linux-kernel - Re: [GIT PULL] Bitmap patches for v5.19-rc1

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Date:   Tue, 31 May 2022 18:15:56 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [GIT PULL] Bitmap patches for v5.19-rc1

On Fri, May 27, 2022 at 09:17:21PM -0700, Linus Torvalds wrote:
> On Fri, May 27, 2022 at 8:44 AM Yury Norov <yury.norov@...il.com> wrote:
> >
> >       bitmap: add bitmap_weight_{cmp, eq, gt, ge, lt, le} functions
> 
> So honestly, I pulled this, looked at the code, and then unpulled it again.
> 
> This is not helping.
> 
> Making changes like this:
> 
> -       if (mm != current->active_mm || cpumask_weight(mm_cpumask(mm)) != 1) {
> +       if (mm != current->active_mm || !cpumask_weight_eq(mm_cpumask(mm), 1)) {
> 
> only makes the code harder to understand.
> 
> And it gets worse:
> 
> -               if (cpumask_weight(mask) > cpumask_weight(sibling_mask(cpu)))
> +               if (cpumask_weight_gt(mask, cpumask_weight(sibling_mask(cpu))))
> 
> is just disgusting. That original line is simple to read and makes
> sense. That new replacement really makes you do "Whaa?"
> 
> Now, I understand that these kinds of helper functions could make for
> slightly more efficient code in that you can break out of the bitmap
> scanning early when you have found enough bits set. I get it.
> 
> BUT.
> 
>  (a) code legibility is really really important
> 
>  (b) the places I found this weren't that performance-critical.
> 
>  (c) in most cases, the bitmaps in question are one single word
> 
> so I'm unpulling this again.
> 
> Now, some other parts of the pull were clear improvements. For
> example, the hyperv changes like this:
> 
> -               if (hc->var_cnt != bitmap_weight((unsigned long
> *)&valid_bank_mask, 64))
> +               if (hc->var_cnt != hweight64(valid_bank_mask))
> 
> were clear improvements where the old code was disgusting, and clearly
> improved by the change.
> 
> But the "bitmap_weight_cmp()" functions (and the cpumask_weight_cmp()
> ones) are just not a direction we want to go.
> 
> The special case of zero (ie "cpumask_weight() == 0" ->
> "bitmap_empty()") is one thing: making that kind of change tends to
> keep the code legible or even make it more understandable. So I didn't
> mind that. But I do mind the pointlessly complex new arbitrary weight
> comparisons, and the kind of mental cost they have.
> 
> There are people in the CS world that think "abstractions are always
> good". Those people are very very wrong.

Ok, I'll send a new pull with all but bitmap_weight_cmp() patches.