Re: [PATCH v2 2/6] powerpc/smp: Enable Asym packing for cores on shared processor

Shrikanth Hegde
2023-10-19T15:57:23Z

On 10/18/23 10:07 PM, Srikar Dronamraju wrote:
> If there are shared processor LPARs, underlying Hypervisor can have more
> virtual cores to handle than actual physical cores.
>
> Starting with Power 9, a core has 2 nearly independent thread groups.
> On a shared processors LPARs, it helps to pack threads to lesser number
> of cores so that the overall system performance and utilization
> improves. PowerVM schedules at a core level. Hence packing to fewer
> cores helps.
>
> For example: Lets says there are two 8-core Shared LPARs that are
> actually sharing a 8 Core shared physical pool, each running 8 threads
> each. Then Consolidating 8 threads to 4 cores on each LPAR would help
> them to perform better. This is because each of the LPAR will get
> 100% time to run applications and there will no switching required by
> the Hypervisor.
>
> To achieve this, enable SD_ASYM_PACKING flag at CACHE, MC and DIE level.

This would have a conflict with tip/master.
DIE has been renamed to PKG and Both changelog and code below should
change DIE to PKG.

>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
> Changelog:
> v1->v2: Using static key instead of a variable.
>
> arch/powerpc/kernel/smp.c | 17 +++++++++++++++--
> 1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 37c41297c9ce..498c2d51fc20 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1009,9 +1009,20 @@ static int powerpc_smt_flags(void)
> */
> static int powerpc_shared_cache_flags(void)
> {
> + if (static_branch_unlikely(&powerpc_asym_packing))
> + return SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
> +
> return SD_SHARE_PKG_RESOURCES;
> }
>
> +static int powerpc_shared_proc_flags(void)
> +{
> + if (static_branch_unlikely(&powerpc_asym_packing))
> + return SD_ASYM_PACKING;
> +
> + return 0;
> +}
> +
> /*
> * We can't just pass cpu_l2_cache_mask() directly because
> * returns a non-const pointer and the compiler barfs on that.
> @@ -1048,8 +1059,8 @@ static struct sched_domain_topology_level powerpc_topology[] = {
> { cpu_smt_mask, powerpc_smt_flags, SD_INIT_NAME(SMT) },
> #endif
> { shared_cache_mask, powerpc_shared_cache_flags, SD_INIT_NAME(CACHE) },
> - { cpu_mc_mask, SD_INIT_NAME(MC) },
> - { cpu_cpu_mask, SD_INIT_NAME(DIE) },
> + { cpu_mc_mask, powerpc_shared_proc_flags, SD_INIT_NAME(MC) },
> + { cpu_cpu_mask, powerpc_shared_proc_flags, SD_INIT_NAME(DIE) },
> { NULL, },
> };
>
> @@ -1687,6 +1698,8 @@ static void __init fixup_topology(void)
> if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
> pr_info_once("Enabling Asymmetric SMT scheduling\n");
> static_branch_enable(&powerpc_asym_packing);
> + } else if (is_shared_processor() && has_big_cores) {
> + static_branch_enable(&powerpc_asym_packing);
> }
>
> #ifdef CONFIG_SCHED_SMT