[PATCH 6.4 103/800] sched/core: Avoid multiple calling update_rq_clock() in __cfsb_csd_unthrottle()

Greg Kroah-Hartman
2023-07-16T19:57:40Z

From: Hao Jia <jiahao.os@bytedance.com>

[ Upstream commit ebb83d84e49b54369b0db67136a5fe1087124dcc ]

After commit 8ad075c2eb1f ("sched: Async unthrottling for cfs
bandwidth"), we may update the rq clock multiple times in the loop of
__cfsb_csd_unthrottle().

A prior (although less common) instance of this problem exists in
unthrottle_offline_cfs_rqs().

Cure both by ensuring update_rq_clock() is called before the loop and
setting RQCF_ACT_SKIP during the loop, to supress further updates.
The alternative would be pulling update_rq_clock() out of
unthrottle_cfs_rq(), but that gives an even bigger mess.

Fixes: 8ad075c2eb1f ("sched: Async unthrottling for cfs bandwidth")
Reviewed-By: Ben Segall <bsegall@google.com>
Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20230613082012.49615-4-jiahao.os@bytedance.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/fair.c | 18 ++++++++++++++++++
kernel/sched/sched.h | 22 ++++++++++++++++++++++
2 files changed, 40 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 373ff5f558844..4da5f35417626 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5576,6 +5576,14 @@ static void __cfsb_csd_unthrottle(void *arg)

rq_lock(rq, &rf);

+ /*
+ * Iterating over the list can trigger several call to
+ * update_rq_clock() in unthrottle_cfs_rq().
+ * Do it once and skip the potential next ones.
+ */
+ update_rq_clock(rq);
+ rq_clock_start_loop_update(rq);
+
/*
* Since we hold rq lock we're safe from concurrent manipulation of
* the CSD list. However, this RCU critical section annotates the
@@ -5595,6 +5603,7 @@ static void __cfsb_csd_unthrottle(void *arg)

rcu_read_unlock();

+ rq_clock_stop_loop_update(rq);
rq_unlock(rq, &rf);
}

@@ -6115,6 +6124,13 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)

lockdep_assert_rq_held(rq);

+ /*
+ * The rq clock has already been updated in the
+ * set_rq_offline(), so we should skip updating
+ * the rq clock again in unthrottle_cfs_rq().
+ */
+ rq_clock_start_loop_update(rq);
+
rcu_read_lock();
list_for_each_entry_rcu(tg, &task_groups, list) {
struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
@@ -6137,6 +6153,8 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
unthrottle_cfs_rq(cfs_rq);
}
rcu_read_unlock();
+
+ rq_clock_stop_loop_update(rq);
}

#else /* CONFIG_CFS_BANDWIDTH */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ec7b3e0a2b207..81ac605b9cd5c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1546,6 +1546,28 @@ static inline void rq_clock_cancel_skipupdate(struct rq *rq)
rq->clock_update_flags &= ~RQCF_REQ_SKIP;
}

+/*
+ * During cpu offlining and rq wide unthrottling, we can trigger
+ * an update_rq_clock() for several cfs and rt runqueues (Typically
+ * when using list_for_each_entry_*)
+ * rq_clock_start_loop_update() can be called after updating the clock
+ * once and before iterating over the list to prevent multiple update.
+ * After the iterative traversal, we need to call rq_clock_stop_loop_update()
+ * to clear RQCF_ACT_SKIP of rq->clock_update_flags.
+ */
+static inline void rq_clock_start_loop_update(struct rq *rq)
+{
+ lockdep_assert_rq_held(rq);
+ SCHED_WARN_ON(rq->clock_update_flags & RQCF_ACT_SKIP);
+ rq->clock_update_flags |= RQCF_ACT_SKIP;
+}
+
+static inline void rq_clock_stop_loop_update(struct rq *rq)
+{
+ lockdep_assert_rq_held(rq);
+ rq->clock_update_flags &= ~RQCF_ACT_SKIP;
+}
+
struct rq_flags {
unsigned long flags;
struct pin_cookie cookie;
--
2.39.2