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

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Message-ID: <5247e354-cb7f-4df7-37a5-95cebed43d4c@rasmusvillemoes.dk>
Date:   Tue, 29 Aug 2023 09:33:29 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH 00/12] bitmap: rework bitmap_{bit,}remap()

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.

Rasmus