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 CEDED or 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.

Correct detection of preempted CPUs is important for detecting idle
CPUs/cores in task scheduler.

Tested-by: Aboorva Devarajan <aboorvad@linux.vnet.ibm.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231019091452.95260-1-srikar@linux.vnet.ibm.com

Author: srikard

arch/powerpc/include/asm/paravirt.h

@@ -71,6 +71,11 @@ static inline void yield_to_any(void)
 {
 	plpar_hcall_norets_notrace(H_CONFER, -1, 0);
 }
+
+static inline bool is_vcpu_idle(int vcpu)
+{
+	return lppaca_of(vcpu).idle;
+}
 #else
 static inline bool is_shared_processor(void)
 {
@@ -100,6 +105,10 @@ static inline void prod_cpu(int cpu)
 	___bad_prod_cpu(); /* This would be a bug */
 }
 
+static inline bool is_vcpu_idle(int vcpu)
+{
+	return false;
+}
 #endif
 
 #define vcpu_is_preempted vcpu_is_preempted
@@ -121,9 +130,23 @@ static inline bool vcpu_is_preempted(int cpu)
 	if (!is_shared_processor())
 		return false;
 
+	/*
+	 * If the hypervisor has dispatched the target CPU on a physical
+	 * processor, then the target CPU is definitely not preempted.
+	 */
+	if (!(yield_count_of(cpu) & 1))
+		return false;
+
+	/*
+	 * If the target CPU has yielded to Hypervisor but OS has not
+	 * requested idle then the target CPU is definitely preempted.
+	 */
+	if (!is_vcpu_idle(cpu))
+		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 +172,29 @@ static inline bool vcpu_is_preempted(int cpu)
 		 */
 		if (cpu_first_thread_sibling(cpu) == first_cpu)
 			return false;
+
+		/*
+		 * If any of the threads of the target CPU's core are not
+		 * preempted or ceded, then consider target 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 (!is_vcpu_idle(i))
+				return true;
+		}
 	}
 #endif
 
-	if (yield_count_of(cpu) & 1)
-		return true;
+	/*
+	 * 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;
 }