linux-kernel - [PATCH sched_ext/for-6.16] sched_ext: Explain the temporary situation around scx

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Message-ID: <aCSyYU6EZzm4fJUa@mtj.duckdns.org>
Date: Wed, 14 May 2025 11:10:25 -0400
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH sched_ext/for-6.16] sched_ext: Explain the temporary
 situation around scx_root dereferences

Naked scx_root dereferences are being used as temporary markers to indicate
that they need to be updated to point to the right scheduler instance.
Explain the situation.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Andrea Righi <arighi@...dia.com>
---
 kernel/sched/ext.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1016,6 +1016,14 @@ enum scx_ops_state {
 #define SCX_OPSS_STATE_MASK	((1LU << SCX_OPSS_QSEQ_SHIFT) - 1)
 #define SCX_OPSS_QSEQ_MASK	(~SCX_OPSS_STATE_MASK)
 
+/*
+ * NOTE: sched_ext is in the process of growing multiple scheduler support and
+ * scx_root usage is in a transitional state. Naked dereferences are safe if the
+ * caller is one of the tasks attached to SCX and explicit RCU dereference is
+ * necessary otherwise. Naked scx_root dereferences trigger sparse warnings but
+ * are used as temporary markers to indicate that the dereferences need to be
+ * updated to point to the associated scheduler instances rather than scx_root.
+ */
 static struct scx_sched __rcu *scx_root;
 
 /*