[PATCH] torture: Forgive SMT-control failures in CPU-hotplug testing

[Joel Fernandes <joelagnelf@nvidia.com>]

The onoff task ends rcutorture with "End of test: RCU_HOTPLUG" if any
CPU-hotplug attempt fails, other than -EBUSY during boot.  Toggling
/sys/devices/system/cpu/smt/control while onoff runs causes two failures
that are not the test's fault.  cpu_up() returns -EPERM for an SMT thread
that SMT control keeps offline, and device_online() and device_offline()
return 1 when the toggle already did the operation.

Repro: in a 16-vCPU guest (8 cores, 2 threads), run
"modprobe rcutorture onoff_interval=1 gp_exp=0" while looping
"echo off/on > smt/control" for 120 seconds.  This ended in RCU_HOTPLUG,
with no reader errors.  With this change it ends in SUCCESS.

Forgive these like boot-time -EBUSY, by not counting the attempt.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/torture.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/kernel/torture.c b/kernel/torture.c
index 8c4e6b2fe8ba..b860c99e9ca7 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -194,6 +194,24 @@ int torture_num_online_cpus(void)
 }
 EXPORT_SYMBOL_GPL(torture_num_online_cpus);
 
+/*
+ * If the CPU-hotplug failure is not the torture test's fault, return a
+ * string saying why it is forgiven.  Otherwise, return NULL.
+ */
+static const char *torture_onoff_forgive(int ret)
+{
+	// PCI probe frequently disables hotplug during boot.
+	if (!rcu_inkernel_boot_has_ended() && ret == -EBUSY)
+		return " (-EBUSY forgiven during boot)";
+	// Hotplug is forbidden, for example by SMT control.
+	if (ret == -EPERM)
+		return " (-EPERM forgiven, hotplug forbidden)";
+	// Something else, such as SMT control, already did the operation.
+	if (ret == 1)
+		return " (already in requested state, forgiven)";
+	return NULL;
+}
+
 /*
  * Attempt to take a CPU offline.  Return false if the CPU is already
  * offline or if it is not subject to CPU-hotplug operations.  The
@@ -204,7 +222,7 @@ bool torture_offline(int cpu, long *n_offl_attempts, long *n_offl_successes,
 {
 	unsigned long delta;
 	int ret;
-	char *s;
+	const char *s;
 	unsigned long starttime;
 
 	if (!cpu_online(cpu) || !cpu_is_hotpluggable(cpu))
@@ -220,12 +238,11 @@ bool torture_offline(int cpu, long *n_offl_attempts, long *n_offl_successes,
 	(*n_offl_attempts)++;
 	ret = remove_cpu(cpu);
 	if (ret) {
-		s = "";
-		if (!rcu_inkernel_boot_has_ended() && ret == -EBUSY) {
-			// PCI probe frequently disables hotplug during boot.
+		s = torture_onoff_forgive(ret);
+		if (s)
 			(*n_offl_attempts)--;
-			s = " (-EBUSY forgiven during boot)";
-		}
+		else
+			s = "";
 		if (verbose)
 			pr_alert("%s" TORTURE_FLAG
 				 "torture_onoff task: offline %d failed%s: errno %d\n",
@@ -266,7 +283,7 @@ bool torture_online(int cpu, long *n_onl_attempts, long *n_onl_successes,
 {
 	unsigned long delta;
 	int ret;
-	char *s;
+	const char *s;
 	unsigned long starttime;
 
 	if (cpu_online(cpu) || !cpu_is_hotpluggable(cpu))
@@ -280,12 +297,11 @@ bool torture_online(int cpu, long *n_onl_attempts, long *n_onl_successes,
 	(*n_onl_attempts)++;
 	ret = add_cpu(cpu);
 	if (ret) {
-		s = "";
-		if (!rcu_inkernel_boot_has_ended() && ret == -EBUSY) {
-			// PCI probe frequently disables hotplug during boot.
+		s = torture_onoff_forgive(ret);
+		if (s)
 			(*n_onl_attempts)--;
-			s = " (-EBUSY forgiven during boot)";
-		}
+		else
+			s = "";
 		if (verbose)
 			pr_alert("%s" TORTURE_FLAG
 				 "torture_onoff task: online %d failed%s: errno %d\n",
-- 
2.54.0