Re-align idle selection logic with some of the latest improvements done
in scx_bpfland.

Signed-off-by: Andrea Righi <[email protected]>

Provide additional task metrics to user-space schedulers via QueuedTask:
 - nvcsw: total amount of voluntary context switches
 - slice: task time slice "budget" (from p->scx.slice)
 - dsq_vtime: current task vtime (from p->scx.dsq_vtime)

In this way user-space schedulers can quickly access these metrics to
implement better scheduling policy.

Signed-off-by: Andrea Righi <[email protected]>

Assign an infinite time slice to the user-space scheduler itself, so
that it can completely drain all the pending tasks and voluntarily
release the CPU when it's done.

This allows to achieve more consistent performance and we can also
remove the speculative user-space scheduler wakeup from ops.stopping().

Signed-off-by: Andrea Righi <[email protected]>

User-space schedulers may still hit some stalls during CPU hotplugging
events.

There is no reason to overcomplicate the code and trying to handle
hotplug events within the scx_rustland_core framework and we can simply
handle a scheduler restart performed by the scx core.

This makes CPU hotplugging more reliable with scx_rustland_core-based
schedulers.

Signed-off-by: Andrea Righi <[email protected]>

Prevent CPUs from going idle when the user-space scheduler has some
pending activities to complete.

Keeping the CPU alive allows to consume tasks from the user-space
scheduler more efficiently, preventing bubbles in the scheduling
pipeline.

To achieve this, trigger a CPU kick from ops.update_idle() and set a
flag in the CPU context to prevent it from going idle. Then keep kicking
the CPU from ops.dispatch() until the flag is cleared, which occurs when
no more tasks are pending or when the CPU exits idle as a task starts
running on it.

This allows to fix the performance regression introduced by the
put_prev_task_scx() behavior change in Linux 6.12 (see #788).

Link: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Andrea Righi <[email protected]>

Do not kick a CPU from rs_select_cpu() (called by the user-space
scheduler), since we may not immediately dispatch the task.

Instead, always try to wake up the task's assigned CPU after dispatching
to a global DSQ, ensuring it can be consumed immediately.

Signed-off-by: Andrea Righi <[email protected]>

With user-space scheduling we don't usually dispatch a task immediately
after selecting an idle CPU, so there's not much benefit at trying to
optimize the WAKE_SYNC scenario (when a task is waking up another task
and releaing the CPU) when picking an idle CPU.

Therefore, get rid of the WAKE_SYNC logic in select_cpu() and rely on
the user-space logic (that has access to the WAKE_SYNC information) to
handle this particular case.

Signed-off-by: Andrea Righi <[email protected]>

Bump up the minor version to reflect the new backward-compatible
functionality added.

Signed-off-by: Andrea Righi <[email protected]>

Use the nvcsw metric from the scx_rustland_core backend, intead of
retrieving this metric in user-space via procfs.

Signed-off-by: Andrea Righi <[email protected]>

Update vruntime adding the used virtual time slice of each task as soon
they are scheduled.

Signed-off-by: Andrea Righi <[email protected]>

scx_rustland is now effectively a deadline-based scheduler and not a
pure vruntime-based scheduler.

Clarify this in the source code. No functional change.

Signed-off-by: Andrea Righi <[email protected]>

Make scx_rlfifo even simpler and keep dispatching tasks even if the CPUs
are all busy.

This allows to better stress test the scx_rustland_core backend, by
using both the per-CPU DSQs and the global shared DSQ.

Signed-off-by: Andrea Righi <[email protected]>