[PATCH v3] tracing: kdb: Allow ftdump to skip all but the last few lines

[Douglas Anderson <dianders@chromium.org>]

The 'ftdump' command in kdb is currently a bit of a last resort, at
least if you have lots of traces turned on.  It's going to print a
whole boatload of lines out your serial port which is probably running
at 115200.  This could easily take many, many minutes.

Usually you're most interested in what's at the _end_ of the ftrace
buffer, AKA what happened most recently.  That means you've got to
wait the full time for the dump.  The 'ftdump' command does attempt to
help you a little bit by allowing you to skip a fixed number of lines.
Unfortunately it provides no way for you to know how many lines you
should skip.

Let's do similar to python and allow you to use a negative number to
indicate that you want to skip all lines except the last few.  This
allows you to quickly see what you want.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v3:
- Optimize counting as per Steven Rostedt.
- Down to 1 patch since patch #1 from v2 landed.

 kernel/trace/trace_kdb.c | 38 +++++++++++++++++++++++++++++---------
 1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 810d78a8d14c..cc6ca6c0d6de 100644
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -17,7 +17,7 @@
 #include "trace.h"
 #include "trace_output.h"
 
-static void ftrace_dump_buf(int skip_lines, long cpu_file)
+static int ftrace_dump_buf(int skip_lines, long cpu_file, bool quiet)
 {
 	/* use static because iter can be a bit big for the stack */
 	static struct trace_iterator iter;
@@ -39,8 +39,6 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
 	/* don't look at user memory in panic mode */
 	tr->trace_flags &= ~TRACE_ITER_SYM_USEROBJ;
 
-	kdb_printf("Dumping ftrace buffer:\n");
-
 	/* reset all but tr, trace, and overruns */
 	memset(&iter.seq, 0,
 		   sizeof(struct trace_iterator) -
@@ -55,6 +53,9 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
 						 cpu, GFP_ATOMIC);
 			ring_buffer_read_start(iter.buffer_iter[cpu]);
 			tracing_iter_reset(&iter, cpu);
+
+			cnt +=
+			ring_buffer_entries_cpu(iter.trace_buffer->buffer, cpu);
 		}
 	} else {
 		iter.cpu_file = cpu_file;
@@ -63,13 +64,21 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
 						 cpu_file, GFP_ATOMIC);
 		ring_buffer_read_start(iter.buffer_iter[cpu_file]);
 		tracing_iter_reset(&iter, cpu_file);
+
+		cnt += ring_buffer_entries_cpu(iter.trace_buffer->buffer,
+					       cpu_file);
 	}
 
-	while (trace_find_next_entry_inc(&iter)) {
-		if (!cnt)
-			kdb_printf("---------------------------------\n");
-		cnt++;
+	if (quiet)
+		goto out;
+
+	kdb_printf("Dumping ftrace buffer (skipping %d lines):\n",
+		   skip_lines);
+
+	if (cnt)
+		kdb_printf("---------------------------------\n");
 
+	while (trace_find_next_entry_inc(&iter)) {
 		if (!skip_lines) {
 			print_trace_line(&iter);
 			trace_printk_seq(&iter.seq);
@@ -99,6 +108,8 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
 			iter.buffer_iter[cpu] = NULL;
 		}
 	}
+
+	return cnt;
 }
 
 /*
@@ -109,6 +120,7 @@ static int kdb_ftdump(int argc, const char **argv)
 	int skip_lines = 0;
 	long cpu_file;
 	char *cp;
+	int cnt;
 
 	if (argc > 2)
 		return KDB_ARGCOUNT;
@@ -129,7 +141,14 @@ static int kdb_ftdump(int argc, const char **argv)
 	}
 
 	kdb_trap_printk++;
-	ftrace_dump_buf(skip_lines, cpu_file);
+
+	/* A negative skip_lines means skip all but the last lines */
+	if (skip_lines < 0) {
+		cnt = ftrace_dump_buf(0, cpu_file, true);
+		skip_lines = max(cnt + skip_lines, 0);
+	}
+
+	ftrace_dump_buf(skip_lines, cpu_file, false);
 	kdb_trap_printk--;
 
 	return 0;
@@ -138,7 +157,8 @@ static int kdb_ftdump(int argc, const char **argv)
 static __init int kdb_ftrace_register(void)
 {
 	kdb_register_flags("ftdump", kdb_ftdump, "[skip_#lines] [cpu]",
-			    "Dump ftrace log", 0, KDB_ENABLE_ALWAYS_SAFE);
+			    "Dump ftrace log; -skip dumps last #lines", 0,
+			    KDB_ENABLE_ALWAYS_SAFE);
 	return 0;
 }
 
-- 
2.21.0.360.g471c308f928-goog