ftrace: soft tracing stop and start

Impact: add way to quickly start stop tracing from the kernel

This patch adds a soft stop and start to the trace. This simply
disables function tracing via the ftrace_disabled flag, and
disables the trace buffers to prevent recording. The tracing
code may still be executed, but the trace will not be recorded.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 79 insertions, 2 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 29ab40a764c8..113aea9447ec 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -43,6 +43,15 @@
 unsigned long __read_mostly     tracing_max_latency = (cycle_t)ULONG_MAX;
 unsigned long __read_mostly     tracing_thresh;
 
+
+/*
+ * Kill all tracing for good (never come back).
+ * It is initialized to 1 but will turn to zero if the initialization
+ * of the tracer is successful. But that is the only place that sets
+ * this back to zero.
+ */
+int tracing_disabled = 1;
+
 static DEFINE_PER_CPU(local_t, ftrace_cpu_disabled);
 
 static inline void ftrace_disable_cpu(void)
@@ -62,8 +71,6 @@ static cpumask_t __read_mostly		tracing_buffer_mask;
 #define for_each_tracing_cpu(cpu)       \
         for_each_cpu_mask(cpu, tracing_buffer_mask)
 
-static int tracing_disabled = 1;
-
 /*
  * ftrace_dump_on_oops - variable to dump ftrace buffer on oops
  *
@@ -613,6 +620,76 @@ static void trace_init_cmdlines(void)
         cmdline_idx = 0;
 }
 
+static int trace_stop_count;
+static DEFINE_SPINLOCK(tracing_start_lock);
+
+/**
+ * tracing_start - quick start of the tracer
+ *
+ * If tracing is enabled but was stopped by tracing_stop,
+ * this will start the tracer back up.
+ */
+void tracing_start(void)
+{
+        struct ring_buffer *buffer;
+        unsigned long flags;
+
+        if (tracing_disabled)
+                return;
+
+        spin_lock_irqsave(&tracing_start_lock, flags);
+        if (--trace_stop_count)
+                goto out;
+
+        if (trace_stop_count < 0) {
+                /* Someone screwed up their debugging */
+                WARN_ON_ONCE(1);
+                trace_stop_count = 0;
+                goto out;
+        }
+
+
+        buffer = global_trace.buffer;
+        if (buffer)
+                ring_buffer_record_enable(buffer);
+
+        buffer = max_tr.buffer;
+        if (buffer)
+                ring_buffer_record_enable(buffer);
+
+        ftrace_start();
+ out:
+        spin_unlock_irqrestore(&tracing_start_lock, flags);
+}
+
+/**
+ * tracing_stop - quick stop of the tracer
+ *
+ * Light weight way to stop tracing. Use in conjunction with
+ * tracing_start.
+ */
+void tracing_stop(void)
+{
+        struct ring_buffer *buffer;
+        unsigned long flags;
+
+        ftrace_stop();
+        spin_lock_irqsave(&tracing_start_lock, flags);
+        if (trace_stop_count++)
+                goto out;
+
+        buffer = global_trace.buffer;
+        if (buffer)
+                ring_buffer_record_disable(buffer);
+
+        buffer = max_tr.buffer;
+        if (buffer)
+                ring_buffer_record_disable(buffer);
+
+ out:
+        spin_unlock_irqrestore(&tracing_start_lock, flags);
+}
+
 void trace_stop_cmdline_recording(void);
 
 static void trace_save_cmdline(struct task_struct *tsk)