Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order

[Vlastimil Babka]

On 1/22/21 9:03 AM, Vincent Guittot wrote:
> On Thu, 21 Jan 2021 at 19:19, Vlastimil Babka <vbabka@xxxxxxx> 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?
>> >
>> >
>>
>