linux-kernel - Re: Deprecating and removing SLOB

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Date:   Fri, 11 Nov 2022 20:46:46 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Pasha Tatashin <pasha.tatashin@...een.com>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Rustam Kovhaev <rkovhaev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Alexander Shiyan <shc_work@...l.ru>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        Janusz Krzysztofik <jmkrzyszt@...il.com>,
        Tony Lindgren <tony@...mide.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        openrisc@...ts.librecores.org, linux-riscv@...ts.infradead.org,
        linux-sh@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Conor.Dooley@...rochip.com,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Paul Cercueil <paul@...pouillou.net>
Subject: Re: Deprecating and removing SLOB

On Fri, Nov 11, 2022 at 11:33:30AM +0100, Vlastimil Babka wrote:
> On 11/8/22 22:44, Pasha Tatashin wrote:
> > On Tue, Nov 8, 2022 at 10:55 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >> Hi,
> >>
> >> as we all know, we currently have three slab allocators. As we discussed
> >> at LPC [1], it is my hope that one of these allocators has a future, and
> >> two of them do not.
> >>
> >> The unsurprising reasons include code maintenance burden, other features
> >> compatible with only a subset of allocators (or more effort spent on the
> >> features), blocking API improvements (more on that below), and my
> >> inability to pronounce SLAB and SLUB in a properly distinguishable way,
> >> without resorting to spelling out the letters.
> >>
> >> I think (but may be proven wrong) that SLOB is the easier target of the
> >> two to be removed, so I'd like to focus on it first.
> >>
> >> I believe SLOB can be removed because:
> >>
> >> - AFAIK nobody really uses it? It strives for minimal memory footprint
> >> by putting all objects together, which has its CPU performance costs
> >> (locking, lack of percpu caching, searching for free space...). I'm not
> >> aware of any "tiny linux" deployment that opts for this. For example,
> >> OpenWRT seems to use SLUB and the devices these days have e.g. 128MB
> >> RAM, not up to 16 MB anymore. I've heard anecdotes that the performance
> >> SLOB impact is too much for those who tried. Googling for
> >> "CONFIG_SLOB=y" yielded nothing useful.
> > 
> > I am all for removing SLOB.
> > 
> > There are some devices with configs where SLOB is enabled by default.
> > Perhaps, the owners/maintainers of those devices/configs should be
> > included into this thread:
> > 
> > tatashin@...een:~/x/linux$ git grep SLOB=y

> > arch/riscv/configs/nommu_k210_defconfig:CONFIG_SLOB=y
> > arch/riscv/configs/nommu_k210_sdcard_defconfig:CONFIG_SLOB=y
> > arch/riscv/configs/nommu_virt_defconfig:CONFIG_SLOB=y

> 
> Turns out that since SLOB depends on EXPERT, many of those lack it so
> running make defconfig ends up with SLUB anyway, unless I miss something.
> Only a subset has both SLOB and EXPERT:
> 
> > git grep CONFIG_EXPERT `git grep -l "CONFIG_SLOB=y"`

> arch/riscv/configs/nommu_virt_defconfig:CONFIG_EXPERT=y

I suppose there's not really a concern with the virt defconfig, but I
did check the output of `make nommu_k210_defconfig" and despite not
having expert it seems to end up CONFIG_SLOB=y in the generated .config.

I do have a board with a k210 so I checked with s/SLOB/SLUB and it still
boots etc, but I have no workloads or w/e to run on it.