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 16:01:26 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

Hi Peter,

On Mon, Apr 25, 2022 at 09:53:35AM +0200, Peter Zijlstra wrote:
> 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.

Thanks for the catch! It was changed from v1 to v2, I should reviewed 
these texts more carefully, and there is another similar typo in the
commit log.

> > +	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' :-)

Yes, will change in the next verson.

Thanks,
Feng

> > +	 * 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
> >