c77b678..a1b39764a6ed 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1203,6 +1203,11 @@ int cpu_core_tag_color_write_u64(struct cgroup_subsys_state *css,
 
 bool cfs_prio_less(struct task_struct *a, struct task_struct *b, bool fi);
 
+int __weak arch_allow_core_sched(void)
+{
+	return true;
+}
+
 #else /* !CONFIG_SCHED_CORE */
 
 static inline bool sched_core_enqueued(struct task_struct *task) { return false; }
-- 
2.29.2.222.g5d2a92d10f8-goog