remove sched notifier for cross-cpu migrations

Linux as a guest on KVM hypervisor, the only user of the pvclock
vsyscall interface, does not require notification on task migration
because:

1. cpu ID number maps 1:1 to per-CPU pvclock time info.
2. per-CPU pvclock time info is updated if the
   underlying CPU changes.
3. that version is increased whenever underlying CPU
   changes.

Which is sufficient to guarantee nanoseconds counter
is calculated properly.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

1 files changed, 0 insertions, 15 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d8eb4525e76..0efd2eefb027 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -976,13 +976,6 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
                 rq->skip_clock_update = 1;
 }
 
-static ATOMIC_NOTIFIER_HEAD(task_migration_notifier);
-
-void register_task_migration_notifier(struct notifier_block *n)
-{
-        atomic_notifier_chain_register(&task_migration_notifier, n);
-}
-
 #ifdef CONFIG_SMP
 void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 {
@@ -1013,18 +1006,10 @@ 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) {
-                struct task_migration_notifier tmn;
-
                 if (p->sched_class->migrate_task_rq)
                         p->sched_class->migrate_task_rq(p, new_cpu);
                 p->se.nr_migrations++;
                 perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, NULL, 0);
-
-                tmn.task = p;
-                tmn.from_cpu = task_cpu(p);
-                tmn.to_cpu = new_cpu;
-
-                atomic_notifier_call_chain(&task_migration_notifier, 0, &tmn);
         }
 
         __set_task_cpu(p, new_cpu);