linux-kernel - [PATCH 09/17] powerpc/smp: Implement arch_scale_cpu

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Message-ID: <20251204175405.1511340-10-srikar@linux.ibm.com>
Date: Thu,  4 Dec 2025 23:23:57 +0530
From: Srikar Dronamraju <srikar@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Peter Zijlstra <peterz@...radead.org>
Cc: Ben Segall <bsegall@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Mel Gorman <mgorman@...e.de>, Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Shrikanth Hegde <sshegde@...ux.ibm.com>,
        Srikar Dronamraju <srikar@...ux.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Swapnil Sapkal <swapnil.sapkal@....com>,
        Thomas Huth <thuth@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        virtualization@...ts.linux.dev, Yicong Yang <yangyicong@...ilicon.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: [PATCH 09/17] powerpc/smp: Implement arch_scale_cpu_capacity for shared LPARs

If a CPU is soft-offlined, then its marked as in-active CPU.
On a shared LPAR, reduce that CPU's capacity. This will force the Linux
kernel scheduler's load balancer to prefer CPUs with higher capacity.

Setting the capacity to be low also helps the scheduler in load
balancing. If there are 2 equal groups of CPUS but if one group has more
inactive CPUs compared to the other, then setting to low capacity will
make the scheduler schedule more tasks on the group with more active
CPUs.

However tasks pinned on that particular CPU (or set of CPUs that are all
marked as inactive) will continue to run on the same CPU.

Signed-off-by: Srikar Dronamraju <srikar@...ux.ibm.com>
---
 arch/powerpc/include/asm/topology.h |  5 +++++
 arch/powerpc/kernel/smp.c           | 17 +++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index f19ca44512d1..031c067fc820 100644
--- a/arch/powerpc/include/asm/topology.h
+++ b/arch/powerpc/include/asm/topology.h
@@ -174,5 +174,10 @@ static inline bool topology_is_core_online(unsigned int cpu)
 }
 #endif
 
+#ifdef CONFIG_PPC_SPLPAR
+#define arch_scale_cpu_capacity arch_scale_cpu_capacity
+unsigned long arch_scale_cpu_capacity(int cpu);
+#endif
+
 #endif /* __KERNEL__ */
 #endif	/* _ASM_POWERPC_TOPOLOGY_H */
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 016dc7dc5bbc..c269b38dcba5 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1811,3 +1811,20 @@ void __noreturn arch_cpu_idle_dead(void)
 }
 
 #endif
+
+#ifdef CONFIG_PPC_SPLPAR
+#define MIN_CAPACITY 1
+
+/*
+ * Assume CPU capacity to be low if CPU number happens be above soft
+ * available limit. This forces load balancer to prefer higher capacity CPUs
+ */
+unsigned long arch_scale_cpu_capacity(int cpu)
+{
+	if (is_shared_processor() && !is_kvm_guest()) {
+		if (!cpu_active(cpu))
+			return MIN_CAPACITY;
+	}
+	return SCHED_CAPACITY_SCALE;
+}
+#endif /* CONFIG_PPC_SPLPAR */
-- 
2.43.7