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

[Will Deacon <will@kernel.org>]

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