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From:   Jann Horn <jannh@google.com>
Date:   Fri, 22 Jan 2021 14:09:33 +0100
Message-ID: <CAG48ez3YavBAO1nQquca6W-RvH4yy2B7oiY4jWn96B_aaA_gVw@mail.gmail.com>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the
 slub page order
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Christoph Lameter <cl@linux.com>,
        Bharata B Rao <bharata@linux.ibm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        aneesh.kumar@linux.ibm.com, Michal Hocko <mhocko@kernel.org>
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On Fri, Jan 22, 2021 at 2:05 PM Jann Horn <jannh@google.com> wrote:
> On Thu, Jan 21, 2021 at 7:19 PM Vlastimil Babka <vbabka@suse.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...)

(The stuff I have in mind will only work on 64-bit though. We are
talking about PPC64 here, right?)