linux-kernel - Re: [PATCH v2 1/2] padata: Mark padata_work_init() as _

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Date:   Mon, 12 Dec 2022 13:06:16 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Nicolas Schier <nicolas@...sle.eu>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev,
        Steffen Klassert <steffen.klassert@...unet.com>,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH v2 1/2] padata: Mark padata_work_init() as __ref

On Mon, Dec 12, 2022 at 02:21:57PM -0500, Daniel Jordan wrote:
> On Mon, Dec 12, 2022 at 10:05:02AM -0700, Nathan Chancellor wrote:
> > On Mon, Dec 12, 2022 at 10:07:24PM +0900, Masahiro Yamada wrote:
> > > I am not sure if the compiler should do this level of optimization
> > > because kernel/padata.c does not seem to be a special case.
> > > Perhaps, we might be hit with more cases that need __ref annotation,
> > > which is only required by LTO.
> > 
> > That's possible. I did only see this once instance in all my builds but
> > allmodconfig + ThinLTO might not be too interesting of a case,
> > since the sanitizers will be enabled, which makes optimization more
> > difficult. I could try to enable ThinLTO with some distribution
> > configurations to see if there are any more instances that crop up.
> 
> Yes, if there were many more instances of this problem it might be worth
> thinking about an LTO-specific solution to fix it closer to the source.

Ack, I will wire up some build tests to see if this optimization occurs
frequently enough to warrant a wider fix.

> > > One note is that, we could discard padata_work_init()
> > > because (1) and (3) are both annotated as __init.
> > > So, another way of fixing is
> > >    static __always_inline void padata_work_init(...)
> > > because the compiler would determine padata_work_init()
> > > would be small enough if the caller and callee belonged to
> > > the same section.
> > > 
> > > I do not have a strong opinion.
> 
> I'm right there with you.  :-)
> 
> > > Honestly, I do not know what the best approach would be to fix this.
> 
> Either approach works, either can include an explanatory comment.
> __ref seems more targeted to the problem at hand.

Right, I suspect __ref is the right way to go for this particular issue.
I will add a comment regardless.

> > > If we go with the __ref annotation, I can pick this, but
> > > at least can you add some comments?
> > > 
> > > 
> > > include/linux/init.h says:
> > > "optimally document why the __ref is needed and why it's OK"
> > > 
> > > 
> > > I think this is the case that needs some comments
> > > because LTO optimization looks too tricky to me.
> > 
> > Sure thing, I will send a v3 either Tuesday or Wednesday with an updated
> > commit message and code comment if we end up going this route.
> 
> A nitpick, but as long as you're respinning, if we stay with this
> approach, could you put __ref just before the function name?  init.h
> says "The markers follow same syntax rules as __init / __initdata" and
> for those it says "You should add __init immediately before the function
> name" though there are plenty of places in the tree that don't do this.

Sure thing!

Cheers,
Nathan