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

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Date:   Thu, 14 Jan 2021 10:27:40 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christoph Lameter <cl@...ux.com>, Jann Horn <jannh@...gle.com>
Cc:     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: SLUB: percpu partial object count is highly inaccurate, causing some
 memory wastage and maybe also worse tail latencies?

On 1/12/21 5:35 PM, Christoph Lameter wrote:
> On Tue, 12 Jan 2021, Jann Horn wrote:
> 
>> [This is not something I intend to work on myself. But since I
>> stumbled over this issue, I figured I should at least document/report
>> it, in case anyone is willing to pick it up.]
> 
> Well yeah all true. There is however a slabinfo tool that has an -s option
> to shrink all slabs.
> 
> 	slabinfo -s
> 
> So you could put that somewhere that executes if the system is
> idle or put it into cron or so.

Hm this would be similar to recommending a periodical echo > drop_caches
operation. We actually discourage from that (and yeah, some tools do that, and
we now report those in dmesg). I believe the kernel should respond to memory
pressure and not OOM prematurely by itself, including SLUB.