linux-kernel - Re: [PATCH v2] minmax: clamp more efficiently by avoiding extra comparison

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Message-ID: <202209231656.AD14FB6@keescook>
Date:   Fri, 23 Sep 2022 17:02:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] minmax: clamp more efficiently by avoiding extra
 comparison

On Fri, Sep 23, 2022 at 03:54:12PM -0700, Andrew Morton wrote:
> On Fri, 23 Sep 2022 17:40:01 +0200 "Jason A. Donenfeld" <Jason@...c4.com> wrote:
> 
> > Currently the clamp algorithm does:
> > 
> > 	if (val > hi)
> > 		val = hi;
> > 	if (val < lo)
> > 		val = lo;
> > 
> > But since hi > lo by definition, this can be made more efficient with:
> > 
> > 	if (val > hi)
> > 		val = hi;
> > 	else if (val < lo)
> > 		val = lo;
> > 
> > So fix up the clamp and clamp_t functions to do this, adding the same
> > argument checking as for min and min_t.
> > 
> 
> The patch adds 140 bytes of text to mm/memblock.o, for example. 
> Presumably from the additional branch.  Larger text means larger cache
> footprint means slower.

Oh, interesting. I had spot-checked one clamp-using function (update_cfs_group)
and it produced the same output just with some register swapping and other
ordering changes. Hmm.

But yes, text is bigger, but bss is smaller. This are my allmodconfig builds:

   text        data         bss          dec       hex  filename
43779952    59510881    28684428    131975261   7ddc85d vmlinux.before
43781295    59510889    28676236    131968420   7ddada4 vmlinux

> So where's the proof that this change gives us a more efficient kernel?

A reasonable question. :)

-- 
Kees Cook