Re: [PATCH v3 08/10] sched/topology: Remove SHARED_CHILD from ASYM_PACKING

[Ionela Voinescu]

Hi Ricardo,

On Monday 06 Feb 2023 at 20:58:36 (-0800), Ricardo Neri wrote:
> Only x86 and Power7 use ASYM_PACKING. They use it differently.
> 
> Power7 has cores of equal priority, but the SMT siblings of a core have
> different priorities. Parent scheduling domains do not need (nor have) the
> ASYM_PACKING flag. SHARED_CHILD is not needed. Using SHARED_PARENT would
> cause the topology debug code to complain.
> 
> X86 has cores of different priority, but all the SMT siblings of the core
> have equal priority. It needs ASYM_PACKING at the MC level, but not at the
> SMT level (it also needs it at upper levels if they have scheduling groups
> of different priority). Removing ASYM_PACKING from the SMT domain causes
> the topology debug code to complain.
> 
> Remove SHARED_CHILD for now. We still need a topology check that satisfies
> both architectures.
> 
> 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
> Suggested-by: Valentin Schneider <vschneid@xxxxxxxxxx>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since v2:
>  * Introduced this patch.
> 
> Changes since v1:
>  * N/A
> ---
>  include/linux/sched/sd_flags.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> index 57bde66d95f7..800238854ba5 100644
> --- a/include/linux/sched/sd_flags.h
> +++ b/include/linux/sched/sd_flags.h
> @@ -132,12 +132,9 @@ SD_FLAG(SD_SERIALIZE, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
>  /*
>   * 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)
> +SD_FLAG(SD_ASYM_PACKING,  SDF_NEEDS_GROUPS)

While this silences the warning one would have gotten when removing
SD_ASYM_PACKING from SMT level, it will still result in sd_asym_packing
being NULL for these systems, which breaks nohz balance. That is because
highest_flag_domain() still stops searching at the first level without
the flag set, in this case SMT, even if levels above have the flag set.

Maybe highest_flag_domain() should be changed to take into account the
metadata flags?

Thanks,
Ionela.

>  
>  /*
>   * Prefer to place tasks in a sibling domain
> -- 
> 2.25.1
> 
>