[RFC 2/2] kernel/idle: Update and use idle-hint in VPA region

[Parth Shah]

In guest, Set idle_hint to 1 when the prev_cpu of a vCPU goes into idle state,
similarly set idle_hint to 0 when exiting an idle state.

Since the idle_hint is in VPA region, the available_idle_cpu() in guest can
read this region every time to find if a vCPU can be scheduled instantly by
the hypervsior or not.

Signed-off-by: Parth Shah <pa...@linux.ibm.com>
---
 arch/powerpc/include/asm/paravirt.h | 12 ++++++++++--
 kernel/sched/idle.c                 |  3 +++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/paravirt.h 
b/arch/powerpc/include/asm/paravirt.h
index bcb7b5f917be..b20e4d25bd00 100644
--- a/arch/powerpc/include/asm/paravirt.h
+++ b/arch/powerpc/include/asm/paravirt.h
@@ -21,6 +21,12 @@ static inline bool is_shared_processor(void)
        return static_branch_unlikely(&shared_processor);
 }
 
+static inline int idle_hint_of(int cpu)
+{
+       __be32 idle_hint = READ_ONCE(lppaca_of(cpu).idle_hint);
+       return be32_to_cpu(idle_hint);
+}
+
 /* If bit 0 is set, the cpu has been preempted */
 static inline u32 yield_count_of(int cpu)
 {
@@ -109,8 +115,10 @@ static inline bool vcpu_is_preempted(int cpu)
        }
 #endif
 
-       if (yield_count_of(cpu) & 1)
-               return true;
+       if (yield_count_of(cpu) & 1) {
+               if (idle_hint_of(cpu) == 0)
+                       return true;
+       }
        return false;
 }
 
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 7ca3d3d86c2a..fdc8d1d474f0 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -7,6 +7,7 @@
  *        tasks which are handled in sched/fair.c )
  */
 #include "sched.h"
+#include <asm/idle_hint.h>
 
 #include <trace/events/power.h>
 
@@ -290,6 +291,7 @@ static void do_idle(void)
                        arch_cpu_idle_dead();
                }
 
+               set_idle_hint(cpu, 1);
                arch_cpu_idle_enter();
                rcu_nocb_flush_deferred_wakeup();
 
@@ -306,6 +308,7 @@ static void do_idle(void)
                        cpuidle_idle_call();
                }
                arch_cpu_idle_exit();
+               set_idle_hint(cpu, 0);
        }
 
        /*
-- 
2.26.3