Re: [PATCH] sched/arm64: store cpu topology before notify_cpu_starting

[Joel Fernandes <joelaf@google.com>]

On Wed, Apr 1, 2020 at 7:27 AM Cheng Jian <cj.chengjian@huawei.com> wrote:
>
> when SCHED_CORE enabled, sched_cpu_starting() uses thread_sibling as
> SMT_MASK to initialize rq->core, but only after store_cpu_topology(),
> the thread_sibling is ready for use.
>
>         notify_cpu_starting()
>             -> sched_cpu_starting()     # use thread_sibling
>
>         store_cpu_topology(cpu)
>             -> update_siblings_masks    # set thread_sibling
>
> Fix this by doing notify_cpu_starting later, just like x86 do.
>
> Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>

Just a high-level question, why does core-scheduling matter on ARM64?
Is it for HPC workloads?

Thanks,

 - Joel


> ---
>  arch/arm64/kernel/smp.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 5407bf5d98ac..a427c14e82af 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -236,13 +236,18 @@ asmlinkage notrace void secondary_start_kernel(void)
>         cpuinfo_store_cpu();
>
>         /*
> -        * Enable GIC and timers.
> +        * Store cpu topology before notify_cpu_starting,
> +        * CPUHP_AP_SCHED_STARTING requires SMT topology
> +        * been initialized for SCHED_CORE.
>          */
> -       notify_cpu_starting(cpu);
> -
>         store_cpu_topology(cpu);
>         numa_add_cpu(cpu);
>
> +       /*
> +        * Enable GIC and timers.
> +        */
> +       notify_cpu_starting(cpu);
> +
>         /*
>          * OK, now it's safe to let the boot CPU continue.  Wait for
>          * the CPU migration code to notice that the CPU is online
> --
> 2.17.1
>