Re: Deprecating and removing SLOB

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 11/10/22 02:57, Conor.Dooley@microchip.com wrote:
> +CC Damien
> 
>> 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/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
> 
> Saw you were not added to the CC Damien & I know you don't want your
> baby broken!

:)

I set SLOB=y for the K210 as the config help mentions it is a bit more
efficient in low memory cases. I did run a few times with SLAB and it
was OK, so removing slob should not be a problem. Can check again.

Cheers.

> 
> 
> On 08/11/2022 21:44, Pasha Tatashin wrote:
>> On Tue, Nov 8, 2022 at 10:55 AM Vlastimil Babka <vbabka@suse.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@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@kvm.asia-northeast3-a.c.our-ratio-313919.internal/#t
>>> [3]
>>> https://lore.kernel.org/all/20210930044202.GP2361455@dread.disaster.area/
>>>
>>>
>>>

-- 
Damien Le Moal
Western Digital Research