From: Will Deacon <will@kernel.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
Bharata B Rao <bharata@linux.ibm.com>,
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>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order
Date: Tue, 26 Jan 2021 23:03:06 +0000 [thread overview]
Message-ID: <20210126230305.GD30941@willie-the-truck> (raw)
In-Reply-To: <66652406-25e4-a9e7-45a1-8ad14d2e8a36@suse.cz>
On Mon, Jan 25, 2021 at 12:20:14PM +0100, Vlastimil Babka wrote:
> On 1/23/21 1:32 PM, Vincent Guittot wrote:
> >> 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
>
> I would have expected num_present_cpus to be 224, 8, 8, respectively.
>
> > 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
>
> Hm, but booting the secondaries is just a software (kernel) action? They are
> already physically there, so it seems to me as if the cpu_present_mask is not
> populated correctly on arm64, and it's just a mirror of cpu_online_mask?
I think the present_mask retains CPUs if they are hotplugged off, whereas
the online mask does not. We can't really do any better on arm64, as there's
no way of telling that a CPU is present until we've seen it.
Will
next prev parent reply other threads:[~2021-01-27 10:18 UTC|newest]
Thread overview: 37+ 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
2021-01-25 11:20 ` Vlastimil Babka
2021-01-26 23:03 ` Will Deacon [this message]
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
2021-01-27 13:38 ` Vlastimil Babka
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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