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

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Message-ID: <aa02cf86-3a83-2e55-3bb6-3ec1c0f71b11@suse.cz>
Date:   Thu, 21 Jan 2021 18:21:18 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Jann Horn <jannh@...gle.com>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 1/12/21 12:12 AM, Jann Horn wrote:
> At first I thought that this wasn't a significant issue because SLUB
> has a reclaim path that can trim the percpu partial lists; but as it
> turns out, that reclaim path is not actually wired up to the page
> allocator's reclaim logic. The SLUB reclaim stuff is only triggered by
> (very rare) subsystem-specific calls into SLUB for specific slabs and
> by sysfs entries. So in userland processes will OOM even if SLUB still
> has megabytes of entirely unused pages lying around.
> 
> It might be a good idea to figure out whether it is possible to
> efficiently keep track of a more accurate count of the free objects on
> percpu partial lists; and if not, maybe change the accounting to
> explicitly track the number of partial pages, and use limits that are
> more appropriate for that? And perhaps the page allocator reclaim path
> should also occasionally rip unused pages out of the percpu partial
> lists?

I'm gonna send a RFC that adds a proper shrinker and thus connects this
shrinking to page reclaim, as a reply to this e-mail.