From: Vincent Guittot <vincent.guittot@linaro.org>
To: Bharata B Rao <bharata@linux.ibm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>,
Christoph Lameter <cl@linux.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>,
Catalin Marinas <Catalin.Marinas@arm.com>,
Will Deacon <will@kernel.org>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order
Date: Sat, 23 Jan 2021 13:32:44 +0100 [thread overview]
Message-ID: <CAKfTPtAjyVmS5VYvU6DBxg4-JEo5bdmWbngf-03YsY18cmWv_g@mail.gmail.com> (raw)
In-Reply-To: <20210123051607.GC2587010@in.ibm.com>
+Adding arch arm64 Maintainers
On Sat, 23 Jan 2021 at 06:16, Bharata B Rao <bharata@linux.ibm.com> wrote:
>
> On Fri, Jan 22, 2021 at 01:03:57PM +0100, Vlastimil Babka 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?
>
> PowerPC PowerNV Host: (160 cpus)
> num_online_cpus 1 num_present_cpus 160 num_possible_cpus 160 nr_cpu_ids 160
>
> PowerPC pseries KVM guest: (-smp 16,maxcpus=160)
> num_online_cpus 1 num_present_cpus 16 num_possible_cpus 160 nr_cpu_ids 160
>
> That's what I see on powerpc, hence I thought num_present_cpus() could
> be the correct one to use in slub page order calculation.
num_present_cpus() is set to 1 on arm64 until secondaries cpus boot
arm64 224cpus acpi host:
num_online_cpus 1 num_present_cpus 1 num_possible_cpus 224 nr_cpu_ids 224
arm64 8cpus DT host:
num_online_cpus 1 num_present_cpus 1 num_possible_cpus 8 nr_cpu_ids 8
arm64 8cpus qemu-system-aarch64 (-smp 8,maxcpus=256)
num_online_cpus 1 num_present_cpus 1 num_possible_cpus 8 nr_cpu_ids 8
Then present and online increase to num_possible_cpus once all cpus are booted
>
> >
> > What about heuristic:
> > - num_online_cpus() > 1 - we trust that and use it
> > - otherwise nr_cpu_ids
> > Would that work? Too arbitrary?
>
> Looking at the following snippet from include/linux/cpumask.h, it
> appears that num_present_cpus() should be reasonable compromise
> between online and possible/nr_cpus_ids to use here.
>
> /*
> * The following particular system cpumasks and operations manage
> * possible, present, active and online cpus.
> *
> * cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
> * cpu_present_mask - has bit 'cpu' set iff cpu is populated
> * cpu_online_mask - has bit 'cpu' set iff cpu available to scheduler
> * cpu_active_mask - has bit 'cpu' set iff cpu available to migration
> *
> * If !CONFIG_HOTPLUG_CPU, present == possible, and active == online.
> *
> * The cpu_possible_mask is fixed at boot time, as the set of CPU id's
> * that it is possible might ever be plugged in at anytime during the
> * life of that system boot. The cpu_present_mask is dynamic(*),
> * representing which CPUs are currently plugged in. And
> * cpu_online_mask is the dynamic subset of cpu_present_mask,
> * indicating those CPUs available for scheduling.
> *
> * If HOTPLUG is enabled, then cpu_possible_mask is forced to have
> * all NR_CPUS bits set, otherwise it is just the set of CPUs that
> * ACPI reports present at boot.
> *
> * If HOTPLUG is enabled, then cpu_present_mask varies dynamically,
> * depending on what ACPI reports as currently plugged in, otherwise
> * cpu_present_mask is just a copy of cpu_possible_mask.
> *
> * (*) Well, cpu_present_mask is dynamic in the hotplug case. If not
> * hotplug, it's a copy of cpu_possible_mask, hence fixed at boot.
> */
>
> So for host systems, present is (usually) equal to possible and for
But "cpu_present_mask varies dynamically, depending on what ACPI
reports as currently plugged in"
So it should varies when secondaries cpus are booted
> guest systems present should indicate the CPUs found to be present
> at boottime. The intention of my original patch was to use this
> metric in slub page order calculation rather than nr_cpus_ids
> or num_cpus_possible() which could be high on guest systems that
> typically support CPU hotplug.
>
> Regards,
> Bharata.
next prev parent reply other threads:[~2021-01-23 12:33 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
2021-01-23 5:16 ` Bharata B Rao
2021-01-23 12:32 ` Vincent Guittot [this message]
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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