tracing/function-graph-tracer: prevent hangs during self-tests

Impact: detect tracing related hangs

Sometimes, with some configs, the function graph tracer can make
the timer interrupt too much slow, hanging the kernel in an endless
loop of timer interrupts servicing.

As suggested by Ingo, this patch brings a watchdog which stops the
selftest after a defined number of functions traced, definitely
disabling this tracer.

For those who want to debug the cause of the function graph trace
hang, you can pass the ftrace_dump_on_oops kernel parameter to dump
the traces after this hang detection.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <1237694675-23509-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 37 insertions, 1 deletions

diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index 38856ba78a92..b56dcf7d3566 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -248,6 +248,28 @@ trace_selftest_startup_function(struct tracer *trace, struct trace_array *tr)
 
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
+
+/* Maximum number of functions to trace before diagnosing a hang */
+#define GRAPH_MAX_FUNC_TEST     100000000
+
+static void __ftrace_dump(bool disable_tracing);
+static unsigned int graph_hang_thresh;
+
+/* Wrap the real function entry probe to avoid possible hanging */
+static int trace_graph_entry_watchdog(struct ftrace_graph_ent *trace)
+{
+        /* This is harmlessly racy, we want to approximately detect a hang */
+        if (unlikely(++graph_hang_thresh > GRAPH_MAX_FUNC_TEST)) {
+                ftrace_graph_stop();
+                printk(KERN_WARNING "BUG: Function graph tracer hang!\n");
+                if (ftrace_dump_on_oops)
+                        __ftrace_dump(false);
+                return 0;
+        }
+
+        return trace_graph_entry(trace);
+}
+
 /*
  * Pretty much the same than for the function tracer from which the selftest
  * has been borrowed.
@@ -259,15 +281,29 @@ trace_selftest_startup_function_graph(struct tracer *trace,
         int ret;
         unsigned long count;
 
-        ret = tracer_init(trace, tr);
+        /*
+         * Simulate the init() callback but we attach a watchdog callback
+         * to detect and recover from possible hangs
+         */
+        tracing_reset_online_cpus(tr);
+        ret = register_ftrace_graph(&trace_graph_return,
+                                    &trace_graph_entry_watchdog);
         if (ret) {
                 warn_failed_init_tracer(trace, ret);
                 goto out;
         }
+        tracing_start_cmdline_record();
 
         /* Sleep for a 1/10 of a second */
         msleep(100);
 
+        /* Have we just recovered from a hang? */
+        if (graph_hang_thresh > GRAPH_MAX_FUNC_TEST) {
+                trace->reset(tr);
+                ret = -1;
+                goto out;
+        }
+
         tracing_stop();
 
         /* check the trace buffer */