linux-kernel - Re: [PATCH v3 01/15] stacktrace/x86: add function for detecting reliable stack traces

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Date:   Mon, 19 Dec 2016 17:25:19 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        Vojtech Pavlik <vojtech@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Chris J Arges <chris.j.arges@...onical.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 01/15] stacktrace/x86: add function for detecting
 reliable stack traces

On Thu, 8 Dec 2016, Josh Poimboeuf wrote:

> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 215612c..b4a6663 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -155,6 +155,7 @@ config X86
>  	select HAVE_PERF_REGS
>  	select HAVE_PERF_USER_STACK_DUMP
>  	select HAVE_REGS_AND_STACK_ACCESS_API
> +	select HAVE_RELIABLE_STACKTRACE		if X86_64 && FRAME_POINTER && STACK_VALIDATION

Tests to measure possible performance penalty of frame pointers were done 
by Mel Gorman. The outcome was quite clear. There IS a measurable 
impact. The percentage depends on the workflow but I think it is safe to 
say that FP usually takes 5-10 percents.

If my understanding is correct there is no single culprit. Register 
pressure is definitely not a problem. We ran simple benchmarks while 
taking a register away from GCC (RBP or a common one). The impact is a 
combination of more cacheline pressure, more memory accesses and the fact 
that the kernel contains a lot of small functions.

Thus, I think that DWARF should be the way to go here.

Other than that the patch looks good to me.

Miroslav