torture: Dump ftrace at shutdown only if requested

If there is a large number of torture tests running concurrently,
all of which are dumping large ftrace buffers at shutdown time, the
resulting dumping can take a very long time, particularly on systems
with rotating-rust storage.  This commit therefore adds a default-off
torture.ftrace_dump_at_shutdown module parameter that enables
shutdown-time ftrace-buffer dumping.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -5096,6 +5096,13 @@
 			Prevent the CPU-hotplug component of torturing
 			until after init has spawned.
 
+	torture.ftrace_dump_at_shutdown= [KNL]
+			Dump the ftrace buffer at torture-test shutdown,
+			even if there were no errors.  This can be a
+			very costly operation when many torture tests
+			are running concurrently, especially on systems
+			with rotating-rust storage.
+
 	tp720=		[HW,PS2]
 
 	tpm_suspend_pcr=[HW,TPM]

kernel/torture.c

@@ -45,6 +45,9 @@ MODULE_AUTHOR("Paul E. McKenney <paulmck@linux.ibm.com>");
 static bool disable_onoff_at_boot;
 module_param(disable_onoff_at_boot, bool, 0444);
 
+static bool ftrace_dump_at_shutdown;
+module_param(ftrace_dump_at_shutdown, bool, 0444);
+
 static char *torture_type;
 static int verbose;
 
@@ -527,7 +530,8 @@ static int torture_shutdown(void *arg)
 		torture_shutdown_hook();
 	else
 		VERBOSE_TOROUT_STRING("No torture_shutdown_hook(), skipping.");
-	rcu_ftrace_dump(DUMP_ALL);
+	if (ftrace_dump_at_shutdown)
+		rcu_ftrace_dump(DUMP_ALL);
 	kernel_power_off();	/* Shut down the system. */
 	return 0;
 }