linux-kernel - [PATCH v2 6/6] powerpc/smp: Avoid asym packing within thread

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Message-ID: <20231018163751.2423181-7-srikar@linux.vnet.ibm.com>
Date:   Wed, 18 Oct 2023 22:07:46 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rohan McLure <rmclure@...ux.ibm.com>,
        Valentin Schneider <vschneid@...hat.com>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 6/6] powerpc/smp: Avoid asym packing within thread_group of a core

PowerVM Hypervisor will schedule at a core granularity. However each
core can have more than one thread_groups. For better utilization in
case of a shared processor, its preferable for the scheduler to pack to
the lowest core. However there is no benefit of moving a thread between
two thread groups of the same core.

Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
---
 arch/powerpc/kernel/smp.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index b1eb11a66902..a710fb32a2a9 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1695,6 +1695,8 @@ void start_secondary(void *unused)
 	BUG();
 }
 
+DEFINE_STATIC_KEY_FALSE(splpar_asym_pack);
+
 static void __init fixup_topology(void)
 {
 	int i;
@@ -1704,6 +1706,7 @@ static void __init fixup_topology(void)
 		static_branch_enable(&powerpc_asym_packing);
 	} else if (is_shared_processor() && has_big_cores) {
 		static_branch_enable(&powerpc_asym_packing);
+		static_branch_enable(&splpar_asym_pack);
 	}
 
 #ifdef CONFIG_SCHED_SMT
@@ -1758,6 +1761,19 @@ void __init smp_cpus_done(unsigned int max_cpus)
 	set_sched_topology(powerpc_topology);
 }
 
+/*
+ * For asym packing, by default lower numbered CPU has higher priority.
+ * On shared processors, pack to lower numbered core. However avoid moving
+ * between thread_groups within the same core.
+ */
+int arch_asym_cpu_priority(int cpu)
+{
+	if (static_branch_unlikely(&splpar_asym_pack))
+		return -cpu / threads_per_core;
+
+	return -cpu;
+}
+
 #ifdef CONFIG_HOTPLUG_CPU
 int __cpu_disable(void)
 {
-- 
2.31.1