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

Srikar Dronamraju
2023-10-18T17:03:45Z

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.

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
--
2.31.1