linux-kernel - Re: [PATCH v2] x86, vmlinux.lds: Add debug option to force all data sections aligned

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Date:   Mon, 25 Apr 2022 09:53:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH v2] x86, vmlinux.lds: Add debug option to force all data
 sections aligned

On Sun, Apr 24, 2022 at 08:25:33PM +0800, Feng Tang wrote:

> diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
> index d3a6f74a94bd..7b8cfe9717f2 100644
> --- a/arch/x86/Kconfig.debug
> +++ b/arch/x86/Kconfig.debug
> @@ -225,6 +225,19 @@ config PUNIT_ATOM_DEBUG
>  	  The current power state can be read from
>  	  /sys/kernel/debug/punit_atom/dev_power_state
> 
> +config DEBUG_FORCE_DATA_SECTION_ALIGNED
> +	bool "Force all data sections to be THREAD_SIZE aligned"

s/THREAD_SIZE/PAGE_SIZE/ as that seems to be what the actual patch does.

> +	depends on EXPERT && !DYNAMIC_DEBUG
> +	help
> +	  There are cases that a commit from one kernel domain changes
> +	  data sections' alignment of other domains, as they are all
> +	  linked together compactly, and cause magic performance bump
> +	  (regression or improvement), which is hard to debug. Enable
> +	  this option will help to verify if the bump is caused by
> +	  data alignment changes.
> +
> +	  It is mainly for debug and performance tuning use.
> +
>  choice
>  	prompt "Choose kernel unwinder"
>  	default UNWINDER_ORC if X86_64
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 7fda7f27e762..6add703de45f 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -155,7 +155,17 @@ SECTIONS
>  	X86_ALIGN_RODATA_END
> 
>  	/* Data */
> -	.data : AT(ADDR(.data) - LOAD_OFFSET) {
> +	.data : AT(ADDR(.data) - LOAD_OFFSET)
> +#ifdef CONFIG_DEBUG_FORCE_DATA_SECTION_ALIGNED
> +	/*
> +	 * In theroy, THREAD_SIZE as the biggest alignment of below sections

I think the more common spelling is: 'theory' :-)

> +	 * should be picked, but since upper 'X86_ALIGN_RODATA_END' can
> +	 * ganrantees the alignment of 'INIT_TASK_DATA', PAGE_SIZE is picked
> +	 * instead to reduce size of kernel binary
> +	 */
> +	SUBALIGN(PAGE_SIZE)
> +#endif
> +	{
>  		/* Start of data section */
>  		_sdata = .;
> 
> --
> 2.27.0
>