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

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Message-ID: <20210125042813.GA2869122@in.ibm.com>
Date:   Mon, 25 Jan 2021 09:58:13 +0530
From:   Bharata B Rao <bharata@...ux.ibm.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Christoph Lameter <cl@...ux.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 Fri, Jan 22, 2021 at 02:05:47PM +0100, Jann Horn wrote:
> 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 can check that, but the current patch was aimed at reducing
the page order of the slub caches so that they don't end up
consuming more memory on 64K systems.

> 
> (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...)

Sounds interesting, will keep a watch to see its effect on powerpc.

Regards,
Bharata.