[PATCH v12 18/19] fgraph: Skip recording calltime/rettime if it is not nneeded

["Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx>]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Skip recording calltime and rettime if the fgraph_ops does not need it.
This is a kind of performance optimization for fprobe. Since the fprobe
user does not use these entries, recording timestamp in fgraph is just
a overhead (e.g. eBPF, ftrace). So introduce the skip_timestamp flag,
and all fgraph_ops sets this flag, skip recording calltime and rettime.

Here is the performance results measured by
 tools/testing/selftests/bpf/benchs/run_bench_trigger.sh

Without this:
kprobe-multi   :    5.700 ± 0.065M/s
kretprobe-multi:    4.239 ± 0.006M/s

With skip-timestamp:
kprobe-multi   :    6.265 ± 0.033M/s	+9.91%
kretprobe-multi:    4.758 ± 0.009M/s	+12.24%

Suggested-by: Jiri Olsa <olsajiri@xxxxxxxxx>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 Changes in v11:
  - Simplify it to be symmetric on push and pop. (Thus the timestamp
    getting place is a bit shifted.)
 Changes in v10:
  - Add likely() to skipping timestamp.
 Changes in v9:
  - Newly added.
---
 include/linux/ftrace.h |    2 ++
 kernel/trace/fgraph.c  |   36 +++++++++++++++++++++++++++++++++---
 kernel/trace/fprobe.c  |    1 +
 3 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index d8a58b940d81..fabf1a0979d4 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -1160,6 +1160,8 @@ struct fgraph_ops {
 	void				*private;
 	trace_func_graph_ent_t		saved_func;
 	int				idx;
+	/* If skip_timestamp is true, this does not record timestamps. */
+	bool				skip_timestamp;
 };
 
 void *fgraph_reserve_data(int idx, int size_bytes);
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index d735a8c872bb..cf3ae59a436e 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -174,6 +174,7 @@ int ftrace_graph_active;
 
 static struct fgraph_ops *fgraph_array[FGRAPH_ARRAY_SIZE];
 static unsigned long fgraph_array_bitmask;
+static bool fgraph_skip_timestamp;
 
 /* LRU index table for fgraph_array */
 static int fgraph_lru_table[FGRAPH_ARRAY_SIZE];
@@ -557,7 +558,11 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
 		return -EBUSY;
 	}
 
-	calltime = trace_clock_local();
+	/* This is not really 'likely' but for keeping the least path to be faster. */
+	if (likely(fgraph_skip_timestamp))
+		calltime = 0LL;
+	else
+		calltime = trace_clock_local();
 
 	offset = READ_ONCE(current->curr_ret_stack);
 	ret_stack = RET_STACK(current, offset);
@@ -728,6 +733,12 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,
 	*ret = ret_stack->ret;
 	trace->func = ret_stack->func;
 	trace->calltime = ret_stack->calltime;
+	/* This is not really 'likely' but for keeping the least path to be faster. */
+	if (likely(!trace->calltime))
+		trace->rettime = 0LL;
+	else
+		trace->rettime = trace_clock_local();
+
 	trace->overrun = atomic_read(&current->trace_overrun);
 	trace->depth = current->curr_ret_depth;
 	/*
@@ -788,7 +799,6 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
 		return (unsigned long)panic;
 	}
 
-	trace.rettime = trace_clock_local();
 	if (fregs)
 		ftrace_regs_set_instruction_pointer(fregs, ret);
 
@@ -1242,6 +1252,24 @@ static void ftrace_graph_disable_direct(bool disable_branch)
 	fgraph_direct_gops = &fgraph_stub;
 }
 
+static void update_fgraph_skip_timestamp(void)
+{
+	int i;
+
+	for (i = 0; i < FGRAPH_ARRAY_SIZE; i++) {
+		struct fgraph_ops *gops = fgraph_array[i];
+
+		if (gops == &fgraph_stub)
+			continue;
+
+		if (!gops->skip_timestamp) {
+			fgraph_skip_timestamp = false;
+			return;
+		}
+	}
+	fgraph_skip_timestamp = true;
+}
+
 int register_ftrace_graph(struct fgraph_ops *gops)
 {
 	int command = 0;
@@ -1267,6 +1295,7 @@ int register_ftrace_graph(struct fgraph_ops *gops)
 	gops->idx = i;
 
 	ftrace_graph_active++;
+	update_fgraph_skip_timestamp();
 
 	if (ftrace_graph_active == 2)
 		ftrace_graph_disable_direct(true);
@@ -1298,6 +1327,7 @@ int register_ftrace_graph(struct fgraph_ops *gops)
 		ftrace_graph_active--;
 		gops->saved_func = NULL;
 		fgraph_lru_release_index(i);
+		update_fgraph_skip_timestamp();
 	}
 out:
 	mutex_unlock(&ftrace_lock);
@@ -1321,8 +1351,8 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
 		goto out;
 
 	fgraph_array[gops->idx] = &fgraph_stub;
-
 	ftrace_graph_active--;
+	update_fgraph_skip_timestamp();
 
 	if (!ftrace_graph_active)
 		command = FTRACE_STOP_FUNC_RET;
diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 5a0b4ef52fa7..b108d26d7ee5 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -345,6 +345,7 @@ NOKPROBE_SYMBOL(fprobe_return);
 static struct fgraph_ops fprobe_graph_ops = {
 	.entryfunc	= fprobe_entry,
 	.retfunc	= fprobe_return,
+	.skip_timestamp = true,
 };
 static int fprobe_graph_active;