linux-kernel - Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order

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Date:   Thu, 21 Jan 2021 10:01:57 +0000 (UTC)
From:   Christoph Lameter <cl@...ux.com>
To:     Bharata B Rao <bharata@...ux.ibm.com>
cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, guro@...com,
        vbabka@...e.cz, shakeelb@...gle.com,
        Johannes Weiner <hannes@...xchg.org>,
        aneesh.kumar@...ux.ibm.com
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the
 slub page order

On Thu, 21 Jan 2021, Bharata B Rao wrote:

> > The problem is that calculate_order() is called a number of times
> > before secondaries CPUs are booted and it returns 1 instead of 224.
> > This makes the use of num_online_cpus() irrelevant for those cases
> >
> > After adding in my command line "slub_min_objects=36" which equals to
> > 4 * (fls(num_online_cpus()) + 1) with a correct num_online_cpus == 224
> > , the regression diseapears:
> >
> > 9 iterations of hackbench -l 16000 -g 16: 3.201sec (+/- 0.90%)
>
> Should we have switched to num_present_cpus() rather than
> num_online_cpus()? If so, the below patch should address the
> above problem.

There is certainly an initcall after secondaries are booted where we could
redo the calculate_order?

Or the num_online_cpus needs to be up to date earlier. Why does this issue
not occur on x86? Does x86 have an up to date num_online_cpus earlier?