netdev - Re: [PATCH RFC] mm+net: allow to set kmem_cache create flag for SLAB_NEVER

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 18 Jan 2023 05:17:15 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Lameter <cl@...two.de>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, penberg@...nel.org,
        vbabka@...e.cz, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        pabeni@...hat.com
Subject: Re: [PATCH RFC] mm+net: allow to set kmem_cache create flag for
 SLAB_NEVER_MERGE

On Tue, Jan 17, 2023 at 03:54:34PM +0100, Christoph Lameter wrote:
> On Tue, 17 Jan 2023, Jesper Dangaard Brouer wrote:
> 
> > When running different network performance microbenchmarks, I started
> > to notice that performance was reduced (slightly) when machines had
> > longer uptimes. I believe the cause was 'skbuff_head_cache' got
> > aliased/merged into the general slub for 256 bytes sized objects (with
> > my kernel config, without CONFIG_HARDENED_USERCOPY).
> 
> Well that is a common effect that we see in multiple subsystems. This is
> due to general memory fragmentation. Depending on the prior load the
> performance could actually be better after some runtime if the caches are
> populated avoiding the page allocator etc.

The page allocator isn't _that_ expensive.  I could see updating several
slabs being more expensive than allocating a new page.

> The merging could actually be beneficial since there may be more partial
> slabs to allocate from and thus avoiding expensive calls to the page
> allocator.

What might be more effective is allocating larger order slabs.  I see
that kmalloc-256 allocates a pair of pages and manages 32 objects within
that pair.  It should perform better in Jesper's scenario if it allocated
4 pages and managed 64 objects per slab.

Simplest way to test that should be booting a kernel with
'slub_min_order=2'.  Does that help matters at all, Jesper?  You could
also try slub_min_order=3.  Going above that starts to get a bit sketchy.