Re: Deprecating and removing SLOB

[Pasha Tatashin]

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

>
> - Last time we discussed it [2], it seemed SLUB memory requirements can
> be brought very close to SLOB's if needed. Of course it can never have
> as small footprint as SLOB due to separate kmem_caches, but the
> difference is not that significant, unless somebody still tries to use
> Linux on very tiny systems (goes back to the previous point).
>
> Besides the smaller maintenance burden, removing SLOB would allow us to
> do a useful API improvement - the ability to use kfree() for both
> objects allocated by kmalloc() and kmem_cache_alloc(). Currently the
> latter has to be freed by kmem_cache_free(), passing a kmem_cache
> pointer in addition to the object pointer. With SLUB and SLAB, it is
> however possible to use kfree() instead, as the kmalloc caches and the
> rest of kmem_caches are the same and kfree() can lookup the kmem_cache
> from object pointer easily for any of those. XFS has apparently did that
> for years without anyone noticing it's broken on SLOB [3], and
> legitimizing and expanding this would help some use cases beside XFS
> (IIRC Matthew mentioned rcu-based freeing for example).
>
> However for SLOB to support kfree() on all allocations, it would need to
> store object size of allocated objects (which it currently does only for
> kmalloc() objects, prepending a size header to the object), but for
> kmem_cache_alloc() allocations as well. This has been attempted in the
> thread [3] but it bloats the memory usage, especially on architectures
> with large ARCH_KMALLOC_MINALIGN, where the prepended header basically
> has to occupy the whole ARCH_KMALLOC_MINALIGN block to be DMA safe.
> There are ongoing efforts to reduce this minalign, but the memory
> footprint would still increase, going against the purpose of SLOB, so
> again it would be easier if we could just remove it.
>
> So with this thread I'm interested in hearing arguments/use cases for
> keeping SLOB. There might be obviously users of SLOB whom this
> conversation will not reach, so I assume the eventual next step would be
> to deprecate it in a way that those users are notified when building a
> new kernel and can raise their voice then. Is there a good proven way
> how to do that for a config option like this one?
>
> Thanks,
> Vlastimil
>
> [1] https://lpc.events/event/16/contributions/1272/ - slides in the
> slabs.pdf linked there
> [2]
> https://lore.kernel.org/all/20211017135708.GA8442@xxxxxxxxxxxxxxxxxxxxxxxxxxx-ratio-313919.internal/#t
> [3]
> https://lore.kernel.org/all/20210930044202.GP2361455@xxxxxxxxxxxxxxxxxxx/
>
>
>