linux-kernel - Re: [PATCH] mm/slub: improve count_partial() for CONFIG_SLUB_CPU

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Message-ID: <alpine.DEB.2.21.2002240126190.13486@www.lameter.com>
Date:   Mon, 24 Feb 2020 01:29:09 +0000 (UTC)
From:   Christopher Lameter <cl@...ux.com>
To:     Wen Yang <wenyang@...ux.alibaba.com>
cc:     Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Xunlei Pang <xlpang@...ux.alibaba.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/slub: improve count_partial() for
 CONFIG_SLUB_CPU_PARTIAL

On Sat, 22 Feb 2020, Wen Yang wrote:

> We also observed that in this scenario, CONFIG_SLUB_CPU_PARTIAL is turned
> on by default, and count_partial() is useless because the returned number
> is far from the reality.

Well its not useless. Its just not counting the partial objects in per cpu
partial slabs. Those are counted by a different counter it.

> Therefore, we can simply return 0, then nr_free is also 0, and eventually
> active_objects == total_objects. We do not introduce any regression, and
> it's preferable to show the unrealistic uniform 100% slab utilization
> rather than some very high but incorrect value.

I suggest that you simply use the number of partial slabs and multiply
them by the number of objects in a slab and use that as a value. Both
values are readily available via /sys/kernel/slab/<...>/

You dont need a patch to do that.