Re: [PATCH v2 5/7] x86/sched: Remove SD_ASYM_PACKING from the "SMT" domain

[Ionela Voinescu]

Hi Ricardo,

On Tuesday 22 Nov 2022 at 12:35:30 (-0800), Ricardo Neri wrote:
> There is no difference between any of the SMT siblings of a physical core.
> asym_packing load balancing is not needed among siblings.
> 
> When balancing load among physical cores, the scheduler now considers the
> state of the siblings when checking the priority of a CPU.
> 
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Len Brown <len.brown@xxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Tim C. Chen <tim.c.chen@xxxxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since v1:
>  * Introduced this patch.
> ---
>  arch/x86/kernel/smpboot.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 3f3ea0287f69..c3de98224cb4 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -545,7 +545,7 @@ static int x86_core_flags(void)
>  #ifdef CONFIG_SCHED_SMT
>  static int x86_smt_flags(void)
>  {
> -	return cpu_smt_flags() | x86_sched_itmt_flags();
> +	return cpu_smt_flags();

Based on:

kernel/sched/topology.c:
sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
rcu_assign_pointer(per_cpu(sd_asym_packing, cpu), sd);

and described at:

include/linux/sched/sd_flags.h:
/*
 * Place busy tasks earlier in the domain
 *
 * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
 *               up, but currently assumed to be set from the base domain
 *               upwards (see update_top_cache_domain()).
 * NEEDS_GROUPS: Load balancing flag.
 */
SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)

doesn't your change result in sd_asym_packing being NULL?

The SD_ASYM_PACKING flag requires all children of a domain to have it set
as well. So having SMT not setting the flag, while CLUSTER and MC having
set the flag would result in a broken topology, right?

Thanks,
Ionela.

>  }
>  #endif
>  #ifdef CONFIG_SCHED_CLUSTER
> -- 
> 2.25.1
> 
>