[PATCH -tip v9 26/26] ftrace: Introduce FTRACE_OPS_FL_SELF_FILTER for ftrace-kprobe

Masami Hiramatsu
2014-04-17T08:19:51Z

Since the kprobes itself owns a hash table to get a kprobe
data structure corresponding to the given ip address, there
is no need to test ftrace hash in ftrace side.
To achive better performance on ftrace-based kprobe,
FTRACE_OPS_FL_SELF_FILTER flag to ftrace_ops which means
that ftrace skips testing its own hash table.

Without this patch, ftrace_lookup_ip() is biggest cycles
consumer when 20,000 kprobes are enabled.
----
Samples: 1K of event 'cycles', Event count (approx.): 340068894
+ 20.77% [k] ftrace_lookup_ip
+ 8.33% [k] kprobe_trace_func
+ 4.83% [k] get_kprobe_cached
----

With this patch, ftrace_lookup_ip() vanished from the
cycles consumer list (of course, there is no caller on
hotpath anymore :))
----
Samples: 1K of event 'cycles', Event count (approx.): 186861492
+ 9.95% [k] kprobe_trace_func
+ 6.00% [k] kprobe_ftrace_handler
+ 5.53% [k] get_kprobe_cached
----

Changes from v7:
- Re-evaluate the performance improvement.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
---
include/linux/ftrace.h | 3 +++
kernel/kprobes.c | 2 +-
kernel/trace/ftrace.c | 3 ++-
3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 9212b01..36e4cb5 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -93,6 +93,8 @@ typedef void (*ftrace_func_t)(unsigned long ip, unsigned long parent_ip,
* INITIALIZED - The ftrace_ops has already been initialized (first use time
* register_ftrace_function() is called, it will initialized the ops)
* DELETED - The ops are being deleted, do not let them be registered again.
+ * SELF_FILTER - The ftrace_ops function filters ip by itself. Do not need to
+ * check hash table on each hit.
*/
enum {
FTRACE_OPS_FL_ENABLED = 1 << 0,
@@ -105,6 +107,7 @@ enum {
FTRACE_OPS_FL_STUB = 1 << 7,
FTRACE_OPS_FL_INITIALIZED = 1 << 8,
FTRACE_OPS_FL_DELETED = 1 << 9,
+ FTRACE_OPS_FL_SELF_FILTER = 1 << 10,
};

/*
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 465e912..af1ff6a 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1020,7 +1020,7 @@ static struct kprobe *alloc_aggr_kprobe(struct kprobe *p)
#ifdef CONFIG_KPROBES_ON_FTRACE
static struct ftrace_ops kprobe_ftrace_ops __read_mostly = {
.func = kprobe_ftrace_handler,
- .flags = FTRACE_OPS_FL_SAVE_REGS,
+ .flags = FTRACE_OPS_FL_SAVE_REGS | FTRACE_OPS_FL_SELF_FILTER,
};
static int kprobe_ftrace_enabled;

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 1fd4b94..4235851 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -4520,7 +4520,8 @@ __ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip,
*/
preempt_disable_notrace();
do_for_each_ftrace_op(op, ftrace_ops_list) {
- if (ftrace_ops_test(op, ip, regs))
+ if (op->flags & FTRACE_OPS_FL_SELF_FILTER ||
+ ftrace_ops_test(op, ip, regs))
op->func(ip, parent_ip, op, regs);
} while_for_each_ftrace_op(op);
preempt_enable_notrace();