tracing: Reset ring buffer when changing trace_clocks

Because the "tsc" clock isn't in nanoseconds, the ring buffer must be
reset when changing clocks so that incomparable timestamps don't end up
in the same trace.

Tested: Confirmed switching clocks resets the trace buffer.

Google-Bug-Id: 6980623
Link: http://lkml.kernel.org/r/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Signed-off-by: David Sharp <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>

Author: David Sharp

kernel/trace/trace.c

@@ -4073,6 +4073,14 @@ static ssize_t tracing_clock_write(struct file *filp, const char __user *ubuf,
 	if (max_tr.buffer)
 		ring_buffer_set_clock(max_tr.buffer, trace_clocks[i].func);
 
+	/*
+	 * New clock may not be consistent with the previous clock.
+	 * Reset the buffer so that it doesn't have incomparable timestamps.
+	 */
+	tracing_reset_online_cpus(&global_trace);
+	if (max_tr.buffer)
+		tracing_reset_online_cpus(&max_tr);
+
 	mutex_unlock(&trace_types_lock);
 
 	*fpos += cnt;