linux-kernel - Re: [PATCH] mtd: cfi: Deiline large functions

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Message-ID: <20150521180346.GD11598@ld-irv-0074>
Date:	Thu, 21 May 2015 11:03:58 -0700
From:	Brian Norris <computersforpeace@...il.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	David Woodhouse <David.Woodhouse@...el.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Jingoo Han <jg1.han@...sung.com>,
	Aaron Sierra <asierra@...-inc.com>,
	Artem Bityutskiy <Artem.Bityutskiy@...ux.intel.com>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: cfi: Deiline large functions

On Thu, May 21, 2015 at 12:13:10PM +0200, Denys Vlasenko wrote:
> On 05/21/2015 10:36 AM, Brian Norris wrote:
> > On Thu, May 21, 2015 at 09:50:38AM +0200, Denys Vlasenko wrote:
> >>>> cfi_udelay(): 74 bytes, 26 callsites
> >>>
> >>> ^^ This is pretty dead-simple. If it's generating bad code, we might
> >>> look at fixing it up instead. Almost all of its call sites are with
> >>> constant input, so it *should* just become:
> >>>
> >>> 	udelay(1);
> >>> 	cond_resched();
> >>>
> >>> in most cases. For the non-constant cases, we might still do an
> >>> out-of-line implementation. Or maybe we just say it's all not worth it,
> >>> and we just stick with what you have. But I'd like to consider
> >>> alternatives to out-lining this one.
> >>
> >> You want to consider not-deinlining (IOW: speed-optimizing)
> > 
> > Inlining isn't always about speed.
> > 
> >> a *fixed time delay function*?
> >>
> >> Think about what delay functions do...
> > 
> > I wasn't really looking at speed. Just memory usage.
> 
> I don't follow.
> 
> A single, not-inlined cfi_udelay(1) call is
> a minimal possible code size. Even
> 
> udelay(1);
> cond_resched();
> 
> ought to be bigger.

That's not really true. If all cases could be inlined to a single
udelay/msleep call, then that would be the minimal code size; you'd save
the non-inlined copy that would just call to msleep/udelay, as well as
save the need for additional EXPORT_SYBMOL_*(). But in most realistic
cases (including this case), your patch is in fact optimal. My follow up
comment (trimmed from below) was intended to concede that I was a little
off-base in my request.

Thanks for putting up, even though some of your comments are tackling a
straw man (I never mentioned performance).

Thanks,
Brian
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