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 Great point, thanks. Ccing. First mail here: https://lore.kernel.org/all/CA%2BCK2bD-uVGJ0%3D9uc7Lt5zwY%2B2PM2RTcfOhxEd65S7TvTrJULA@xxxxxxxxxxxxxx/ >> >> - 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/ >> >> >>
