We currently provide scx_ops_error() as a way for ext.c to cause a
scheduler to be evicted due to erroneous behavior (for example, due to
returning an invalid CPU from ops.select_cpu()). Now that we have a
method for exiting gracefully with an exit code from BPF programs, we
can similarly provide an scx_ops_exit() macro that allows ext.c to exit
and pipe an exit code up to user space.

This patch adds that macro. A subsequent patch will use it to exit and
plumb up hotplug events.

Signed-off-by: David Vernet <[email protected]>

A scheduler may implement ops.cpu_online() and ops.cpu_offline() for CPU
onlining and offlining events respectively. If a scheduler does _not_
implement these callbacks, it's indicative that they do not support
hotplug. Given that a scheduler that doesn't support hotplug is
essentially guaranteed to perform incorrectly if a hotplug event occurs,
let's update ext.c to do the sane thing, and exit the scheduler
automatically.

Given that there may be other events in the future that could cause the
scheduler to initiate an exit, we create a new enum scx_exit_code type
that reserves the top bit of the exit_code field in struct
scx_exit_info, and defines SCX_ECODE_RESTART. We could as an alternative
instead just return something like -EAGAIN to signify that user space
can try restarting the scheduler.

Signed-off-by: David Vernet <[email protected]>

Now that we have bits reserved for system exit code reasons and actions,
as well as bits available for user by user space, let's add some
ease-of-use macro to user_exit_info.h. A subsequent patch will add
selftests that use these macros.

Signed-off-by: David Vernet <[email protected]>

We've recently added some logic related to hotplug:

- If a hotplug event occurs and a scheduler hasn't implemented a
  callback for it, we automatically exit the scheduler with specific,
  built-in exit codes

- With scx_bpf_exit(), a scheduler can choose to manually exit the
  scheduler in a hotplug event, or do something else. In any case, the
  scheduler should _not_ be automatically exited by the kernel

Let's add selftests to validate these conditions.

Signed-off-by: David Vernet <[email protected]>