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