On Tue, Dec 13, 2022 at 11:04:33AM +0800, Baoquan He wrote: > On 12/12/22 at 05:11am, Dennis Zhou wrote: > > Hello, > > > > On Mon, Dec 12, 2022 at 11:54:28AM +0100, Vlastimil Babka wrote: > > > On 11/27/22 12:05, Hyeonggon Yoo wrote: > > > > On Mon, Nov 21, 2022 at 06:12:00PM +0100, Vlastimil Babka wrote: > > > >> SLUB gets most of its scalability by percpu slabs. However for > > > >> CONFIG_SLUB_TINY the goal is minimal memory overhead, not scalability. > > > >> Thus, #ifdef out the whole kmem_cache_cpu percpu structure and > > > >> associated code. Additionally to the slab page savings, this reduces > > > >> percpu allocator usage, and code size. > > > > > > > > [+Cc Dennis] > > > > > > +To: Baoquan also. > > Thanks for adding me. > > > > > > > > Wondering if we can reduce (or zero) early reservation of percpu area > > > > when #if !defined(CONFIG_SLUB) || defined(CONFIG_SLUB_TINY)? > > > > > > Good point. I've sent a PR as it was [1], but (if merged) we can still > > > improve that during RC series, if it means more memory saved thanks to less > > > percpu usage with CONFIG_SLUB_TINY. > > > > > > [1] > > > https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/tag/?h=slab-for-6.2-rc1 > > > > The early reservation area not used at boot is then used to serve normal > > percpu allocations. Percpu allocates additional chunks based on a free > > page float count and is backed page by page, not all at once. I get > > slabs is the main motivator of early reservation, but if there are other > > users of percpu, then shrinking the early reservation area is a bit > > moot. > > Agree. Before kmem_cache_init() is done, anyone calling alloc_percpu() > can only get allocation done from early reservatoin of percpu area. > So, unless we can make sure nobody need to call alloc_percpu() before > kmem_cache_init() now and future. Thank you both for explaination. just googled and found random /proc/meminfo output of K210 board (6MB RAM, dual-core) Given that even K210 board uses around 100kB of percpu area, might not be worth thing to do :( https://gist.github.com/pdp7/0fd86d39e07ad7084f430c85a7a567f4?permalink_comment_id=3179983#gistcomment-3179983 > The only drawback of early reservation is it's not so flexible. We can > only dynamically create chunk to increase percpu areas when early > reservation is run out, but can't shrink early reservation if system > doesn't need that much. > > So we may need weigh the two ideas: > - Not allowing to alloc_percpu() before kmem_cache_init(); > - Keep early reservation, and think of a economic value for > CONFIG_SLUB_TINY. > > start_kernel() > ->setup_per_cpu_areas(); > ...... > ->mm_init(); > ...... > -->kmem_cache_init(); > > > __alloc_percpu() > -->pcpu_alloc() > --> succeed to allocate from early reservation > or > -->pcpu_create_chunk() > -->pcpu_alloc_chunk() > -->pcpu_mem_zalloc() > -- Thanks, Hyeonggon
