linux-kernel - [PATCH 14/17] powerpc/smp: Assume preempt if CPU is inactive.

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Message-ID: <20251204175405.1511340-15-srikar@linux.ibm.com>
Date: Thu,  4 Dec 2025 23:24:02 +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 14/17] powerpc/smp: Assume preempt if CPU is inactive.

When a vCPU is marked inactive, it qualifies as preempted vCPU.
And when a vCPU is marked active, we should hope that its not going to
be preempted. Also with lower steal times, the chances of active vCPU
being preempted reduces too.

Signed-off-by: Srikar Dronamraju <srikar@...ux.ibm.com>
---
 arch/powerpc/include/asm/paravirt.h | 62 +++++------------------------
 1 file changed, 9 insertions(+), 53 deletions(-)

diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
index b78b82d66057..93c4e4f57cb3 100644
--- a/arch/powerpc/include/asm/paravirt.h
+++ b/arch/powerpc/include/asm/paravirt.h
@@ -145,6 +145,15 @@ static inline bool vcpu_is_preempted(int cpu)
 	if (!is_shared_processor())
 		return false;
 
+#ifdef CONFIG_PPC_SPLPAR
+	/*
+	 * Assume the target CPU to be preempted if it is above soft
+	 * entitlement limit
+	 */
+	if (!is_kvm_guest())
+		return !cpu_active(cpu);
+#endif
+
 	/*
 	 * If the hypervisor has dispatched the target CPU on a physical
 	 * processor, then the target CPU is definitely not preempted.
@@ -159,59 +168,6 @@ static inline bool vcpu_is_preempted(int cpu)
 	if (!is_vcpu_idle(cpu))
 		return true;
 
-#ifdef CONFIG_PPC_SPLPAR
-	if (!is_kvm_guest()) {
-		int first_cpu, i;
-
-		/*
-		 * The result of vcpu_is_preempted() is used in a
-		 * speculative way, and is always subject to invalidation
-		 * by events internal and external to Linux. While we can
-		 * be called in preemptable context (in the Linux sense),
-		 * we're not accessing per-cpu resources in a way that can
-		 * race destructively with Linux scheduler preemption and
-		 * migration, and callers can tolerate the potential for
-		 * error introduced by sampling the CPU index without
-		 * pinning the task to it. So it is permissible to use
-		 * raw_smp_processor_id() here to defeat the preempt debug
-		 * warnings that can arise from using smp_processor_id()
-		 * in arbitrary contexts.
-		 */
-		first_cpu = cpu_first_thread_sibling(raw_smp_processor_id());
-
-		/*
-		 * The PowerVM hypervisor dispatches VMs on a whole core
-		 * basis. So we know that a thread sibling of the executing CPU
-		 * cannot have been preempted by the hypervisor, even if it
-		 * has called H_CONFER, which will set the yield bit.
-		 */
-		if (cpu_first_thread_sibling(cpu) == first_cpu)
-			return false;
-
-		/*
-		 * The specific target CPU was marked by guest OS as idle, but
-		 * then also check all other cpus in the core for PowerVM
-		 * because it does core scheduling and one of the vcpu
-		 * of the core getting preempted by hypervisor implies
-		 * other vcpus can also be considered preempted.
-		 */
-		first_cpu = cpu_first_thread_sibling(cpu);
-		for (i = first_cpu; i < first_cpu + threads_per_core; i++) {
-			if (i == cpu)
-				continue;
-			if (vcpu_is_dispatched(i))
-				return false;
-			if (!is_vcpu_idle(i))
-				return true;
-		}
-	}
-#endif
-
-	/*
-	 * None of the threads in target CPU's core are running but none of
-	 * them were preempted too. Hence assume the target CPU to be
-	 * non-preempted.
-	 */
 	return false;
 }
 
-- 
2.43.7