sched/fair: Start a cfs_rq on throttled hierarchy with PELT clock throttled

sched/fair: Start a cfs_rq on throttled hierarchy with PELT clock throttled

Matteo reported hitting the assert_list_leaf_cfs_rq() warning from
enqueue_task_fair() post commit fe8d238e646e ("sched/fair: Propagate
load for throttled cfs_rq") which transitioned to using
cfs_rq_pelt_clock_throttled() check for leaf cfs_rq insertions in
propagate_entity_cfs_rq().

The "cfs_rq->pelt_clock_throttled" flag is used to indicate if the
hierarchy has its PELT frozen. If a cfs_rq's PELT is marked frozen, all
its descendants should have their PELT frozen too or weird things can
happen as a result of children accumulating PELT signals when the
parents have their PELT clock stopped.

Another side effect of this is the loss of integrity of the leaf cfs_rq
list. As debugged by Aaron, consider the following hierarchy:

    root(#)
   /    \
  A(#)   B(*)
         |
         C <--- new cgroup
         |
         D <--- new cgroup

  # - Already on leaf cfs_rq list
  * - Throttled with PELT frozen

The newly created cgroups don't have their "pelt_clock_throttled" signal
synced with cgroup B. Next, the following series of events occur:

1. online_fair_sched_group() for cgroup D will call
   propagate_entity_cfs_rq(). (Same can happen if a throttled task is
   moved to cgroup C and enqueue_task_fair() returns early.)

   propagate_entity_cfs_rq() adds the cfs_rq of cgroup C to
   "rq->tmp_alone_branch" since its PELT clock is not marked throttled
   and cfs_rq of cgroup B is not on the list.

   cfs_rq of cgroup B is skipped since its PELT is throttled.

   root cfs_rq already exists on cfs_rq leading to
   list_add_leaf_cfs_rq() returning early.

   The cfs_rq of cgroup C is left dangling on the
   "rq->tmp_alone_branch".

2. A new task wakes up on cgroup A. Since the whole hierarchy is already
   on the leaf cfs_rq list, list_add_leaf_cfs_rq() keeps returning early
   without any modifications to "rq->tmp_alone_branch".

   The final assert_list_leaf_cfs_rq() in enqueue_task_fair() sees the
   dangling reference to cgroup C's cfs_rq in "rq->tmp_alone_branch".

   !!! Splat !!!

Syncing the "pelt_clock_throttled" indicator with parent cfs_rq is not
enough since the new cfs_rq is not yet enqueued on the hierarchy. A
dequeue on other subtree on the throttled hierarchy can freeze the PELT
clock for the parent hierarchy without setting the indicators for this
newly added cfs_rq which was never enqueued.

Since there are no tasks on the new hierarchy, start a cfs_rq on a
throttled hierarchy with its PELT clock throttled. The first enqueue, or
the distribution (whichever happens first) will unfreeze the PELT clock
and queue the cfs_rq on the leaf cfs_rq list.

While at it, add an assert_list_leaf_cfs_rq() in
propagate_entity_cfs_rq() to catch such cases in the future.

Closes: https://lore.kernel.org/lkml/58a587d694f33c2ea487c700b0d046fa@codethink.co.uk/
Fixes: e1fad12dcb66 ("sched/fair: Switch to task based throttle model")
Reported-by: Matteo Martelli <matteo.martelli@codethink.co.uk>
Suggested-by: Aaron Lu <ziqianlu@bytedance.com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Aaron Lu <ziqianlu@bytedance.com>
Tested-by: Aaron Lu <ziqianlu@bytedance.com>
Tested-by: Matteo Martelli <matteo.martelli@codethink.co.uk>
Link: https://patch.msgid.link/20251021053522.37583-1-kprateek.nayak@amd.com