[PATCH] sched: Don't try to catch up excess steal time.

[Suleiman Souhlal <suleiman@google.com>]

When steal time exceeds the measured delta when updating clock_task, we
currently try to catch up the excess in future updates.
However, this results in inaccurate run times for the future clock_task
measurements, as they end up getting additional steal time that did not
actually happen, from the previous excess steal time being paid back.

For example, suppose a task in a VM runs for 10ms and had 15ms of steal
time reported while it ran. clock_task rightly doesn't advance. Then, a
different task runs on the same rq for 10ms without any time stolen.
Because of the current catch up mechanism, clock_sched inaccurately ends
up advancing by only 5ms instead of 10ms even though there wasn't any
actual time stolen. The second task is getting charged for less time
than it ran, even though it didn't deserve it.
In other words, tasks can end up getting more run time than they should
actually get.

So, we instead don't make future updates pay back past excess stolen time.

Signed-off-by: Suleiman Souhlal <suleiman@google.com>
---
 kernel/sched/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bcf2c4cc0522..42b37da2bda6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -728,13 +728,15 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
 #endif
 #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
 	if (static_key_false((&paravirt_steal_rq_enabled))) {
-		steal = paravirt_steal_clock(cpu_of(rq));
+		u64 prev_steal;
+
+		steal = prev_steal = paravirt_steal_clock(cpu_of(rq));
 		steal -= rq->prev_steal_time_rq;
 
 		if (unlikely(steal > delta))
 			steal = delta;
 
-		rq->prev_steal_time_rq += steal;
+		rq->prev_steal_time_rq = prev_steal;
 		delta -= steal;
 	}
 #endif
-- 
2.46.0.rc2.264.g509ed76dc8-goog