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

Frederic Weisbecker
2018-11-14T02:47:01Z

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@kernel.org>
Cc: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
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