On Sat, 1 Oct 2016, Srinivas Pandruvada wrote:

+static int sched_itmt_update_handler(struct ctl_table *table, int write,
+			      void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	int ret;
+	unsigned int old_sysctl;
+
+	mutex_lock(&itmt_update_mutex);
+
+	if (!sched_itmt_capable) {
+		mutex_unlock(&itmt_update_mutex);
+		return 0;

This should return a proper error code.

 void sched_set_itmt_support(bool itmt_supported)
 {
 	mutex_lock(&itmt_update_mutex);
 
-	if (itmt_supported != sched_itmt_capable)
-		sched_itmt_capable = itmt_supported;
+	if (itmt_supported == sched_itmt_capable) {
+		mutex_unlock(&itmt_update_mutex);
+		return;
+	}
+	sched_itmt_capable = itmt_supported;
+
+	if (itmt_supported) {
+		itmt_sysctl_header =
+			register_sysctl_table(itmt_root_table);
+		if (!itmt_sysctl_header) {
+			mutex_unlock(&itmt_update_mutex);
+			return;

So you now have a state of capable which cannot be enabled. Whats the
point?

+		}
+		/*
+		 * ITMT capability automatically enables ITMT
+		 * scheduling for small systems (single node).
+		 */
+		if (topology_num_packages() == 1)
+			sysctl_sched_itmt_enabled = 1;
+	} else {
+		if (itmt_sysctl_header)
+			unregister_sysctl_table(itmt_sysctl_header);
+	}
+
+	if (sysctl_sched_itmt_enabled) {
+		/* disable sched_itmt if we are no longer ITMT capable */
+		if (!itmt_supported)

How do you get here if itmt is not supported? 

+			sysctl_sched_itmt_enabled = 0;
+		x86_topology_update = true;
+		rebuild_sched_domains();
+	}
 
 	mutex_unlock(&itmt_update_mutex);

Thanks,

	tglx