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From: Vlastimil Babka <vbabka@suse.cz>
To: Vincent Guittot <vincent.guittot@linaro.org>
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 13:03:57 +0100	[thread overview]
Message-ID: <786571e7-b9a2-4cdb-06d5-aa4a4b439b7e@suse.cz> (raw)
In-Reply-To: <CAKfTPtDy3Ynk2nGCTWiXjz9-4vuSHB3pGuafoTUBPFNO1ac3PA@mail.gmail.com>

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?

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?
>> >
>> >
>>
> 



  reply	other threads:[~2021-01-22 12:04 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 [this message]
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
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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