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

[Vincent Guittot]

On Fri, 22 Jan 2021 at 13:03, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> 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?

num_present_cpus() starts to 1 until secondary cpus boot in the arm64 case

>
> 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?
> >> >
> >> >
> >>
> >
>