sched_ext: Implement scx_bpf_kick_cpu() and task preemption support

It's often useful to wake up and/or trigger reschedule on other CPUs. This
patch adds scx_bpf_kick_cpu() kfunc helper that BPF scheduler can call to
kick the target CPU into the scheduling path.

As a sched_ext task relinquishes its CPU only after its slice is depleted,
this patch also adds SCX_KICK_PREEMPT and SCX_ENQ_PREEMPT which clears the
slice of the target CPU's current task to guarantee that sched_ext's
scheduling path runs on the CPU.

If SCX_KICK_IDLE is specified, the target CPU is kicked iff the CPU is idle
to guarantee that the target CPU will go through at least one full sched_ext
scheduling cycle after the kicking. This can be used to wake up idle CPUs
without incurring unnecessary overhead if it isn't currently idle.

As a demonstration of how backward compatibility can be supported using BPF
CO-RE, tools/sched_ext/include/scx/compat.bpf.h is added. It provides
__COMPAT_scx_bpf_kick_cpu_IDLE() which uses SCX_KICK_IDLE if available or
becomes a regular kicking otherwise. This allows schedulers to use the new
SCX_KICK_IDLE while maintaining support for older kernels. The plan is to
temporarily use compat helpers to ease API updates and drop them after a few
kernel releases.

v5: - SCX_KICK_IDLE added. Note that this also adds a compat mechanism for
      schedulers so that they can support kernels without SCX_KICK_IDLE.
      This is useful as a demonstration of how new feature flags can be
      added in a backward compatible way.

    - kick_cpus_irq_workfn() reimplemented so that it touches the pending
      cpumasks only as necessary to reduce kicking overhead on machines with
      a lot of CPUs.

    - tools/sched_ext/include/scx/compat.bpf.h added.

v4: - Move example scheduler to its own patch.

v3: - Make scx_example_central switch all tasks by default.

    - Convert to BPF inline iterators.

v2: - Julia Lawall reported that scx_example_central can overflow the
      dispatch buffer and malfunction. As scheduling for other CPUs can't be
      handled by the automatic retry mechanism, fix by implementing an
      explicit overflow and retry handling.

    - Updated to use generic BPF cpumask helpers.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 85fb5dc725ef..3b2809b980ac 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -134,6 +134,10 @@ struct sched_ext_entity {
 	 * scx_bpf_dispatch() but can also be modified directly by the BPF
 	 * scheduler. Automatically decreased by SCX as the task executes. On
 	 * depletion, a scheduling event is triggered.
+	 *
+	 * This value is cleared to zero if the task is preempted by
+	 * %SCX_KICK_PREEMPT and shouldn't be used to determine how long the
+	 * task ran. Use p->se.sum_exec_runtime instead.
 	 */
 	u64			slice;