On 11/10/22 02:57, Conor.Dooley@xxxxxxxxxxxxx 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@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/ >>> >>> >>> -- Damien Le Moal Western Digital Research
