linux-kernel - Re: [PATCH 00/12] bitmap: rework bitmap

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Date:   Tue, 29 Aug 2023 16:38:41 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/12] bitmap: rework bitmap_{bit,}remap()

On Tue, Aug 29, 2023 at 09:33:29AM +0200, Rasmus Villemoes wrote:
> On 28/08/2023 20.43, Yury Norov wrote:
> > This series adds a test, const-time optimizaton and fixes O(N^2)
> > complexity problem for the functions. It's based on discussion in
> > bitmap: optimize bitmap_remap() series [1], but there's much more work
> > here, so I decided to give it a separete run, and don't name it as v2.
> > 
> > bitmap_remap() API has just one user in generic code, and few more in
> > drivers, so this may look like an overkill. But the work gives ~10x
> > better performance for a 1000-bit bitmaps, which is typical for nodemasks
> > in major distros like Ubuntu.
> 
> Can you find just _one_ project on Debian Code Search or elsewhere that
> actually uses mbind(2), that could possibly ever trigger the use of that
> bitmap_remap stuff? Also, the bitmap may be order 10, but that's just
> because the kitchen sink distros are silly, real machines have nowhere
> near that number of nodes, so even if mbind is used, the bitmaps
> involved will never actually have anything beyond index ~64.
> 
> I think this is all total overkill for non-existing problems, and when
> it takes me 20 seconds to find the first bug, I really don't think it's
> worth the churn. I'm not giving a thorough review on the rest of the
> series, nor commenting on followups.

I posted one patch to replace these APIs with something else, more particular
for GPIO case(s). Have you chance to look at that? With that taking in, I'm
fully agree on the above statement (as we lose the user of this complicated
thingy which is a niche of the NUMA as you mentioned already).

-- 
With Best Regards,
Andy Shevchenko