[PATCH 0/2] sched_ext: Fix entry-path contract breaks in core.c

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

Hello,

wake_up_new_task() consults select_task_rq() but activates the new task
without ENQUEUE_RQ_SELECTED, so on the fork path ops.enqueue() never
sees SCX_ENQ_CPU_SELECTED and SCX_EV_SELECT_CPU_FALLBACK never counts a
fork-path fallback; pass the flag on the initial enqueue to match the
wakeup path. rt_mutex_setprio() is the only entry path into sched_ext
that does not call sched_proxy_block_task(): an ext task PI-boosted by
an rt waiter and blocked on a second kernel mutex is routed back to ext
on deboost carrying the queued+blocked state that
scx_prepare_setscheduler() promises never happens; block a retained
donor before the deboost routes it back.

0001 passes the flag on the initial enqueue to match the wakeup path,
and 0002 blocks a retained donor before the deboost routes it back to
ext.

Verified in qemu on tj/sched_ext for-next (5b740a72b55c), same kernel
and config with and without each patch, comparing stock and patched
binaries; the touched core.c regions are byte-identical on this base,
so the runs carry over: with a probe forcing select_cpu() fallbacks,
40k forks produce 0 fallback events on the baseline kernel while the
wakeup side counts normally, and fork and wakeup paths both count 1:1
with patch 1; and a driver-model trigger hands the scheduler ~39966
queued+blocked donors across 40000 PI cycles on the baseline kernel
with 0 with patch 2, PI chain traffic unchanged.

Based on sched_ext/for-next (5b740a72b55c).

Tao Cui (2):
  sched/core: Pass ENQUEUE_RQ_SELECTED on initial enqueue
  sched_ext: Block a retained donor before PI deboost routes it back to
    sched_ext

 kernel/sched/core.c    | 17 ++++++++++++++++-
 kernel/sched/ext/ext.h |  2 ++
 2 files changed, 18 insertions(+), 1 deletion(-)

-- 
2.53.0