Message-ID: <93079aba-362e-5d1e-e9b4-dfe3a84da750@opensource.wdc.com>
Date:   Mon, 14 Nov 2022 14:48:29 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Conor Dooley <conor@...nel.org>, 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, Paul Cercueil <paul@...pouillou.net>
Subject: Re: Deprecating and removing SLOB

On 11/14/22 10:55, Damien Le Moal wrote:
> On 11/12/22 05:46, Conor Dooley wrote:
>> 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.
> 
> I sent a patch to change the k210 defconfig to using SLUB. However...
> 
> The current default config using SLOB gives about 630 free memory pages
> after boot (cat /proc/vmstat). Switching to SLUB, this is down to about
> 400 free memory pages (CONFIG_SLUB_CPU_PARTIAL is off).
> 
> This is with a buildroot kernel 5.19 build including a shell and sd-card
> boot. With SLUB, I get clean boots and a shell prompt as expected. But I
> definitely see more errors with shell commands failing due to allocation
> failures for the shell process fork. So as far as the K210 is concerned,
> switching to SLUB is not ideal.
> 
> I would not want to hold on kernel mm improvements because of this toy
> k210 though, so I am not going to prevent SLOB deprecation. I just wish
> SLUB itself used less memory :)

Did further tests with kernel 6.0.1:
* SLOB: 630 free pages after boot, shell working (occasional shell fork
failure happen though)
* SLAB: getting memory allocation for order 7 failures on boot already
(init process). Shell barely working (high frequency of shell command fork
failures)
* SLUB: getting memory allocation for order 7 failures on boot. I do get a
shell prompt but cannot run any shell command that involves forking a new
process.

So if we want to keep the k210 support functional with a shell, we need
slob. If we reduce that board support to only one application started as
the init process, then I guess anything is OK.

-- 
Damien Le Moal
Western Digital Research

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