linux-kernel - [PATCH 2/2 sched_ext/for-6.12] sched_ext: Use ktime_get_ns() instead of rq_clock_task() in touch_core

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Message-ID: <ZtGkgCEkgNLzjxUC@slm.duckdns.org>
Date: Fri, 30 Aug 2024 00:52:48 -1000
From: Tejun Heo <tj@...nel.org>
To: David Vernet <void@...ifault.com>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	kernel-team@...a.com
Subject: [PATCH 2/2 sched_ext/for-6.12] sched_ext: Use ktime_get_ns() instead
 of rq_clock_task() in touch_core_sched()

Since sched_ext: Unpin and repin rq lock from balance_scx(), sched_ext's
balance path terminates rq_pin in the outermost function. This is simpler
and in line with what other balance functions are doing but it loses control
over rq->clock_update_flags which makes assert_clock_udpated() trigger if
other CPUs pins the rq lock.

The only place this matters is touch_core_sched() which uses the timestamp
to order tasks from sibling rq's. For now, switch to ktime_get_ns(). Later,
it'd be better to use per-core dispatch sequence number.

Signed-off-by: Tejun Heo <tj@...nel.org>
Fixes: 3cf78c5d01d6 ("sched_ext: Unpin and repin rq lock from balance_scx()")
Cc: Peter Zijlstra <peterz@...radead.org>
---
 kernel/sched/ext.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1453,13 +1453,20 @@ static void schedule_deferred(struct rq
  */
 static void touch_core_sched(struct rq *rq, struct task_struct *p)
 {
+	lockdep_assert_rq_held(rq);
+
 #ifdef CONFIG_SCHED_CORE
 	/*
 	 * It's okay to update the timestamp spuriously. Use
 	 * sched_core_disabled() which is cheaper than enabled().
+	 *
+	 * TODO: ktime_get_ns() is used because rq_clock_task() can't be used as
+	 * SCX balance path doesn't pin the rq. As this is used to determine
+	 * ordering between tasks of sibling CPUs, it'd be better to use
+	 * per-core dispatch sequence instead.
 	 */
 	if (!sched_core_disabled())
-		p->scx.core_sched_at = rq_clock_task(rq);
+		p->scx.core_sched_at = ktime_get_ns();
 #endif
 }
 
@@ -1476,7 +1483,6 @@ static void touch_core_sched(struct rq *
 static void touch_core_sched_dispatch(struct rq *rq, struct task_struct *p)
 {
 	lockdep_assert_rq_held(rq);
-	assert_clock_updated(rq);
 
 #ifdef CONFIG_SCHED_CORE
 	if (SCX_HAS_OP(core_sched_before))