linux-kernel - Re: SLUB: percpu partial object count is highly inaccurate, causing some memory wastage and maybe also worse tail latencies?

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]

Message-ID: <alpine.DEB.2.22.394.2101140902210.38517@www.lameter.com>
Date:   Thu, 14 Jan 2021 09:04:26 +0000 (UTC)
From:   Christoph Lameter <cl@...ux.com>
To:     Jann Horn <jannh@...gle.com>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: SLUB: percpu partial object count is highly inaccurate, causing
 some memory wastage and maybe also worse tail latencies?

On Wed, 13 Jan 2021, Jann Horn wrote:

> Some brainstorming:
>
> Maybe you could have an atomic counter in kmem_cache_cpu that tracks
> the number of empty frozen pages that are associated with a specific
> CPU? So the freeing slowpath would do its cmpxchg_double, and if the


The latencies of these functions are so low that any additional counter
will have significant performance impacts. An atomic counter would be waay
out there.

> You could additionally have a plain percpu counter, not tied to the

The performance critical counters are already all per cpu. I enhanced the
percpu subsystem specifically to support latency critical operations in
the fast path of the slab allocators.