RE: [patch 00/37] cpu/hotplug, x86: Reworked parallel CPU bringup

[Michael Kelley (LINUX)]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Sent: Friday, April 14, 2023 4:44 PM

[snip]

> 
> Conclusion
> ----------
> 
> Adding the basic parallel bringup mechanism as provided by this series
> makes a lot of sense. Improving particular issues as pointed out in the
> analysis makes sense too.
> 
> But trying to solve an application specific problem fully in the kernel
> with tons of complexity, without exploring straight forward and simple
> approaches first, does not make any sense at all.
> 
> Thanks,
> 
> 	tglx
> 
> ---
>  Documentation/admin-guide/kernel-parameters.txt |   20
>  Documentation/core-api/cpu_hotplug.rst          |   13
>  arch/Kconfig                                    |   23 +
>  arch/arm/Kconfig                                |    1
>  arch/arm/include/asm/smp.h                      |    2
>  arch/arm/kernel/smp.c                           |   18
>  arch/arm64/Kconfig                              |    1
>  arch/arm64/include/asm/smp.h                    |    2
>  arch/arm64/kernel/smp.c                         |   14
>  arch/csky/Kconfig                               |    1
>  arch/csky/include/asm/smp.h                     |    2
>  arch/csky/kernel/smp.c                          |    8
>  arch/mips/Kconfig                               |    1
>  arch/mips/cavium-octeon/smp.c                   |    1
>  arch/mips/include/asm/smp-ops.h                 |    1
>  arch/mips/kernel/smp-bmips.c                    |    1
>  arch/mips/kernel/smp-cps.c                      |   14
>  arch/mips/kernel/smp.c                          |    8
>  arch/mips/loongson64/smp.c                      |    1
>  arch/parisc/Kconfig                             |    1
>  arch/parisc/kernel/process.c                    |    4
>  arch/parisc/kernel/smp.c                        |    7
>  arch/riscv/Kconfig                              |    1
>  arch/riscv/include/asm/smp.h                    |    2
>  arch/riscv/kernel/cpu-hotplug.c                 |   14
>  arch/x86/Kconfig                                |   45 --
>  arch/x86/include/asm/apic.h                     |    5
>  arch/x86/include/asm/cpu.h                      |    5
>  arch/x86/include/asm/cpumask.h                  |    5
>  arch/x86/include/asm/processor.h                |    1
>  arch/x86/include/asm/realmode.h                 |    3
>  arch/x86/include/asm/sev-common.h               |    3
>  arch/x86/include/asm/smp.h                      |   26 -
>  arch/x86/include/asm/topology.h                 |   23 -
>  arch/x86/include/asm/tsc.h                      |    2
>  arch/x86/kernel/acpi/sleep.c                    |    9
>  arch/x86/kernel/apic/apic.c                     |   22 -
>  arch/x86/kernel/callthunks.c                    |    4
>  arch/x86/kernel/cpu/amd.c                       |    2
>  arch/x86/kernel/cpu/cacheinfo.c                 |   21
>  arch/x86/kernel/cpu/common.c                    |   50 --
>  arch/x86/kernel/cpu/topology.c                  |    3
>  arch/x86/kernel/head_32.S                       |   14
>  arch/x86/kernel/head_64.S                       |  121 +++++
>  arch/x86/kernel/sev.c                           |    2
>  arch/x86/kernel/smp.c                           |    3
>  arch/x86/kernel/smpboot.c                       |  508 ++++++++----------------
>  arch/x86/kernel/topology.c                      |   98 ----
>  arch/x86/kernel/tsc.c                           |   20
>  arch/x86/kernel/tsc_sync.c                      |   36 -
>  arch/x86/power/cpu.c                            |   37 -
>  arch/x86/realmode/init.c                        |    3
>  arch/x86/realmode/rm/trampoline_64.S            |   27 +
>  arch/x86/xen/enlighten_hvm.c                    |   11
>  arch/x86/xen/smp_hvm.c                          |   16
>  arch/x86/xen/smp_pv.c                           |   56 +-
>  drivers/acpi/processor_idle.c                   |    4
>  include/linux/cpu.h                             |    4
>  include/linux/cpuhotplug.h                      |   17
>  kernel/cpu.c                                    |  397 +++++++++++++++++-
>  kernel/smp.c                                    |    2
>  kernel/smpboot.c                                |  163 -------
>  62 files changed, 953 insertions(+), 976 deletions(-)
> 

I smoke-tested several Linux guest configurations running on Hyper-V,
using the "kernel/git/tglx/devel.git hotplug" tree as updated on April 26th.
No functional issues, but encountered one cosmetic issue (details below).

Configurations tested:
*  16 vCPUs and 32 vCPUs
*  1 NUMA node and 2 NUMA nodes
*  Parallel bring-up enabled and disabled via kernel boot line
*  "Normal" VMs and SEV-SNP VMs running with a paravisor on Hyper-V.
    This config can use parallel bring-up because most of the SNP-ness is
    hidden in the paravisor.  I was glad to see this work properly.

There's not much difference in performance with and without parallel
bring-up on the 32 vCPU VM.   Without parallel, the time is about 26
milliseconds.  With parallel, it's about 24 ms.   So bring-up is already
fast in the virtual environment.

The cosmetic issue is in the dmesg log, and arises because Hyper-V
enumerates SMT CPUs differently from many other environments.  In
a Hyper-V guest, the SMT threads in a core are numbered as <even, odd>
pairs.  Guest CPUs #0 & #1 are SMT threads in core, as are #2 & #3, etc.  With
parallel bring-up, here's the dmesg output:

[    0.444345] smp: Bringing up secondary CPUs ...
[    0.445139] .... node  #0, CPUs:    #2  #4  #6  #8 #10 #12 #14 #16 #18 #20 #22 #24 #26 #28 #30
[    0.454112] x86: Booting SMP configuration:
[    0.456035]       #1  #3  #5  #7  #9 #11 #13 #15 #17 #19 #21 #23 #25 #27 #29 #31
[    0.466120] smp: Brought up 1 node, 32 CPUs
[    0.467036] smpboot: Max logical packages: 1
[    0.468035] smpboot: Total of 32 processors activated (153240.06 BogoMIPS)

The function announce_cpu() is specifically testing for CPU #1 to output the
"Booting SMP configuration" message.  In a Hyper-V guest, CPU #1 is the second
SMT thread in a core, so it isn't started until all the even-numbered CPUs are
started.

I don't know if this cosmetic issue is worth fixing, but I thought I'd point it out.

In any case,

Tested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>