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: NMarcelo Tosatti <mtosatti@redhat.com>
Acked-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

arch/x86/include/asm/pvclock.h

@@ -93,7 +93,6 @@ unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
 
 struct pvclock_vsyscall_time_info {
 	struct pvclock_vcpu_time_info pvti;
-	u32 migrate_count;
 } __attribute__((__aligned__(SMP_CACHE_BYTES)));
 
 #define PVTI_SIZE sizeof(struct pvclock_vsyscall_time_info)

arch/x86/kernel/pvclock.c

@@ -128,46 +128,7 @@ void pvclock_read_wallclock(struct pvclock_wall_clock *wall_clock,
 	set_normalized_timespec(ts, now.tv_sec, now.tv_nsec);
 }
 
-static struct pvclock_vsyscall_time_info *pvclock_vdso_info;
-
-static struct pvclock_vsyscall_time_info *
-pvclock_get_vsyscall_user_time_info(int cpu)
-{
-	if (!pvclock_vdso_info) {
-		BUG();
-		return NULL;
-	}
-
-	return &pvclock_vdso_info[cpu];
-}
-
-struct pvclock_vcpu_time_info *pvclock_get_vsyscall_time_info(int cpu)
-{
-	return &pvclock_get_vsyscall_user_time_info(cpu)->pvti;
-}
-
 #ifdef CONFIG_X86_64
-static int pvclock_task_migrate(struct notifier_block *nb, unsigned long l,
-			        void *v)
-{
-	struct task_migration_notifier *mn = v;
-	struct pvclock_vsyscall_time_info *pvti;
-
-	pvti = pvclock_get_vsyscall_user_time_info(mn->from_cpu);
-
-	/* this is NULL when pvclock vsyscall is not initialized */
-	if (unlikely(pvti == NULL))
-		return NOTIFY_DONE;
-
-	pvti->migrate_count++;
-
-	return NOTIFY_DONE;
-}
-
-static struct notifier_block pvclock_migrate = {
-	.notifier_call = pvclock_task_migrate,
-};
-
 /*
  * Initialize the generic pvclock vsyscall state.  This will allocate
  * a/some page(s) for the per-vcpu pvclock information, set up a
@@ -181,17 +142,12 @@ int __init pvclock_init_vsyscall(struct pvclock_vsyscall_time_info *i,
 
 	WARN_ON (size != PVCLOCK_VSYSCALL_NR_PAGES*PAGE_SIZE);
 
-	pvclock_vdso_info = i;
-
 	for (idx = 0; idx <= (PVCLOCK_FIXMAP_END-PVCLOCK_FIXMAP_BEGIN); idx++) {
 		__set_fixmap(PVCLOCK_FIXMAP_BEGIN + idx,
 			     __pa(i) + (idx*PAGE_SIZE),
 			     PAGE_KERNEL_VVAR);
 	}
 
-
-	register_task_migration_notifier(&pvclock_migrate);
-
 	return 0;
 }
 #endif

arch/x86/vdso/vclock_gettime.c

@@ -85,15 +85,18 @@ static notrace cycle_t vread_pvclock(int *mode)
 	cycle_t ret;
 	u64 last;
 	u32 version;
-	u32 migrate_count;
 	u8 flags;
 	unsigned cpu, cpu1;
 
 
 	/*
-	 * When looping to get a consistent (time-info, tsc) pair, we
-	 * also need to deal with the possibility we can switch vcpus,
-	 * so make sure we always re-fetch time-info for the current vcpu.
+	 * Note: hypervisor must guarantee that:
+	 * 1. cpu ID number maps 1:1 to per-CPU pvclock time info.
+	 * 2. that per-CPU pvclock time info is updated if the
+	 *    underlying CPU changes.
+	 * 3. that version is increased whenever underlying CPU
+	 *    changes.
+	 *
 	 */
 	do {
 		cpu = __getcpu() & VGETCPU_CPU_MASK;
@@ -104,8 +107,6 @@ static notrace cycle_t vread_pvclock(int *mode)
 
 		pvti = get_pvti(cpu);
 
-		migrate_count = pvti->migrate_count;
-
 		version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
 
 		/*
@@ -117,8 +118,7 @@ static notrace cycle_t vread_pvclock(int *mode)
 		cpu1 = __getcpu() & VGETCPU_CPU_MASK;
 	} while (unlikely(cpu != cpu1 ||
 			  (pvti->pvti.version & 1) ||
-			  pvti->pvti.version != version ||
-			  pvti->migrate_count != migrate_count));
+			  pvti->pvti.version != version));
 
 	if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
 		*mode = VCLOCK_NONE;

include/linux/sched.h

@@ -107,14 +107,6 @@ extern unsigned long this_cpu_load(void);
 extern void calc_global_load(unsigned long ticks);
 extern void update_cpu_load_nohz(void);
 
-/* Notifier for when a task gets migrated to a new CPU */
-struct task_migration_notifier {
-	struct task_struct *task;
-	int from_cpu;
-	int to_cpu;
-};
-extern void register_task_migration_notifier(struct notifier_block *n);
-
 extern unsigned long get_parent_ip(unsigned long addr);
 
 extern void dump_cpu_task(int cpu);

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);