From: <Conor.Dooley@microchip.com>
To: <damien.lemoal@opensource.wdc.com>
Cc: <cl@linux.com>, <rientjes@google.com>, <iamjoonsoo.kim@lge.com>,
<penberg@kernel.org>, <42.hyeyoo@gmail.com>,
<willy@infradead.org>, <roman.gushchin@linux.dev>,
<pasha.tatashin@soleen.com>, <torvalds@linux-foundation.org>,
<linux-mm@kvack.org>, <vbabka@suse.cz>,
<linux-kernel@vger.kernel.org>, <catalin.marinas@arm.com>,
<rkovhaev@gmail.com>, <akpm@linux-foundation.org>
Subject: Re: Deprecating and removing SLOB
Date: Wed, 9 Nov 2022 17:57:08 +0000 [thread overview]
Message-ID: <efa623fb-686f-072e-df0d-9f5727ae1b1f@microchip.com> (raw)
In-Reply-To: <CA+CK2bD-uVGJ0=9uc7Lt5zwY+2PM2RTcfOhxEd65S7TvTrJULA@mail.gmail.com>
+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!
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/
>>
>>
>>
next prev parent reply other threads:[~2022-11-09 18:00 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-08 15:55 Deprecating and removing SLOB Vlastimil Babka
2022-11-08 18:18 ` Christophe Leroy
2022-11-08 19:17 ` Andrew Morton
2022-11-08 18:46 ` Roman Gushchin
2022-11-08 20:13 ` Yosry Ahmed
2022-11-09 9:09 ` Vlastimil Babka
2022-11-08 21:44 ` Pasha Tatashin
2022-11-09 9:00 ` Vlastimil Babka
2022-11-09 15:50 ` Aaro Koskinen
2022-11-09 16:45 ` Geert Uytterhoeven
2022-11-09 17:45 ` Mike Rapoport
2022-11-09 21:16 ` Janusz Krzysztofik
2022-11-09 17:57 ` Conor.Dooley [this message]
2022-11-09 23:00 ` Damien Le Moal
2022-11-11 10:25 ` Vlastimil Babka
2022-11-12 1:40 ` Damien Le Moal
2022-11-11 10:33 ` Vlastimil Babka
2022-11-11 20:46 ` Conor Dooley
2022-11-12 1:40 ` Damien Le Moal
2022-11-14 1:55 ` Damien Le Moal
2022-11-14 5:48 ` Damien Le Moal
2022-11-14 9:36 ` Vlastimil Babka
2022-11-14 11:35 ` Damien Le Moal
2022-11-14 14:47 ` Hyeonggon Yoo
2022-11-15 4:24 ` Damien Le Moal
2022-11-15 4:28 ` Damien Le Moal
2022-11-16 7:57 ` Matthew Wilcox
2022-11-16 8:02 ` Damien Le Moal
2022-11-16 17:51 ` Vlastimil Babka
2022-11-17 0:22 ` Damien Le Moal
2022-11-21 4:30 ` Damien Le Moal
2022-11-21 17:02 ` Vlastimil Babka
2022-11-14 11:50 ` Hyeonggon Yoo
2022-11-09 20:56 Paul Cercueil
2022-11-09 21:39 ` Linus Torvalds
2022-11-09 23:48 ` Aaro Koskinen
2022-11-09 23:51 ` Aaro Koskinen
2022-11-10 4:40 ` Theodore Ts'o
2022-11-10 7:31 ` Vlastimil Babka
2022-11-10 7:54 ` Feng Tang
2022-11-10 16:20 ` Matthew Wilcox
2022-11-11 9:37 ` David Laight
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