Re: [PATCH v12 00/11] Parallel CPU bringup for x86_64

Oleksandr Natalenko
2023-02-26T18:31:11Z

Hello.

On neděle 26. února 2023 12:07:51 CET Usama Arif wrote:
> The main code change over v11 is the build error fix by Brian Gerst and
> acquiring tr_lock in trampoline_64.S whenever the stack is setup.
>
> The git history is also rewritten to move the commits that removed
> initial_stack, early_gdt_descr and initial_gs earlier in the patchset.
>
> Thanks,
> Usama
>
> Changes across versions:
> v2: Cut it back to just INIT/SIPI/SIPI in parallel for now, nothing more
> v3: Clean up x2apic patch, add MTRR optimisation, lock topology update
> in preparation for more parallelisation.
> v4: Fixes to the real mode parallelisation patch spotted by SeanC, to
> avoid scribbling on initial_gs in common_cpu_up(), and to allow all
> 24 bits of the physical X2APIC ID to be used. That patch still needs
> a Signed-off-by from its original author, who once claimed not to
> remember writing it at all. But now we've fixed it, hopefully he'll
> admit it now :)
> v5: rebase to v6.1 and remeasure performance, disable parallel bringup
> for AMD CPUs.
> v6: rebase to v6.2-rc6, disabled parallel boot on amd as a cpu bug and
> reused timer calibration for secondary CPUs.
> v7: [David Woodhouse] iterate over all possible CPUs to find any existing
> cluster mask in alloc_clustermask. (patch 1/9)
> Keep parallel AMD support enabled in AMD, using APIC ID in CPUID leaf
> 0x0B (for x2APIC mode) or CPUID leaf 0x01 where 8 bits are sufficient.
> Included sanity checks for APIC id from 0x0B. (patch 6/9)
> Removed patch for reusing timer calibration for secondary CPUs.
> commit message and code improvements.
> v8: Fix CPU0 hotplug by setting up the initial_gs, initial_stack and
> early_gdt_descr.
> Drop trampoline lock and bail if APIC ID not found in find_cpunr.
> Code comments improved and debug prints added.
> v9: Drop patch to avoid repeated saves of MTRR at boot time.
> rebased and retested at v6.2-rc8.
> added kernel doc for no_parallel_bringup and made do_parallel_bringup
> __ro_after_init.
> v10: Fixed suspend/resume not working with parallel smpboot.
> rebased and retested to 6.2.
> fixed checkpatch errors.
> v11: Added patches from Brian Gerst to remove the global variables initial_gs,
> initial_stack, and early_gdt_descr from the 64-bit boot code
> (https://lore.kernel.org/all/20230222221301.245890-1-brgerst@gmail.com/).
> v12: Fixed compilation errors, acquire tr_lock for every stack setup in
> trampoline_64.S.
> Rearranged commits for a cleaner git history.
>
> Brian Gerst (3):
> x86/smpboot: Remove initial_stack on 64-bit
> x86/smpboot: Remove early_gdt_descr on 64-bit
> x86/smpboot: Remove initial_gs
>
> David Woodhouse (8):
> x86/apic/x2apic: Allow CPU cluster_mask to be populated in parallel
> cpu/hotplug: Move idle_thread_get() to <linux/smpboot.h>
> cpu/hotplug: Add dynamic parallel bringup states before
> CPUHP_BRINGUP_CPU
> x86/smpboot: Reference count on smpboot_setup_warm_reset_vector()
> x86/smpboot: Split up native_cpu_up into separate phases and document
> them
> x86/smpboot: Support parallel startup of secondary CPUs
> x86/smpboot: Send INIT/SIPI/SIPI to secondary CPUs in parallel
> x86/smpboot: Serialize topology updates for secondary bringup
>
> .../admin-guide/kernel-parameters.txt | 3 +
> arch/x86/include/asm/processor.h | 6 +-
> arch/x86/include/asm/realmode.h | 4 +-
> arch/x86/include/asm/smp.h | 15 +-
> arch/x86/include/asm/topology.h | 2 -
> arch/x86/kernel/acpi/sleep.c | 15 +-
> arch/x86/kernel/apic/apic.c | 2 +-
> arch/x86/kernel/apic/x2apic_cluster.c | 126 ++++---
> arch/x86/kernel/asm-offsets.c | 1 +
> arch/x86/kernel/cpu/common.c | 6 +-
> arch/x86/kernel/head_64.S | 129 +++++--
> arch/x86/kernel/smpboot.c | 350 +++++++++++++-----
> arch/x86/realmode/init.c | 3 +
> arch/x86/realmode/rm/trampoline_64.S | 27 +-
> arch/x86/xen/smp_pv.c | 4 +-
> arch/x86/xen/xen-head.S | 2 +-
> include/linux/cpuhotplug.h | 2 +
> include/linux/smpboot.h | 7 +
> kernel/cpu.c | 31 +-
> kernel/smpboot.h | 2 -
> 20 files changed, 537 insertions(+), 200 deletions(-)

With `CONFIG_FORCE_NR_CPUS=y` this results in:

```
ld: vmlinux.o: in function `secondary_startup_64_no_verify':
(.head.text+0x10c): undefined reference to `nr_cpu_ids'
```

That's because in `arch/x86/kernel/head_64.S` `secondary_startup_64_no_verify()` refers to `nr_cpu_ids` under `#ifdef CONFIG_SMP`, but this symbol is available under the following conditions:

```
38 #if (NR_CPUS == 1) || defined(CONFIG_FORCE_NR_CPUS)
39 #define nr_cpu_ids ((unsigned int)NR_CPUS)
40 #else
41 extern unsigned int nr_cpu_ids;
42 #endif

1090 #if (NR_CPUS > 1) && !defined(CONFIG_FORCE_NR_CPUS)
1091 /* Setup number of possible processor ids */
1092 unsigned int nr_cpu_ids __read_mostly = NR_CPUS;
1093 EXPORT_SYMBOL(nr_cpu_ids);
1094 #endif
```

So having `CONFIG_SMP=y` and, for instance, `CONFIG_NR_CPUS=320`, it is possible to compile out `EXPORT_SYMBOL(nr_cpu_ids);` if `CONFIG_FORCE_NR_CPUS=y` is set.

--
Oleksandr Natalenko (post-factum)