Subject: Re: [PATCH 2/2] powerpc: Enable ASYM_SMT on interleaved big-core systems

From: Michael Neuling <[email-masked]>
Date: Mon, 14 May 2018 13:22:07 +1000

On Fri, 2018-05-11 at 16:47 +0530, Gautham R. Shenoy wrote:
> From: "Gautham R. Shenoy" <[email-masked]>
>=20
> Each of the SMT4 cores forming a fused-core are more or less
> independent units. Thus when multiple tasks are scheduled to run on
> the fused core, we get the best performance when the tasks are spread
> across the pair of SMT4 cores.
>=20
> Since the threads in the pair of SMT4 cores of an interleaved big-core
> are numbered {0,2,4,6} and {1,3,5,7} respectively, enable ASYM_SMT on
> such interleaved big-cores that will bias the load-balancing of tasks
> on smaller numbered threads, which will automatically result in
> spreading the tasks uniformly across the associated pair of SMT4
> cores.
>=20
> Signed-off-by: Gautham R. Shenoy <[email-masked]>
> ---
>  arch/powerpc/kernel/smp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>=20
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 9ca7148..0153f01 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1082,7 +1082,7 @@ static int powerpc_smt_flags(void)
>  {
>  	int flags =3D SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
> =20
> -	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
> +	if (cpu_has_feature(CPU_FTR_ASYM_SMT) || has_interleaved_big_core) {

Shouldn't we just set CPU_FTR_ASYM_SMT and leave this code unchanged?

>  		printk_once(KERN_INFO "Enabling Asymmetric SMT
> scheduling\n");
>  		flags |=3D SD_ASYM_PACKING;
>  	}