From: Vincent Guittot <vincent.guittot@linaro.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>,
Bharata B Rao <bharata@linux.ibm.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Andrew Morton <akpm@linux-foundation.org>,
guro@fb.com, Shakeel Butt <shakeelb@google.com>,
Johannes Weiner <hannes@cmpxchg.org>,
aneesh.kumar@linux.ibm.com, Jann Horn <jannh@google.com>,
Michal Hocko <mhocko@kernel.org>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order
Date: Fri, 22 Jan 2021 14:16:11 +0100 [thread overview]
Message-ID: <CAKfTPtAEFSHBR3_oK6PpH-GWhtZBV9unyBh=n5DVT36eHvo6Dg@mail.gmail.com> (raw)
In-Reply-To: <786571e7-b9a2-4cdb-06d5-aa4a4b439b7e@suse.cz>
On Fri, 22 Jan 2021 at 13:03, Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/22/21 9:03 AM, Vincent Guittot wrote:
> > On Thu, 21 Jan 2021 at 19:19, 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?
> >>
> >
> > From hackbench description:
> > Hackbench is both a benchmark and a stress test for the Linux kernel
> > scheduler. It's main
> > job is to create a specified number of pairs of schedulable
> > entities (either threads or
> > traditional processes) which communicate via either sockets or
> > pipes and time how long it
> > takes for each pair to send data back and forth.
>
> Yep, so I wonder which slab entities this is stressing that much.
>
> >> Things would be easier if we could trust *on all arches* either
> >>
> >> - num_present_cpus() to count what the hardware really physically has during
> >> boot, even if not yet onlined, at the time we init slab. This would still not
> >> handle later hotplug (probably mostly in a VM scenario, not that somebody would
> >> bring bunch of actual new cpu boards to a running bare metal system?).
> >>
> >> - num_possible_cpus()/nr_cpu_ids not to be excessive (broken BIOS?) on systems
> >> where it's not really possible to plug more CPU's. In a VM scenario we could
> >> still have an opposite problem, where theoretically "anything is possible" but
> >> the virtual cpus are never added later.
> >
> > On all the system that I have tested num_possible_cpus()/nr_cpu_ids
> > were correctly initialized
> >
> > large arm64 acpi system
> > small arm64 DT based system
> > VM on x86 system
>
> So it's just powerpc that has this issue with too large nr_cpu_ids? Is it caused
> by bios or the hypervisor? How does num_present_cpus() look there?
num_present_cpus() starts to 1 until secondary cpus boot in the arm64 case
>
> What about heuristic:
> - num_online_cpus() > 1 - we trust that and use it
> - otherwise nr_cpu_ids
> Would that work? Too arbitrary?
>
>
> >> We could also start questioning the very assumption that number of cpus should
> >> affect slab page size in the first place. Should it? After all, each CPU will
> >> have one or more slab pages privately cached, as we discuss in the other
> >> thread... So why make the slab pages also larger?
> >>
> >> > 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?
> >> >
> >> >
> >>
> >
>
next prev parent reply other threads:[~2021-01-22 13:16 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-18 8:27 [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order Bharata B Rao
2020-11-18 11:25 ` Vlastimil Babka
2020-11-18 19:34 ` Roman Gushchin
2020-11-18 19:53 ` David Rientjes
2021-01-20 17:36 ` Vincent Guittot
2021-01-21 5:30 ` Bharata B Rao
2021-01-21 9:09 ` Vincent Guittot
2021-01-21 10:01 ` Christoph Lameter
2021-01-21 10:48 ` Vincent Guittot
2021-01-21 18:19 ` Vlastimil Babka
2021-01-22 8:03 ` Vincent Guittot
2021-01-22 12:03 ` Vlastimil Babka
2021-01-22 13:16 ` Vincent Guittot [this message]
2021-01-23 5:16 ` Bharata B Rao
2021-01-23 12:32 ` Vincent Guittot
2021-01-25 11:20 ` Vlastimil Babka
2021-01-26 23:03 ` Will Deacon
2021-01-27 9:10 ` Christoph Lameter
2021-01-27 11:04 ` Vlastimil Babka
2021-02-03 11:10 ` Bharata B Rao
2021-02-04 7:32 ` Vincent Guittot
2021-02-04 9:07 ` Christoph Lameter
2021-02-04 9:33 ` Vlastimil Babka
2021-02-08 13:41 ` [PATCH] mm, slub: better heuristic for number of cpus when calculating slab order Vlastimil Babka
2021-02-08 14:54 ` Vincent Guittot
2021-02-10 14:07 ` Mel Gorman
2021-01-22 13:05 ` [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order Jann Horn
2021-01-22 13:09 ` Jann Horn
2021-01-22 15:27 ` Vlastimil Babka
2021-01-25 4:28 ` Bharata B Rao
2021-01-26 8:52 ` Michal Hocko
2021-01-26 13:38 ` Vincent Guittot
2021-01-26 13:59 ` Michal Hocko
2021-01-28 13:45 ` Mel Gorman
2021-01-28 13:57 ` Michal Hocko
2021-01-28 14:42 ` Mel Gorman
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