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SubjectRe: [PATCH v3 08/10] sched/topology: Remove SHARED_CHILD from ASYM_PACKING
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@google.com>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Len Brown <len.brown@intel.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Tim C. Chen <tim.c.chen@intel.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> Suggested-by: Valentin Schneider <vschneid@redhat.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> 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
>
>

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