linux-kernel - [PATCH v3 1/5] powerpc/smp: Enable Asym packing for cores on shared processor

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Message-ID: <20231026101843.56784-2-srikar@linux.vnet.ibm.com>
Date:   Thu, 26 Oct 2023 15:48:36 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Rohan McLure <rmclure@...ux.ibm.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: [PATCH v3 1/5] powerpc/smp: Enable Asym packing for cores on shared processor

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@...ux.vnet.ibm.com>
---
Changelog:
v1->v2: Using Jump label instead of a variable.
v2 -> v3:
- Handle comments on commit message (Michael Ellerman)
- Rework using existing cpu_has_features static key (Michael Ellerman)
- Added a comment on why we do asym_packing at core (Peter Zijlstra)

 arch/powerpc/kernel/smp.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 5826f5108a12..dbf0a584804b 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -988,18 +988,22 @@ static int __init init_thread_group_cache_map(int cpu, int cache_property)
 }
 
 static bool shared_caches;
+/*
+ * On shared processor LPARs scheduled on a big core (which has two or more
+ * independent thread groups per core), prefer lower numbered CPUs, so
+ * that workload consolidates to lesser number of cores.
+ */
+static __ro_after_init DEFINE_STATIC_KEY_FALSE(splpar_asym_pack);
 
 #ifdef CONFIG_SCHED_SMT
 /* 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) ||
+			static_branch_unlikely(&splpar_asym_pack))
+		return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
 
-	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;
 }
 #endif
 
@@ -1011,9 +1015,20 @@ static int powerpc_smt_flags(void)
  */
 static int powerpc_shared_cache_flags(void)
 {
+	if (static_branch_unlikely(&splpar_asym_pack))
+		return SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
+
 	return SD_SHARE_PKG_RESOURCES;
 }
 
+static int powerpc_shared_proc_flags(void)
+{
+	if (static_branch_unlikely(&splpar_asym_pack))
+		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.
@@ -1050,8 +1065,8 @@ 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, },
 };
 
@@ -1686,7 +1701,13 @@ static void __init fixup_topology(void)
 {
 	int i;
 
+	if (is_shared_processor() && has_big_cores)
+		static_branch_enable(&splpar_asym_pack);
+
 #ifdef CONFIG_SCHED_SMT
+	if (cpu_has_feature(CPU_FTR_ASYM_SMT))
+		pr_info_once("Enabling Asymmetric SMT scheduling\n");
+
 	if (has_big_cores) {
 		pr_info("Big cores detected but using small core scheduling\n");
 		powerpc_topology[smt_idx].mask = smallcore_smt_mask;
-- 
2.31.1