ftrace: ftrace_dump_on_oops=[tracer]

Impact: add new (optional) debug boot option

In order to facilitate early boot trouble, allow one to specify a tracer
on the kernel boot line.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 46 insertions, 20 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 1bbcaa8982b6..4862284d3119 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -765,6 +765,14 @@ and is between 256 and 4096 characters. It is defined in the file
                         parameter will force ia64_sal_cache_flush to call
                         ia64_pal_cache_flush instead of SAL_CACHE_FLUSH.
 
+        ftrace=[tracer]
+                        [ftrace] will set and start the specified tracer
+                        as early as possible in order to facilitate early
+                        boot debugging.
+
+        ftrace_dump_on_oops
+                        [ftrace] will dump the trace buffers on oops.
+
         gamecon.map[2|3]=
                         [HW,JOY] Multisystem joystick and NES/SNES/PSX pad
                         support via parallel port (up to 5 devices per port)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index bdb1df00fb10..482583eb8001 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -79,6 +79,15 @@ static int tracing_disabled = 1;
  */
 int ftrace_dump_on_oops;
 
+static int tracing_set_tracer(char *buf);
+
+static int __init set_ftrace(char *str)
+{
+        tracing_set_tracer(str);
+        return 1;
+}
+__setup("ftrace", set_ftrace);
+
 static int __init set_ftrace_dump_on_oops(char *str)
 {
         ftrace_dump_on_oops = 1;
@@ -2394,29 +2403,11 @@ tracing_set_trace_read(struct file *filp, char __user *ubuf,
         return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
 }
 
-static ssize_t
-tracing_set_trace_write(struct file *filp, const char __user *ubuf,
-                        size_t cnt, loff_t *ppos)
+static int tracing_set_tracer(char *buf)
 {
         struct trace_array *tr = &global_trace;
         struct tracer *t;
-        char buf[max_tracer_type_len+1];
-        int i;
-        size_t ret;
-
-        ret = cnt;
-
-        if (cnt > max_tracer_type_len)
-                cnt = max_tracer_type_len;
-
-        if (copy_from_user(&buf, ubuf, cnt))
-                return -EFAULT;
-
-        buf[cnt] = 0;
-
-        /* strip ending whitespace. */
-        for (i = cnt - 1; i > 0 && isspace(buf[i]); i--)
-                buf[i] = 0;
+        int ret = 0;
 
         mutex_lock(&trace_types_lock);
         for (t = trace_types; t; t = t->next) {
@@ -2440,6 +2431,33 @@ tracing_set_trace_write(struct file *filp, const char __user *ubuf,
  out:
         mutex_unlock(&trace_types_lock);
 
+        return ret;
+}
+
+static ssize_t
+tracing_set_trace_write(struct file *filp, const char __user *ubuf,
+                        size_t cnt, loff_t *ppos)
+{
+        char buf[max_tracer_type_len+1];
+        int i;
+        size_t ret;
+
+        if (cnt > max_tracer_type_len)
+                cnt = max_tracer_type_len;
+
+        if (copy_from_user(&buf, ubuf, cnt))
+                return -EFAULT;
+
+        buf[cnt] = 0;
+
+        /* strip ending whitespace. */
+        for (i = cnt - 1; i > 0 && isspace(buf[i]); i--)
+                buf[i] = 0;
+
+        ret = tracing_set_tracer(buf);
+        if (!ret)
+                ret = cnt;
+
         if (ret > 0)
                 filp->f_pos += ret;