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

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Date:   Fri, 22 Jan 2021 14:05:47 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Lameter <cl@...ux.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        aneesh.kumar@...ux.ibm.com, Michal Hocko <mhocko@...nel.org>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the
 slub page order

On Thu, Jan 21, 2021 at 7:19 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> On 1/21/21 11:01 AM, Christoph Lameter 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%)
>
> I'm surprised that hackbench is that sensitive to slab performance, anyway. It's
> supposed to be a scheduler benchmark? What exactly is going on?

Uuuh, I think powerpc doesn't have cmpxchg_double?

"vgrep cmpxchg_double arch/" just spits out arm64, s390 and x86? And
<https://liblfds.org/mediawiki/index.php?title=Article:CAS_and_LL/SC_Implementation_Details_by_Processor_family>
says under "POWERPC": "no DW LL/SC"

So powerpc is probably hitting the page-bitlock-based implementation
all the time for stuff like __slub_free()? Do you have detailed
profiling results from "perf top" or something like that?

(I actually have some WIP patches and a design document for getting
rid of cmpxchg_double in struct page that I hacked together in the
last couple days; I'm currently in the process of sending them over to
some other folks in the company who hopefully have cycles to
review/polish/benchmark them so that they can be upstreamed, assuming
that those folks think they're important enough. I don't have the
cycles for it...)