Re: [PATCH v4 1/5] powerpc/smp: Enable Asym packing for cores on shared processor

Aneesh Kumar K.V
2023-11-15T06:35:55Z

Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:

> If there are shared processor LPARs, underlying Hypervisor can have more
> virtual cores to handle than actual physical cores.
>
> Starting with Power 9, a big core (aka SMT8 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 big 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
> when the system is running in shared processor mode and has big cores.
>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
> Changelog:
> v3 -> v4:
> - Dont use splpar_asym_pack with SMT
> - Conflict resolution due to rebase
> (DIE changed to PKG)
> v2 -> v3:
> - Handle comments from Michael Ellerman.
> - Rework using existing cpu_has_features static key
> v1->v2: Using Jump label instead of a variable.
>
> arch/powerpc/kernel/smp.c | 37 +++++++++++++++++++++++++++++--------
> 1 file changed, 29 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index ab691c89d787..69a3262024f1 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -993,16 +993,20 @@ static bool shared_caches;
> /* cpumask of CPUs with asymmetric SMT dependency */
> static int powerpc_smt_flags(void)
> {
> - int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
> + if (!cpu_has_feature(CPU_FTR_ASYM_SMT))
> + return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
>
> - if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
> - printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
> - flags |= SD_ASYM_PACKING;
> - }
> - return flags;
> + return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
> }
> #endif
>

Only relevant change there is dropping printk_once(). Rest of the
changes are not needed?

-aneesh