sched_ext: Hand over cgroups at sub-scheduler enable/disable

Sub-schedulers don't get cgroups yet: every task_group is inited on the root
sched and the routing added by the previous patches always resolves to it.
Add the handover: an enabling sub-scheduler takes over the cgroups in its
subtree and a disabling one returns them to its parent.

scx_cgroup_claim_subtree() runs while the sub enables, after the subtree's
cgrp->scx_sched's are set and before any task is claimed. It inits each
subtree task_group on the sub, exits it from the parent and updates
tg->scx.sched. A failed ops.cgroup_init() unwinds the sub-side inits and
aborts the enable with the parent untouched.

Disabling reverses it with scx_cgroup_return_subtree(): exit each cgroup
from the sub, then re-init it on the parent with the current tg->scx.*
values, resyncing weight and bandwidth changes made while the sub had it.
When a re-init fails, the parent is failed and the remaining task_groups
still transfer uninited and get no cgroup ops - the same punting done for
tasks. The dying parent's own disable moves them onward.

The handover walks include dying but not yet offlined task_groups, the same
as root's bulk walks: a removed cgroup keeps hosting scheduling events until
its dying tasks finish their final context switches, and its
ops.cgroup_exit() must follow the last of them. tg on/offlining is excluded
through cgroup_lock(), so either ordering against an rmdir of a subtree
cgroup delivers balanced init/exit pairs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index a6db5d300f30..78b2f289cb98 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -298,6 +298,19 @@ static inline bool scx_rcu_cpu_stall(const struct cpumask *stalled_mask) { retur
 
 struct scx_task_group {
 #ifdef CONFIG_EXT_GROUP_SCHED
+	/*
+	 * The sched this tg is on, NULL if none. SCX_TG_INITED tracks whether
+	 * ops.cgroup_init() succeeded on it. When a child sched exits and its
+	 * tgs move to the parent, a failed init leaves the tg on the parent
+	 * with INITED clear (see scx_cgroup_return_subtree()).
+	 *
+	 * This is tracked separately from cgrp->scx_sched because the tg
+	 * hierarchy can diverge from the cgroup2 hierarchy in both lifetime and
+	 * shape. A tg stays online past its cgroup's removal while the
+	 * cgrp->scx_sched rewrites visit only live cgroups, leaving a removed
+	 * cgroup's pointer stale. The cpu controller can also be mounted on
+	 * cgroup1.
+	 */
 	struct scx_sched	*sched;
 
 	u32			flags;		/* SCX_TG_* */