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 11:48:33 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Christoph Lameter <cl@...ux.com>
Cc:     Bharata B Rao <bharata@...ux.ibm.com>,
        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, Shakeel Butt <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 at 11:01, Christoph Lameter <cl@...ux.com> wrote:
>
> 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?

I have added a printk in calculate_order() :
        pr_info(" SLUB calculate_order cmd  %d min %d online %d
present %d possible %d cpus %d", slub_min_objects, min_objects,
num_online_cpus(), num_present_cpus(), num_possible_cpus(),
nr_cpu_ids);

And checked with
qemu-system-x86_64 -kernel bzImage -nographic -smp 4 -append "console=ttyS0"

[    0.064927]  SLUB calculate_order cmd  0 min 8 online 1 present 4
possible 4 cpus 4
[    0.064970] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=4, Nodes=1

The num_online_cpus has the same behavior as on my platform and
num_online_cpus =  1 when kmem_cache_init() is called

Only the num_present_cpus = 4 from the beginning but that's probably
just because it runs in a VM

Also it's interesting to notice that num_possible_cpus and nr_cpu_ids
are set to the correct value

>
>