linux-kernel - [PATCH] powerpc/paravirt: Improve vcpu_is

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Date:   Mon,  9 Oct 2023 10:47:40 +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>,
        Ajay Kaher <akaher@...are.com>,
        Alexey Makhalov <amakhalov@...are.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        Nicholas Piggin <npiggin@...il.com>,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH] powerpc/paravirt: Improve vcpu_is_preempted

PowerVM Hypervisor dispatches on a whole core basis. In a shared LPAR, a
CPU from a core that is preempted may have a larger latency. In
such a scenario, its preferable to choose a different CPU to run.

If one of the CPUs in the core is active, i.e neither CEDED nor
preempted, then consider this CPU as not preempted.

Also if any of the CPUs in the core has yielded but OS has not requested
CEDE or CONFER, then consider this CPU to be preempted.

Cc: Ajay Kaher <akaher@...are.com>
Cc: Alexey Makhalov <amakhalov@...are.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Juergen Gross <jgross@...e.com>
Cc: linux-kernel@...r.kernel.org
Cc: linuxppc-dev@...ts.ozlabs.org
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: virtualization@...ts.linux-foundation.org
Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
---
 arch/powerpc/include/asm/paravirt.h | 33 ++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h
index e08513d73119..a980756f58df 100644
--- a/arch/powerpc/include/asm/paravirt.h
+++ b/arch/powerpc/include/asm/paravirt.h
@@ -121,9 +121,19 @@ static inline bool vcpu_is_preempted(int cpu)
 	if (!is_shared_processor())
 		return false;
 
+	if (!(yield_count_of(cpu) & 1))
+		return false;
+
+	/*
+	 * If CPU has yielded but OS has not requested idle then this CPU is
+	 * definitely preempted.
+	 */
+	if (!lppaca_of(cpu).idle)
+		return true;
+
 #ifdef CONFIG_PPC_SPLPAR
 	if (!is_kvm_guest()) {
-		int first_cpu;
+		int first_cpu, i;
 
 		/*
 		 * The result of vcpu_is_preempted() is used in a
@@ -149,11 +159,28 @@ static inline bool vcpu_is_preempted(int cpu)
 		 */
 		if (cpu_first_thread_sibling(cpu) == first_cpu)
 			return false;
+
+		/*
+		 * If any of the threads of this core is not preempted or
+		 * ceded, then consider this CPU to be non-preempted
+		 */
+		first_cpu = cpu_first_thread_sibling(cpu);
+		for (i = first_cpu; i < first_cpu + threads_per_core; i++) {
+			if (i == cpu)
+				continue;
+			if (!(yield_count_of(i) & 1))
+				return false;
+			if (!lppaca_of(i).idle)
+				return true;
+		}
 	}
 #endif
 
-	if (yield_count_of(cpu) & 1)
-		return true;
+	/*
+	 * None of the threads in this thread group are running but none of
+	 * them were preempted too. Hence assume the thread to be
+	 * non-preempted.
+	 */
 	return false;
 }
 
-- 
2.31.1