[PATCH 0/2] sched/fair: Cut per-group work from sched domain rebuilds

[Qiliang Yuan <qiliang.yuan@linux.dev>]

A sched domain rebuild, from CPU hotplug or a cpuset partition change,
moves every active CPU to a new root domain through rq_attach_root().
For each of them rq_offline_fair() and rq_online_fair() walk all task
groups with the rq lock held and interrupts disabled, so a rebuild
costs O(nr_cpus * nr_task_groups). Dietmar pointed at the offline side
when f60a631ab9ed was reviewed [1].

Patch 1 skips rq_offline_fair() for CPUs that stay active. Patch 2
skips the runtime_enabled re-sync in rq_online_fair() unless the CPU
actually went down.

With 4000 cpu cgroups on a 32-CPU VM booted with rt_group_sched=0
(median of 20 partition switches and 16 hotplug operations, all three
kernels measured in the same run):

  operation         base (ms)  patch 1 (ms)  patch 1+2 (ms)  change
  partition root        13.28          7.68            1.21    -91%
  cpu offline           44.95         30.96           13.44    -70%
  cpu online            36.84         28.48           12.82    -65%

With both patches these times no longer grow with the number of
groups. With RT_GROUP_SCHED enabled, the rt.c runtime walks remain.

[1] https://lore.kernel.org/r/dbd36b16-4acb-415a-9bbd-258b2e022475@arm.com

Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
Qiliang Yuan (2):
      sched/fair: Skip rq_offline_fair() work for CPUs that stay active
      sched/fair: Avoid re-syncing runtime_enabled on root domain changes

 kernel/sched/fair.c  | 20 +++++++++++++++++---
 kernel/sched/sched.h |  2 ++
 2 files changed, 19 insertions(+), 3 deletions(-)
---
base-commit: 067c5c24cef23d913b67eaaf71ae67e7169a6620
change-id: 20261111-bug-sched-fair-offline-active-8d063e735d3f

Best regards,
-- 
Qiliang Yuan <qiliang.yuan@linux.dev>