On Thu, Oct 19, 2023 at 03:38:40PM +1100, Michael Ellerman wrote:

Srikar Dronamraju [off-list ref] writes:

quoted

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.

You need to be clearer here that you're talking about "big cores", not
SMT4 cores as seen on bare metal systems.

What is a 'big core' ? I'm thinking big.LITTLE, but I didn't think Power
went that route (yet?).. help?

quoted

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.

.. when the system is running in shared processor mode and has big cores.

cheers

quoted

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;
+}

Can you leave the future reader a clue in the form of a comment around
here perhaps? Explaining *why* things are as they are etc..

quoted

+
 /*
  * 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