linux-kernel - Re: [mm, slub] 8ff60eb052: stress-ng.rawpkt.ops_per

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Date:   Wed, 10 Mar 2021 07:59:51 +0100 (CET)
From:   Christoph Lameter <cl@...two.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     kernel test robot <oliver.sang@...el.com>,
        Jann Horn <jannh@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Feng Tang <feng.tang@...el.com>, zhengjun.xing@...el.com
Subject: Re: [mm, slub] 8ff60eb052: stress-ng.rawpkt.ops_per_sec -47.9%
 regression

On Tue, 9 Mar 2021, Linus Torvalds wrote:

> So when you wrote:
>
>     However, the current code accidentally stops looking at the partial list
>     completely in that case.  Especially on kernels without CONFIG_NUMA set,
>     this means that get_partial() fails and new_slab_objects() falls back to
>     new_slab(), allocating new pages.  This could lead to an unnecessary
>     increase in memory fragmentation.
>
> it really looks like this might well have been very intentional
> indeed. Or at least very beneficial for _some_ loads.
>
> Comments?

Yes the thought was that adding an additional page when contention is
there on the page objects will increase possible concurrency while
avoiding locks and increase the ability to allocate / free concurrently
from a multitude of objects.