[PATCH v5 3/3] tracing: kdb: Allow ftdump to skip all but the last few entries

[Douglas Anderson]

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 data 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
entries.  Unfortunately it provides no way for you to know how many
entries 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 entries except the last few.  This
allows you to quickly see what you want.

Note that we also change the printout in ftdump to print the
(positive) number of entries actually skipped since that could be
helpful to know when you've specified a negative skip count.

Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
Acked-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
---

Changes in v5:
- Only print skipping info if we skipped something (Daniel/Steven)
- Add Daniel Thompson Ack.

Changes in v4:
- Now uses trace_total_entries() / trace_total_entries_cpu().
- Based upon new patch that renames "lines" to "entries".

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

 kernel/trace/trace_kdb.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 4b666643d69f..a9fc4f828036 100644
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -40,6 +40,8 @@ static void ftrace_dump_buf(int skip_entries, long cpu_file)
 	tr->trace_flags &= ~TRACE_ITER_SYM_USEROBJ;
 
 	kdb_printf("Dumping ftrace buffer:\n");
+	if (skip_entries)
+		kdb_printf("(skipping %d entries)\n", skip_entries);
 
 	/* reset all but tr, trace, and overruns */
 	memset(&iter.seq, 0,
@@ -109,6 +111,7 @@ static int kdb_ftdump(int argc, const char **argv)
 	int skip_entries = 0;
 	long cpu_file;
 	char *cp;
+	int cnt;
 
 	if (argc > 2)
 		return KDB_ARGCOUNT;
@@ -129,6 +132,16 @@ static int kdb_ftdump(int argc, const char **argv)
 	}
 
 	kdb_trap_printk++;
+
+	/* A negative skip_entries means skip all but the last entries */
+	if (skip_entries < 0) {
+		if (cpu_file == RING_BUFFER_ALL_CPUS)
+			cnt = trace_total_entries(NULL);
+		else
+			cnt = trace_total_entries_cpu(NULL, cpu_file);
+		skip_entries = max(cnt + skip_entries, 0);
+	}
+
 	ftrace_dump_buf(skip_entries, cpu_file);
 	kdb_trap_printk--;
 
@@ -138,7 +151,8 @@ static int kdb_ftdump(int argc, const char **argv)
 static __init int kdb_ftrace_register(void)
 {
 	kdb_register_flags("ftdump", kdb_ftdump, "[skip_#entries] [cpu]",
-			    "Dump ftrace log", 0, KDB_ENABLE_ALWAYS_SAFE);
+			    "Dump ftrace log; -skip dumps last #entries", 0,
+			    KDB_ENABLE_ALWAYS_SAFE);
 	return 0;
 }
 
-- 
2.21.0.225.g810b269d1ac-goog