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

[Valentin Schneider]

(+LAKML, +Sudeep)

On Wed, Apr 01 2020, Cheng Jian 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.
>

I haven't been following the sched core stuff closely; can't this
rq->core assignment be done in sched_cpu_activate() instead? We already
look at the cpu_smt_mask() in there, and it is valid (we go through the
entirety of secondary_start_kernel() before getting anywhere near
CPUHP_AP_ACTIVE).

I don't think this breaks anything, but without this dependency in
sched_cpu_starting() then there isn't really a reason for this move.

> Signed-off-by: Cheng Jian <cj.chengjian@xxxxxxxxxx>
> ---
>  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