sched_ext: Allow scx_bpf_reenqueue_local() to be called from anywhere

The ops.cpu_acquire/release() callbacks miss events under multiple conditions.
There are two distinct task dispatch gaps that can cause cpu_released flag
desynchronization:

1. balance-to-pick_task gap: This is what was originally reported. balance_scx()
   can enqueue a task, but during consume_remote_task() when the rq lock is
   released, a higher priority task can be enqueued and ultimately picked while
   cpu_released remains false. This gap is closeable via RETRY_TASK handling.

2. ttwu-to-pick_task gap: ttwu() can directly dispatch a task to a CPU's local
   DSQ. By the time the sched path runs on the target CPU, higher class tasks may
   already be queued. In such cases, nothing on sched_ext side will be invoked,
   and the only solution would be a hook invoked regardless of sched class, which
   isn't desirable.

Rather than adding invasive core hooks, BPF schedulers can use generic BPF
mechanisms like tracepoints. From SCX scheduler's perspective, this is congruent
with other mechanisms it already uses and doesn't add further friction.

The main use case for cpu_release() was calling scx_bpf_reenqueue_local() when
a CPU gets preempted by a higher priority scheduling class. However, the old
scx_bpf_reenqueue_local() could only be called from cpu_release() context.

Add a new version of scx_bpf_reenqueue_local() that can be called from any
context by deferring the actual re-enqueue operation. This eliminates the need
for cpu_acquire/release() ops entirely. Schedulers can now use standard BPF
mechanisms like the sched_switch tracepoint to detect and handle CPU preemption.

Update scx_qmap to demonstrate the new approach using sched_switch instead of
cpu_release, with compat support for older kernels. Mark cpu_acquire/release()
as deprecated. The old scx_bpf_reenqueue_local() variant will be removed in
v6.23.

Reported-by: Wen-Fang Liu <liuwenfang@honor.com>
Link: https://lore.kernel.org/all/8d64c74118c6440f81bcf5a4ac6b9f00@honor.com/
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions