From: Vlastimil Babka <vbabka@suse.cz>
To: Vincent Guittot <vincent.guittot@linaro.org>,
Bharata B Rao <bharata@linux.ibm.com>
Cc: 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: Mon, 25 Jan 2021 12:20:14 +0100 [thread overview]
Message-ID: <66652406-25e4-a9e7-45a1-8ad14d2e8a36@suse.cz> (raw)
In-Reply-To: <CAKfTPtAjyVmS5VYvU6DBxg4-JEo5bdmWbngf-03YsY18cmWv_g@mail.gmail.com>
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?
next prev parent reply other threads:[~2021-01-25 11:20 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
2021-01-25 11:20 ` Vlastimil Babka [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=66652406-25e4-a9e7-45a1-8ad14d2e8a36@suse.cz \
--to=vbabka@suse.cz \
--cc=Catalin.Marinas@arm.com \
--cc=akpm@linux-foundation.org \
--cc=aneesh.kumar@linux.ibm.com \
--cc=bharata@linux.ibm.com \
--cc=cl@linux.com \
--cc=guro@fb.com \
--cc=hannes@cmpxchg.org \
--cc=iamjoonsoo.kim@lge.com \
--cc=jannh@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@kernel.org \
--cc=rientjes@google.com \
--cc=shakeelb@google.com \
--cc=vincent.guittot@linaro.org \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).