Re: Deprecating and removing SLOB

[Vlastimil Babka]

On 11/8/22 22:44, Pasha Tatashin wrote:
> On Tue, Nov 8, 2022 at 10:55 AM Vlastimil Babka <vbabka@xxxxxxx> 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@soleen:~/x/linux$ git grep SLOB=y
> arch/arm/configs/clps711x_defconfig:CONFIG_SLOB=y
> arch/arm/configs/collie_defconfig:CONFIG_SLOB=y
> arch/arm/configs/multi_v4t_defconfig:CONFIG_SLOB=y
> arch/arm/configs/omap1_defconfig:CONFIG_SLOB=y
> arch/arm/configs/pxa_defconfig:CONFIG_SLOB=y
> arch/arm/configs/tct_hammer_defconfig:CONFIG_SLOB=y
> arch/arm/configs/xcep_defconfig:CONFIG_SLOB=y
> arch/openrisc/configs/or1ksim_defconfig:CONFIG_SLOB=y
> arch/openrisc/configs/simple_smp_defconfig:CONFIG_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
> arch/sh/configs/rsk7201_defconfig:CONFIG_SLOB=y
> arch/sh/configs/rsk7203_defconfig:CONFIG_SLOB=y
> arch/sh/configs/se7206_defconfig:CONFIG_SLOB=y
> arch/sh/configs/shmin_defconfig:CONFIG_SLOB=y
> arch/sh/configs/shx3_defconfig:CONFIG_SLOB=y
> kernel/configs/tiny.config: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/arm/configs/collie_defconfig:CONFIG_EXPERT=y
arch/arm/configs/omap1_defconfig:CONFIG_EXPERT=y
arch/arm/configs/tct_hammer_defconfig:CONFIG_EXPERT=y
arch/arm/configs/xcep_defconfig:CONFIG_EXPERT=y
arch/openrisc/configs/or1ksim_defconfig:CONFIG_EXPERT=y
arch/openrisc/configs/simple_smp_defconfig:CONFIG_EXPERT=y
arch/riscv/configs/nommu_virt_defconfig:CONFIG_EXPERT=y
arch/x86/configs/x86_64_defconfig:CONFIG_EXPERT=y

So how about the following for -next and 6.2? I did try depending on BROKEN
as suggested, but didn't find a way to override it by enabling CONFIG_BROKEN=y
without modifying a Kconfig file, so this seems more graceful to me?

----8<----