ftrace,kdb: Allow dumping a specific cpu's buffer with ftdump

In systems with more than one processor it is desirable to look at the
per cpu trace buffers.
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
CC: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/trace_kdb.c

@@ -17,7 +17,7 @@
 #include "trace.h"
 #include "trace_output.h"
 
-static void ftrace_dump_buf(int skip_lines)
+static void ftrace_dump_buf(int skip_lines, long cpu_file)
 {
 	/* use static because iter can be a bit big for the stack */
 	static struct trace_iterator iter;
@@ -44,13 +44,20 @@ static void ftrace_dump_buf(int skip_lines)
 	iter.iter_flags |= TRACE_FILE_LAT_FMT;
 	iter.pos = -1;
 
-	for_each_tracing_cpu(cpu) {
-		iter.buffer_iter[cpu] =
+	if (cpu_file == TRACE_PIPE_ALL_CPU) {
+		for_each_tracing_cpu(cpu) {
+			iter.buffer_iter[cpu] =
 			ring_buffer_read_prepare(iter.tr->buffer, cpu);
-		ring_buffer_read_start(iter.buffer_iter[cpu]);
-		tracing_iter_reset(&iter, cpu);
+			ring_buffer_read_start(iter.buffer_iter[cpu]);
+			tracing_iter_reset(&iter, cpu);
+		}
+	} else {
+		iter.cpu_file = cpu_file;
+		iter.buffer_iter[cpu_file] =
+			ring_buffer_read_prepare(iter.tr->buffer, cpu_file);
+		ring_buffer_read_start(iter.buffer_iter[cpu_file]);
+		tracing_iter_reset(&iter, cpu_file);
 	}
-
 	if (!trace_empty(&iter))
 		trace_find_next_entry_inc(&iter);
 	while (!trace_empty(&iter)) {
@@ -91,9 +98,10 @@ static void ftrace_dump_buf(int skip_lines)
 static int kdb_ftdump(int argc, const char **argv)
 {
 	int skip_lines = 0;
+	long cpu_file;
 	char *cp;
 
-	if (argc > 1)
+	if (argc > 2)
 		return KDB_ARGCOUNT;
 
 	if (argc) {
@@ -102,8 +110,17 @@ static int kdb_ftdump(int argc, const char **argv)
 			skip_lines = 0;
 	}
 
+	if (argc == 2) {
+		cpu_file = simple_strtol(argv[2], &cp, 0);
+		if (*cp || cpu_file >= NR_CPUS || cpu_file < 0 ||
+		    !cpu_online(cpu_file))
+			return KDB_BADINT;
+	} else {
+		cpu_file = TRACE_PIPE_ALL_CPU;
+	}
+
 	kdb_trap_printk++;
-	ftrace_dump_buf(skip_lines);
+	ftrace_dump_buf(skip_lines, cpu_file);
 	kdb_trap_printk--;
 
 	return 0;
@@ -111,8 +128,8 @@ static int kdb_ftdump(int argc, const char **argv)
 
 static __init int kdb_ftrace_register(void)
 {
-	kdb_register_repeat("ftdump", kdb_ftdump, "", "Dump ftrace log",
-			    0, KDB_REPEAT_NONE);
+	kdb_register_repeat("ftdump", kdb_ftdump, "[skip_#lines] [cpu]",
+			    "Dump ftrace log", 0, KDB_REPEAT_NONE);
 	return 0;
 }