[PATCH] sched: add CONFIG_TASK_MIGRATION_NOTIFIER

[Paolo Bonzini]

The task migration notifier is only used in x86 paravirt.  Make it
possible to compile it out.

While at it, move some code around to ensure tmn is filled from CPU
registers.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/Kconfig    | 1 +
 init/Kconfig        | 3 +++
 kernel/sched/core.c | 9 ++++++++-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d43e7e1c784b..9af252c8698d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -649,6 +649,7 @@ if HYPERVISOR_GUEST
 
 config PARAVIRT
 	bool "Enable paravirtualization code"
+	select TASK_MIGRATION_NOTIFIER
 	---help---
 	  This changes the kernel so it can modify itself when it is run
 	  under a hypervisor, potentially improving performance significantly
diff --git a/init/Kconfig b/init/Kconfig
index 3b9df1aa35db..891917123338 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2016,6 +2016,9 @@ source "block/Kconfig"
 config PREEMPT_NOTIFIERS
 	bool
 
+config TASK_MIGRATION_NOTIFIER
+	bool
+
 config PADATA
 	depends on SMP
 	bool
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f9123a82cbb6..c07a53aa543c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1016,12 +1016,14 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
 		rq_clock_skip_update(rq, true);
 }
 
+#ifdef CONFIG_TASK_MIGRATION_NOTIFIER
 static ATOMIC_NOTIFIER_HEAD(task_migration_notifier);
 
 void register_task_migration_notifier(struct notifier_block *n)
 {
 	atomic_notifier_chain_register(&task_migration_notifier, n);
 }
+#endif
 
 #ifdef CONFIG_SMP
 void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
@@ -1053,18 +1055,23 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 	trace_sched_migrate_task(p, new_cpu);
 
 	if (task_cpu(p) != new_cpu) {
+#ifdef CONFIG_TASK_MIGRATION_NOTIFIER
 		struct task_migration_notifier tmn;
+		int from_cpu = task_cpu(p);
+#endif
 
 		if (p->sched_class->migrate_task_rq)
 			p->sched_class->migrate_task_rq(p, new_cpu);
 		p->se.nr_migrations++;
 		perf_sw_event_sched(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 0);
 
+#ifdef CONFIG_TASK_MIGRATION_NOTIFIER
 		tmn.task = p;
-		tmn.from_cpu = task_cpu(p);
+		tmn.from_cpu = from_cpu;
 		tmn.to_cpu = new_cpu;
 
 		atomic_notifier_call_chain(&task_migration_notifier, 0, &tmn);
+#endif
 	}
 
 	__set_task_cpu(p, new_cpu);
-- 
2.3.5

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