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Subject[PATCH 2/3] 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>
---
include/linux/ftrace.h | 5 +++
kernel/trace/trace.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 84 insertions(+), 2 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 6eb966d..41054a3 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -219,6 +219,9 @@ static inline void __ftrace_enabled_restore(int enabled)
#ifdef CONFIG_TRACING
extern int ftrace_dump_on_oops;

+extern void tracing_start(void);
+extern void tracing_stop(void);
+
extern void
ftrace_special(unsigned long arg1, unsigned long arg2, unsigned long arg3);

@@ -249,6 +252,8 @@ ftrace_special(unsigned long arg1, unsigned long arg2, unsigned long arg3) { }
static inline int
ftrace_printk(const char *fmt, ...) __attribute__ ((format (printf, 1, 0)));

+static inline void tracing_start(void) { }
+static inline void tracing_stop(void) { }
static inline int
ftrace_printk(const char *fmt, ...)
{
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c4900b8..411ab5c 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)
--
1.5.6.5
--


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