sched/ext: Remove sched_fork() hack

[ Upstream commit 0f0d1b8e5010bfe1feeb4d78d137e41946a5370d ]

Instead of solving the underlying problem of the double invocation of
__sched_fork() for idle tasks, sched-ext decided to hack around the issue
by partially clearing out the entity struct to preserve the already
enqueued node. A provided analysis and solution has been ignored for four
months.

Now that someone else has taken care of cleaning it up, remove the
disgusting hack and clear out the full structure. Remove the comment in the
structure declaration as well, as there is no requirement for @node being
the last element anymore.

Fixes: f0e1a0643a ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/87ldy82wkc.ffs@tglx
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/linux/sched/ext.h

@@ -199,7 +199,6 @@ struct sched_ext_entity {
 #ifdef CONFIG_EXT_GROUP_SCHED
 	struct cgroup		*cgrp_moving_from;
 #endif
-	/* must be the last field, see init_scx_entity() */
 	struct list_head	tasks_node;
 };
 

kernel/sched/ext.c

@@ -3567,12 +3567,7 @@ static void scx_ops_exit_task(struct task_struct *p)
 
 void init_scx_entity(struct sched_ext_entity *scx)
 {
-	/*
-	 * init_idle() calls this function again after fork sequence is
-	 * complete. Don't touch ->tasks_node as it's already linked.
-	 */
-	memset(scx, 0, offsetof(struct sched_ext_entity, tasks_node));
-
+	memset(scx, 0, sizeof(*scx));
 	INIT_LIST_HEAD(&scx->dsq_list.node);
 	RB_CLEAR_NODE(&scx->dsq_priq);
 	scx->sticky_cpu = -1;