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