linux-kernel - Re: [PATCH 10/12] mm, slub: remove percpu slabs with CONFIG_SLUB

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Date:   Mon, 12 Dec 2022 05:11:11 -0800
From:   Dennis Zhou <dennis@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>, Baoquan He <bhe@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>, patches@...ts.linux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/12] mm, slub: remove percpu slabs with CONFIG_SLUB_TINY

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.
> 
> > 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.

Thanks,
Dennis

> 
> >> This change builds on recent commit c7323a5ad078 ("mm/slub: restrict
> >> sysfs validation to debug caches and make it safe"), as caches with
> >> enabled debugging also avoid percpu slabs and all allocations and
> >> freeing ends up working with the partial list. With a bit more
> >> refactoring by the preceding patches, use the same code paths with
> >> CONFIG_SLUB_TINY.
> >> 
> >> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> > 
>