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

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Message-ID: <20210924015744.h7egzvyxm5cifjzj@treble>
Date:   Thu, 23 Sep 2021 18:57:44 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH] x86, vmlinux.lds: Add debug option to force all data
 sections aligned

On Thu, Sep 23, 2021 at 10:57:20PM +0800, Feng Tang wrote:
> For binary size, I just tested 5.14 kernel with a default desktop
> config from Ubuntu (I didn't use the normal rhel-8.3 config used
> by 0Day, which is more for server):
> 
> v5.14
> ------------------------
> text		data		bss	    dec		hex	filename
> 16010221	14971391	6098944	37080556	235cdec	vmlinux
> 
> v5.14 + 64B-function-align
> --------------------------
> text		data		bss	    dec		hex	filename
> 18107373	14971391	6098944	39177708	255cdec	vmlinux
> 
> v5.14 + data-align(THREAD_SIZE 16KB)
> --------------------------
> text		data		bss	    dec		hex	filename
> 16010221	57001791	6008832	79020844	4b5c32c	vmlinux

That data size increase is indeed excessive.  However I wonder if some
other approach (other than SUBALIGN) could be taken.  For example, a 4k
alignment for each compilation unit's .data section.  That might require
some linker magic at the built-in.o linking level.

Anyway, I suspect the data alignment issues are less common than
function alignment.  It might be fine to leave the data alignment as a
debug feature for now, as this current patch does.

> > On a similar vein I think we should re-explore permanently enabling
> > cacheline-sized function alignment i.e. making something like
> > CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B the default.  Ingo did some
> > research on that a while back:
> > 
> >    https://lkml.kernel.org/r/20150519213820.GA31688@gmail.com
> 
> Thanks for sharing this, from which I learned a lot, and I hope I
> knew this thread when we first check strange regressions in 2019 :)
> 
> > At the time, the main reported drawback of -falign-functions=64 was that
> > even small functions got aligned.  But now I think that can be mitigated
> > with some new options like -flimit-function-alignment and/or
> > -falign-functions=64,X (for some carefully-chosen value of X).
> 
> Will study more about these options. 
> 
> If they have much less size increase and no regression in performance,
> then maybe it could be turned on by default.

Agreed!  I think/hope it would be a net positive change.

I've also been burned by such issues -- like a random one-line code
change causing a measurable performance regression due to changed
i-cache behavior in unrelated code.  It doesn't only affect 0-day tests,
it also affects real users.

-- 
Josh