Re: [PATCH 1/2] x86/dumpstack: Optimize save_stack_trace

[Frederic Weisbecker]

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);

Nice improvement but how about doing that with the return value of
stacktrace_ops::address() instead?

print_context_stack_bp() uses that for example. This behaviour could
be extended.