linux-kernel - Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

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Message-Id: <20250611184817.bf9fee25d6947a9bcf60b6f9@linux-foundation.org>
Date: Wed, 11 Jun 2025 18:48:17 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: corbet@....net, colyli@...nel.org, kent.overstreet@...ux.dev,
 robertpang@...gle.com, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-bcache@...r.kernel.org,
 jserv@...s.ncku.edu.tw, stable@...r.kernel.org
Subject: Re: [PATCH 0/8] Fix bcache regression with equality-aware heap APIs

On Wed, 11 Jun 2025 05:55:08 +0800 Kuan-Wei Chiu <visitorckw@...il.com> wrote:

> This patch series introduces equality-aware variants of the min heap
> API that use a top-down heapify strategy to improve performance when
> many elements are equal under the comparison function. It also updates
> the documentation accordingly and modifies bcache to use the new APIs
> to fix a performance regression caused by the switch to the generic min
> heap library.
> 
> In particular, invalidate_buckets_lru() in bcache suffered from
> increased comparison overhead due to the bottom-up strategy introduced
> in commit 866898efbb25 ("bcache: remove heap-related macros and switch
> to generic min_heap"). The regression is addressed by switching to the
> equality-aware variants and using the inline versions to avoid function
> call overhead in this hot path.
> 
> Cc: stable@...r.kernel.org

To justify a -stable backport this performance regression would need to
have a pretty significant impact upon real-world userspace.  Especially
as the patchset is large.

Unfortunately the changelog provides no indication of the magnitude of
the userspace impact.   Please tell us this, in detail.

Also, if we are to address this regression in -stable kernels then
reverting 866898efbb25 is an obvious way - it is far far safer.  So
please also tell us why the proposed patchset is a better way for us to
go.

(Also, each patch should have a fixes:866898efbb25 to help direct the
backporting efforts)


I'll add the patches to mm.git to get you some testing but from what
I'm presently seeing the -stable backporting would be unwise.