Re: [PATCH 1/2] x86/dumpstack: Optimize save_stack_trace
From: Josh Poimboeuf
Date: Fri Jul 08 2016 - 10:08:29 EST
On Mon, Jul 04, 2016 at 07:27:54PM +0900, Byungchul Park wrote:
> I suggested this patch on https://lkml.org/lkml/2016/6/20/22. However,
> I want to proceed saperately since it's somewhat independent from each
> other. Frankly speaking, I want this patchset to be accepted at first so
> that the crossfeature can use this optimized save_stack_trace_norm()
> which makes crossrelease work smoothly.
>
> ----->8-----
> From 1ceb4cee520cfc562d5d63471f6db4e9a8d9ff42 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul.park@xxxxxxx>
> Date: Mon, 4 Jul 2016 18:31:09 +0900
> Subject: [PATCH 1/2] x86/dumpstack: Optimize save_stack_trace
>
> 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:
>
> [ 2.326940] save_stack_trace() takes 83931 ns
> [ 2.326389] save_stack_trace() takes 62576 ns
> [ 2.327575] save_stack_trace() takes 58826 ns
> [ 2.327000] save_stack_trace() takes 88980 ns
> [ 2.327424] save_stack_trace() takes 59831 ns
> [ 2.327575] save_stack_trace() takes 58482 ns
> [ 2.327597] save_stack_trace() takes 87114 ns
> [ 2.327931] save_stack_trace() takes 121140 ns
> [ 2.327434] save_stack_trace() takes 64321 ns
> [ 2.328632] save_stack_trace() takes 84997 ns
> [ 2.328000] save_stack_trace() takes 115037 ns
> [ 2.328460] save_stack_trace() takes 72292 ns
> [ 2.328632] save_stack_trace() takes 61236 ns
> [ 2.328567] save_stack_trace() takes 76666 ns
> [ 2.328867] save_stack_trace() takes 79525 ns
> [ 2.328460] save_stack_trace() takes 64902 ns
> [ 2.329585] save_stack_trace() takes 58760 ns
> [ 2.329000] save_stack_trace() takes 91349 ns
> [ 2.329414] save_stack_trace() takes 60069 ns
> [ 2.329585] save_stack_trace() takes 61012 ns
> [ 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:
>
> [ 2.780735] save_stack_trace() takes 19902 ns
> [ 2.780718] save_stack_trace() takes 20240 ns
> [ 2.781692] save_stack_trace() takes 45215 ns
> [ 2.781477] save_stack_trace() takes 20191 ns
> [ 2.781694] save_stack_trace() takes 20044 ns
> [ 2.782589] save_stack_trace() takes 20292 ns
> [ 2.782706] save_stack_trace() takes 20024 ns
> [ 2.782706] save_stack_trace() takes 19881 ns
> [ 2.782881] save_stack_trace() takes 24577 ns
> [ 2.782706] save_stack_trace() takes 19901 ns
> [ 2.783621] save_stack_trace() takes 24381 ns
> [ 2.783621] save_stack_trace() takes 20205 ns
> [ 2.783760] save_stack_trace() takes 19956 ns
> [ 2.783718] save_stack_trace() takes 20280 ns
> [ 2.784179] save_stack_trace() takes 20099 ns
> [ 2.784835] save_stack_trace() takes 20055 ns
> [ 2.785922] save_stack_trace() takes 20157 ns
> [ 2.785922] save_stack_trace() takes 20140 ns
> [ 2.786178] save_stack_trace() takes 20040 ns
> [ 2.786877] save_stack_trace() takes 20102 ns
> [ 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@xxxxxxx>
> ---
> 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 70bbe39..fc572e7 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 9c30acf..89f68f3 100644
> --- a/arch/x86/kernel/dumpstack.c
> +++ b/arch/x86/kernel/dumpstack.c
> @@ -115,6 +115,8 @@ print_context_stack(struct thread_info *tinfo,
> print_ftrace_graph_addr(addr, data, ops, tinfo, graph);
> }
> stack++;
> + if (ops->end_walk && ops->end_walk(data))
> + break;
Instead of adding a new callback, why not just check the ops->address()
return value? It already returns an error if the array is full.
I think that would be cleaner and would help prevent more callback
sprawl.
> }
> return bp;
> }
> @@ -139,6 +141,8 @@ print_context_stack_bp(struct thread_info *tinfo,
> frame = frame->next_frame;
> ret_addr = &frame->return_address;
> print_ftrace_graph_addr(addr, data, ops, tinfo, graph);
> + if (ops->end_walk && ops->end_walk(data))
> + break;
Same here, and print_context_stack_bp() already checks the
ops->address() return value anyway.
--
Josh