[PATCH 5.9 182/252] sched: Fix rq->nr_iowait ordering

Greg Kroah-Hartman
2020-11-23T12:57:08Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit ec618b84f6e15281cc3660664d34cd0dd2f2579e ]

schedule() ttwu()
deactivate_task(); if (p->on_rq && ...) // false
atomic_dec(&task_rq(p)->nr_iowait);
if (prev->in_iowait)
atomic_inc(&rq->nr_iowait);

Allows nr_iowait to be decremented before it gets incremented,
resulting in more dodgy IO-wait numbers than usual.

Note that because we can now do ttwu_queue_wakelist() before
p->on_cpu==0, we lose the natural ordering and have to further delay
the decrement.

Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Link: https://lkml.kernel.org/r/20201117093829.GD3121429@hirez.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/core.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b1e0da56abcac..c4da7e17b9061 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2505,7 +2505,12 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
#ifdef CONFIG_SMP
if (wake_flags & WF_MIGRATED)
en_flags |= ENQUEUE_MIGRATED;
+ else
#endif
+ if (p->in_iowait) {
+ delayacct_blkio_end(p);
+ atomic_dec(&task_rq(p)->nr_iowait);
+ }

activate_task(rq, p, en_flags);
ttwu_do_wakeup(rq, p, wake_flags, rf);
@@ -2892,11 +2897,6 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
if (READ_ONCE(p->on_rq) && ttwu_runnable(p, wake_flags))
goto unlock;

- if (p->in_iowait) {
- delayacct_blkio_end(p);
- atomic_dec(&task_rq(p)->nr_iowait);
- }
-
#ifdef CONFIG_SMP
/*
* Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be
@@ -2967,6 +2967,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)

cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
if (task_cpu(p) != cpu) {
+ if (p->in_iowait) {
+ delayacct_blkio_end(p);
+ atomic_dec(&task_rq(p)->nr_iowait);
+ }
+
wake_flags |= WF_MIGRATED;
psi_ttwu_dequeue(p);
set_task_cpu(p, cpu);
--
2.27.0