Message-ID: <44da078c-b630-a249-bf50-67df83cd8347@suse.cz>
Date:   Mon, 14 Nov 2022 10:36:31 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Conor Dooley <conor@...nel.org>
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 06:48, Damien Le Moal wrote:
> 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...

Thanks!

>> 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).

Thanks for the testing! How much RAM does the system have btw? I found 8MB
somewhere, is that correct?
So 230 pages that's a ~920 kB difference. Last time we saw less  dramatic
difference [1]. But that was looking at Slab pages, not free pages. The
extra overhead could be also in percpu allocations, code etc.

>> 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.

In [1] it was possible to save some more memory with more tuning. Some of
that required boot parameters and other code changes. In another reply [2] I
considered adding something like SLUB_TINY to take care of all that, so
looks like it would make sense to proceed with that.

[1]
https://lore.kernel.org/all/Yg9xSWEaTZLA+hYt@ip-172-31-19-208.ap-northeast-1.compute.internal/
[2] https://lore.kernel.org/all/eebc9dc8-6a45-c099-61da-230d06cb3157@suse.cz/

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