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Date:   Mon, 12 Dec 2022 11:54:28 +0100
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Subject: Re: [PATCH 10/12] mm, slub: remove percpu slabs with CONFIG_SLUB_TINY
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To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>, Baoquan He <bhe@redhat.com>
Cc:     Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Pekka Enberg <penberg@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>, patches@lists.linux.dev,
        Dennis Zhou <dennis@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20221121171202.22080-1-vbabka@suse.cz>
 <20221121171202.22080-11-vbabka@suse.cz> <Y4NEkF8DI1uXFRv4@hyeyoo>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <Y4NEkF8DI1uXFRv4@hyeyoo>
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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

>> 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@suse.cz>
>