[PATCH MANUALSEL 4.14] sched/cputime: Fix getrusage(RUSAGE_THREAD) with nohz_full

[Sasha Levin]

From: Frederic Weisbecker <frederic@xxxxxxxxxx>

[ Upstream commit e7f2be115f0746b969c0df14c0d182f65f005ca5 ]

getrusage(RUSAGE_THREAD) with nohz_full may return shorter utime/stime
than the actual time.

task_cputime_adjusted() snapshots utime and stime and then adjust their
sum to match the scheduler maintained cputime.sum_exec_runtime.
Unfortunately in nohz_full, sum_exec_runtime is only updated once per
second in the worst case, causing a discrepancy against utime and stime
that can be updated anytime by the reader using vtime.

To fix this situation, perform an update of cputime.sum_exec_runtime
when the cputime snapshot reports the task as actually running while
the tick is disabled. The related overhead is then contained within the
relevant situations.

Reported-by: Hasegawa Hitomi <hasegawa-hitomi@xxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Hasegawa Hitomi <hasegawa-hitomi@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
Acked-by: Phil Auld <pauld@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20211026141055.57358-3-frederic@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 include/linux/sched/cputime.h |  5 +++--
 kernel/sched/cputime.c        | 12 +++++++++---
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched/cputime.h b/include/linux/sched/cputime.h
index d0677f6739f63..787f3221b575b 100644
--- a/include/linux/sched/cputime.h
+++ b/include/linux/sched/cputime.h
@@ -18,15 +18,16 @@
 #endif /* CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
 
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
-extern void task_cputime(struct task_struct *t,
+extern bool task_cputime(struct task_struct *t,
 			 u64 *utime, u64 *stime);
 extern u64 task_gtime(struct task_struct *t);
 #else
-static inline void task_cputime(struct task_struct *t,
+static inline bool task_cputime(struct task_struct *t,
 				u64 *utime, u64 *stime)
 {
 	*utime = t->utime;
 	*stime = t->stime;
+	return false;
 }
 
 static inline u64 task_gtime(struct task_struct *t)
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 45c2cd37fe6b0..2d6c3372d8795 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -664,7 +664,8 @@ void task_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st)
 		.sum_exec_runtime = p->se.sum_exec_runtime,
 	};
 
-	task_cputime(p, &cputime.utime, &cputime.stime);
+	if (task_cputime(p, &cputime.utime, &cputime.stime))
+		cputime.sum_exec_runtime = task_sched_runtime(p);
 	cputime_adjust(&cputime, &p->prev_cputime, ut, st);
 }
 EXPORT_SYMBOL_GPL(task_cputime_adjusted);
@@ -857,19 +858,21 @@ u64 task_gtime(struct task_struct *t)
  * add up the pending nohz execution time since the last
  * cputime snapshot.
  */
-void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
+bool task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
 {
 	struct vtime *vtime = &t->vtime;
 	unsigned int seq;
 	u64 delta;
+	int ret;
 
 	if (!vtime_accounting_enabled()) {
 		*utime = t->utime;
 		*stime = t->stime;
-		return;
+		return false;
 	}
 
 	do {
+		ret = false;
 		seq = read_seqcount_begin(&vtime->seqcount);
 
 		*utime = t->utime;
@@ -879,6 +882,7 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
 		if (vtime->state == VTIME_INACTIVE || is_idle_task(t))
 			continue;
 
+		ret = true;
 		delta = vtime_delta(vtime);
 
 		/*
@@ -890,5 +894,7 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
 		else if (vtime->state == VTIME_SYS)
 			*stime += vtime->stime + delta;
 	} while (read_seqcount_retry(&vtime->seqcount, seq));
+
+	return ret;
 }
 #endif /* CONFIG_VIRT_CPU_ACCOUNTING_GEN */
-- 
2.33.0