+Adding arch arm64 Maintainers On Sat, 23 Jan 2021 at 06:16, Bharata B Rao <bharata@xxxxxxxxxxxxx> wrote: > > On Fri, Jan 22, 2021 at 01:03:57PM +0100, Vlastimil Babka 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? > > 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 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 > guest systems present should indicate the CPUs found to be present > at boottime. The intention of my original patch was to use this > metric in slub page order calculation rather than nr_cpus_ids > or num_cpus_possible() which could be high on guest systems that > typically support CPU hotplug. > > Regards, > Bharata.
