Currently, x86 implementation of save_stack_trace() is walking all stack
region word by word regardless of what the trace->max_entries is.
However, it's unnecessary to walk after already fulfilling caller's
requirement, say, if trace->nr_entries >= trace->max_entries is true.

I measured its overhead and printed its difference of sched_clock() with
my QEMU x86 machine. The latency was improved over 70% when
trace->max_entries = 5.

Before this patch:

[    2.329573] save_stack_trace() takes 76820 ns
[    2.329863] save_stack_trace() takes 62131 ns
[    2.330000] save_stack_trace() takes 99476 ns
[    2.329846] save_stack_trace() takes 62419 ns
[    2.330000] save_stack_trace() takes 88918 ns
[    2.330253] save_stack_trace() takes 73669 ns
[    2.330520] save_stack_trace() takes 67876 ns
[    2.330671] save_stack_trace() takes 75963 ns
[    2.330983] save_stack_trace() takes 95079 ns
[    2.330451] save_stack_trace() takes 62352 ns

After this patch:

[    2.795000] save_stack_trace() takes 21147 ns
[    2.795397] save_stack_trace() takes 20230 ns
[    2.795397] save_stack_trace() takes 31274 ns
[    2.795739] save_stack_trace() takes 19706 ns
[    2.796484] save_stack_trace() takes 20266 ns
[    2.796484] save_stack_trace() takes 20902 ns
[    2.797000] save_stack_trace() takes 38110 ns
[    2.797510] save_stack_trace() takes 20224 ns
[    2.798181] save_stack_trace() takes 20172 ns
[    2.798837] save_stack_trace() takes 20824 ns

Signed-off-by: Byungchul Park <redacted>
---
 arch/x86/include/asm/stacktrace.h | 1 +
 arch/x86/kernel/dumpstack.c       | 4 ++++
 arch/x86/kernel/dumpstack_32.c    | 2 ++
 arch/x86/kernel/stacktrace.c      | 7 +++++++
 4 files changed, 14 insertions(+)

diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
index 0944218..f6d0694 100644
--- a/arch/x86/include/asm/stacktrace.h
+++ b/arch/x86/include/asm/stacktrace.h

@@ -41,6 +41,7 @@ struct stacktrace_ops {
 	/* On negative return stop dumping */
 	int (*stack)(void *data, char *name);
 	walk_stack_t	walk_stack;
+	int (*end_walk)(void *data);
 };
 
 void dump_trace(struct task_struct *tsk, struct pt_regs *regs,

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index ef8017c..274d42a 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c

@@ -113,6 +113,8 @@ print_context_stack(struct task_struct *task,
 			print_ftrace_graph_addr(addr, data, ops, task, graph);
 		}
 		stack++;
+		if (ops->end_walk && ops->end_walk(data))
+			break;
 	}
 	return bp;
 }

@@ -138,6 +140,8 @@ print_context_stack_bp(struct task_struct *task,
 		frame = frame->next_frame;
 		ret_addr = &frame->return_address;
 		print_ftrace_graph_addr(addr, data, ops, task, graph);
+		if (ops->end_walk && ops->end_walk(data))
+			break;
 	}
 
 	return (unsigned long)frame;

diff --git a/arch/x86/kernel/dumpstack_32.c b/arch/x86/kernel/dumpstack_32.c
index fef917e..762d1fd 100644
--- a/arch/x86/kernel/dumpstack_32.c
+++ b/arch/x86/kernel/dumpstack_32.c

@@ -69,6 +69,8 @@ void dump_trace(struct task_struct *task, struct pt_regs *regs,
 
 		bp = ops->walk_stack(task, stack, bp, ops, data,
 				     end_stack, &graph);
+		if (ops->end_walk && ops->end_walk(data))
+			break;
 
 		/* Stop if not on irq stack */
 		if (!end_stack)

diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index 9ee98ee..a44de4d 100644
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c

@@ -47,10 +47,17 @@ save_stack_address_nosched(void *data, unsigned long addr, int reliable)
 	return __save_stack_address(data, addr, reliable, true);
 }
 
+static int save_stack_end(void *data)
+{
+	struct stack_trace *trace = data;
+	return trace->nr_entries >= trace->max_entries;
+}
+
 static const struct stacktrace_ops save_stack_ops = {
 	.stack		= save_stack_stack,
 	.address	= save_stack_address,
 	.walk_stack	= print_context_stack,
+	.end_walk	= save_stack_end,
 };
 
 static const struct stacktrace_ops save_stack_ops_nosched = {

-- 
1.9.1

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