[PATCH 18/25] vtime: Track nice-ness on top of context switch

[Frederic Weisbecker]

We need to read the nice value of the task running on any CPU, possibly
remotely, in order to correctly support kcpustat on nohz_full.
Unfortunately we can't just read task_nice(tsk) when tsk runs on another
CPU because its nice value may be concurrently changed. There could be a
risk that a recently modified nice value is thought to apply for a longer
while than is supposed to.

For example if a task runs at T0 with nice = -10, then its nice value
is changed at T0 + 1 second with nice = 10, a reader at T0 + 1 second
could think that the task had this "nice == 10" value since the beginning
(T0) and spuriously account 1 second nice time on kcpustat instead of 1
second user time.

So we need to track the nice value changes under vtime seqcount. Start
with context switches and account the vtime nice-ness on top of it.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Yauheni Kaliuta <yauheni.kaliuta@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/linux/sched.h  |  1 +
 kernel/sched/cputime.c | 44 +++++++++++++++++++++++++++++++++++---------
 2 files changed, 36 insertions(+), 9 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 27e0544..356326f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -280,6 +280,7 @@ enum vtime_state {
 struct vtime {
 	seqcount_t		seqcount;
 	unsigned long long	starttime;
+	int			nice;
 	enum vtime_state	state;
 	unsigned int		cpu;
 	u64			utime;
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 8f5dee2..07c2e7f 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -735,13 +735,42 @@ static void vtime_account_system(struct task_struct *tsk,
 static void vtime_account_guest(struct task_struct *tsk,
 				struct vtime *vtime)
 {
+	enum cpu_usage_stat index;
+
 	vtime->gtime += get_vtime_delta(vtime);
-	if (vtime->gtime >= TICK_NSEC) {
-		account_guest_time(tsk, vtime->gtime);
-		vtime->gtime = 0;
-	}
+
+	if (vtime->gtime < TICK_NSEC)
+		return;
+
+	if (vtime->nice)
+		index = CPUTIME_GUEST_NICE;
+	else
+		index = CPUTIME_GUEST;
+
+	account_guest_time_index(tsk, vtime->gtime, index);
+	vtime->gtime = 0;
 }
 
+static void vtime_account_user(struct task_struct *tsk,
+			       struct vtime *vtime)
+{
+	enum cpu_usage_stat index;
+
+	vtime->utime += get_vtime_delta(vtime);
+
+	if (vtime->utime < TICK_NSEC)
+		return;
+
+	if (vtime->nice)
+		index = CPUTIME_NICE;
+	else
+		index = CPUTIME_USER;
+
+	account_user_time_index(tsk, vtime->utime, index);
+	vtime->utime = 0;
+}
+
+
 static void __vtime_account_kernel(struct task_struct *tsk,
 				   struct vtime *vtime)
 {
@@ -779,11 +808,7 @@ void vtime_user_exit(struct task_struct *tsk)
 	struct vtime *vtime = &tsk->vtime;
 
 	write_seqcount_begin(&vtime->seqcount);
-	vtime->utime += get_vtime_delta(vtime);
-	if (vtime->utime >= TICK_NSEC) {
-		account_user_time(tsk, vtime->utime);
-		vtime->utime = 0;
-	}
+	vtime_account_user(tsk, vtime);
 	vtime->state = VTIME_SYS;
 	write_seqcount_end(&vtime->seqcount);
 }
@@ -864,6 +889,7 @@ void vtime_task_switch_generic(struct task_struct *prev)
 		vtime->state = VTIME_SYS;
 	vtime->starttime = sched_clock();
 	vtime->cpu = smp_processor_id();
+	vtime->nice = (task_nice(current) > 0) ? 1 : 0;
 	write_seqcount_end(&vtime->seqcount);
 
 	rcu_assign_pointer(kcpustat->curr, current);
-- 
2.7.4