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SubjectRe: [PATCH v2 5/7] x86/sched: Remove SD_ASYM_PACKING from the "SMT" domain
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@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
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> 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
>
>

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