[PATCH v4 01/15] x86/dumpstack: Optimize save_stack_trace

Byungchul Park
2016-12-09T05:16:39Z

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 <byungchul.park@lge.com>
---
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