tracing: Add tp_printk cmdline to have tracepoints go to printk()

Add the kernel command line tp_printk option that will have tracepoints
that are active sent to printk() as well as to the trace buffer.

Passing "tp_printk" will activate this. To turn it off, the sysctl
/proc/sys/kernel/tracepoint_printk can have '0' echoed into it. Note,
this only works if the cmdline option is used. Echoing 1 into the sysctl
file without the cmdline option will have no affect.

Note, this is a dangerous option. Having high frequency tracepoints send
their data to printk() can possibly cause a live lock. This is another
reason why this is only active if the command line option is used.

Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1412121539300.16494@nanos

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

4 files changed, 57 insertions, 0 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 4aada6d9fe74..bb50c2187194 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -622,6 +622,13 @@ static struct ctl_table kern_table[] = {
                 .mode           = 0644,
                 .proc_handler   = proc_dointvec,
         },
+        {
+                .procname       = "tracepoint_printk",
+                .data           = &tracepoint_printk,
+                .maxlen         = sizeof(tracepoint_printk),
+                .mode           = 0644,
+                .proc_handler   = proc_dointvec,
+        },
 #endif
 #ifdef CONFIG_KEXEC
         {
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ec3ca694665f..e890d2d4ec89 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -63,6 +63,10 @@ static bool __read_mostly tracing_selftest_running;
  */
 bool __read_mostly tracing_selftest_disabled;
 
+/* Pipe tracepoints to printk */
+struct trace_iterator *tracepoint_print_iter;
+int tracepoint_printk;
+
 /* For tracers that don't implement custom flags */
 static struct tracer_opt dummy_tracer_opt[] = {
         { }
@@ -193,6 +197,13 @@ static int __init set_trace_boot_clock(char *str)
 }
 __setup("trace_clock=", set_trace_boot_clock);
 
+static int __init set_tracepoint_printk(char *str)
+{
+        if ((strcmp(str, "=0") != 0 && strcmp(str, "=off") != 0))
+                tracepoint_printk = 1;
+        return 1;
+}
+__setup("tp_printk", set_tracepoint_printk);
 
 unsigned long long ns2usecs(cycle_t nsec)
 {
@@ -6878,6 +6889,12 @@ out:
 
 void __init trace_init(void)
 {
+        if (tracepoint_printk) {
+                tracepoint_print_iter =
+                        kmalloc(sizeof(*tracepoint_print_iter), GFP_KERNEL);
+                if (WARN_ON(!tracepoint_print_iter))
+                        tracepoint_printk = 0;
+        }
         tracer_alloc_buffers();
         init_ftrace_syscalls();
         trace_event_init();     
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index c138c149d6ef..8de48bac1ce2 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1313,5 +1313,6 @@ void trace_event_init(void);
 static inline void __init trace_event_init(void) { }
 #endif
 
+extern struct trace_iterator *tracepoint_print_iter;
 
 #endif /* _LINUX_KERNEL_TRACE_H */
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index fd9deb0e03f0..9f7175a3df71 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -212,8 +212,40 @@ void *ftrace_event_buffer_reserve(struct ftrace_event_buffer *fbuffer,
 }
 EXPORT_SYMBOL_GPL(ftrace_event_buffer_reserve);
 
+static DEFINE_SPINLOCK(tracepoint_iter_lock);
+
+static void output_printk(struct ftrace_event_buffer *fbuffer)
+{
+        struct ftrace_event_call *event_call;
+        struct trace_event *event;
+        unsigned long flags;
+        struct trace_iterator *iter = tracepoint_print_iter;
+
+        if (!iter)
+                return;
+
+        event_call = fbuffer->ftrace_file->event_call;
+        if (!event_call || !event_call->event.funcs ||
+            !event_call->event.funcs->trace)
+                return;
+
+        event = &fbuffer->ftrace_file->event_call->event;
+
+        spin_lock_irqsave(&tracepoint_iter_lock, flags);
+        trace_seq_init(&iter->seq);
+        iter->ent = fbuffer->entry;
+        event_call->event.funcs->trace(iter, 0, event);
+        trace_seq_putc(&iter->seq, 0);
+        printk("%s", iter->seq.buffer);
+
+        spin_unlock_irqrestore(&tracepoint_iter_lock, flags);
+}
+
 void ftrace_event_buffer_commit(struct ftrace_event_buffer *fbuffer)
 {
+        if (tracepoint_printk)
+                output_printk(fbuffer);
+
         event_trigger_unlock_commit(fbuffer->ftrace_file, fbuffer->buffer,
                                     fbuffer->event, fbuffer->entry,
                                     fbuffer->flags, fbuffer->pc);