sched/fair: Optimize and simplify rq leaf_cfs_rq_list

We notice the rq leaf_cfs_rq_list has two problems when do bugfix
backports and some test profiling.

1. cfs_rqs under throttled subtree could be added to the list, and
   make their fully decayed ancestors on the list, even though not needed.

2. #1 also make the leaf_cfs_rq_list management complex and error prone,
   this is the list of related bugfix so far:

   commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
   commit fe61468b2cbc ("sched/fair: Fix enqueue_task_fair warning")
   commit b34cb07dde7c ("sched/fair: Fix enqueue_task_fair() warning some more")
   commit 39f23ce07b93 ("sched/fair: Fix unthrottle_cfs_rq() for leaf_cfs_rq list")
   commit 0258bdfaff5b ("sched/fair: Fix unfairness caused by missing load decay")
   commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
   commit fdaba61ef8a2 ("sched/fair: Ensure that the CFS parent is added after unthrottling")
   commit 2630cde26711 ("sched/fair: Add ancestors of unthrottled undecayed cfs_rq")

commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
delete every cfs_rq under throttled subtree from rq->leaf_cfs_rq_list,
and delete the throttled_hierarchy() test in update_blocked_averages(),
which optimized update_blocked_averages().

But those later bugfix add cfs_rqs under throttled subtree back to
rq->leaf_cfs_rq_list again, with their fully decayed ancestors, for
the integrity of rq->leaf_cfs_rq_list.

This patch takes another method, skip all cfs_rqs under throttled
hierarchy when list_add_leaf_cfs_rq(), to completely make cfs_rqs
under throttled subtree off the leaf_cfs_rq_list.

So we don't need to consider throttled related things in
enqueue_entity(), unthrottle_cfs_rq() and enqueue_task_fair(),
which simplify the code a lot. Also optimize update_blocked_averages()
since cfs_rqs under throttled hierarchy and their ancestors
won't be on the leaf_cfs_rq_list.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20220601021848.76943-1-zhouchengming@bytedance.com

0 files changed, 0 insertions, 0 deletions