sched_ext: Implement sched_ext_ops.cpu_acquire/release()

Scheduler classes are strictly ordered and when a higher priority class has
tasks to run, the lower priority ones lose access to the CPU. Being able to
monitor and act on these events are necessary for use cases includling
strict core-scheduling and latency management.

This patch adds two operations ops.cpu_acquire() and .cpu_release(). The
former is invoked when a CPU becomes available to the BPF scheduler and the
opposite for the latter. This patch also implements
scx_bpf_reenqueue_local() which can be called from .cpu_release() to trigger
requeueing of all tasks in the local dsq of the CPU so that the tasks can be
reassigned to other available CPUs.

scx_pair is updated to use .cpu_acquire/release() along with
%SCX_KICK_WAIT to make the pair scheduling guarantee strict even when a CPU
is preempted by a higher priority scheduler class.

scx_qmap is updated to use .cpu_acquire/release() to empty the local
dsq of a preempted CPU. A similar approach can be adopted by BPF schedulers
that want to have a tight control over latency.

v4: Use the new SCX_KICK_IDLE to wake up a CPU after re-enqueueing.

v3: Drop the const qualifier from scx_cpu_release_args.task. BPF enforces
    access control through the verifier, so the qualifier isn't actually
    operative and only gets in the way when interacting with various
    helpers.

v2: Add p->scx.kf_mask annotation to allow calling scx_bpf_reenqueue_local()
    from ops.cpu_release() nested inside ops.init() and other sleepable
    operations.

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

3 files changed, 39 insertions, 3 deletions

diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index 421118bc56ff..8686f84497db 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -34,6 +34,7 @@ void scx_bpf_dispatch(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flag
 u32 scx_bpf_dispatch_nr_slots(void) __ksym;
 void scx_bpf_dispatch_cancel(void) __ksym;
 bool scx_bpf_consume(u64 dsq_id) __ksym;
+u32 scx_bpf_reenqueue_local(void) __ksym;
 void scx_bpf_kick_cpu(s32 cpu, u64 flags) __ksym;
 s32 scx_bpf_dsq_nr_queued(u64 dsq_id) __ksym;
 void scx_bpf_destroy_dsq(u64 dsq_id) __ksym;
diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index 879fc9c788e5..4a87377558c8 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -11,6 +11,8 @@
  *
  * - BPF-side queueing using PIDs.
  * - Sleepable per-task storage allocation using ops.prep_enable().
+ * - Using ops.cpu_release() to handle a higher priority scheduling class taking
+ *   the CPU away.
  *
  * This scheduler is primarily for demonstration and testing of sched_ext
  * features and unlikely to be useful for actual workloads.
@@ -90,7 +92,7 @@ struct {
 } cpu_ctx_stor SEC(".maps");
 
 /* Statistics */
-u64 nr_enqueued, nr_dispatched, nr_dequeued;
+u64 nr_enqueued, nr_dispatched, nr_reenqueued, nr_dequeued;
 
 s32 BPF_STRUCT_OPS(qmap_select_cpu, struct task_struct *p,
 		   s32 prev_cpu, u64 wake_flags)
@@ -164,6 +166,22 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
 		return;
 	}
 
+	/*
+	 * If the task was re-enqueued due to the CPU being preempted by a
+	 * higher priority scheduling class, just re-enqueue the task directly
+	 * on the global DSQ. As we want another CPU to pick it up, find and
+	 * kick an idle CPU.
+	 */
+	if (enq_flags & SCX_ENQ_REENQ) {
+		s32 cpu;
+
+		scx_bpf_dispatch(p, SHARED_DSQ, 0, enq_flags);
+		cpu = scx_bpf_pick_idle_cpu(p->cpus_ptr, 0);
+		if (cpu >= 0)
+			scx_bpf_kick_cpu(cpu, SCX_KICK_IDLE);
+		return;
+	}
+
 	ring = bpf_map_lookup_elem(&queue_arr, &idx);
 	if (!ring) {
 		scx_bpf_error("failed to find ring %d", idx);
@@ -257,6 +275,22 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)
 	}
 }
 
+void BPF_STRUCT_OPS(qmap_cpu_release, s32 cpu, struct scx_cpu_release_args *args)
+{
+	u32 cnt;
+
+	/*
+	 * Called when @cpu is taken by a higher priority scheduling class. This
+	 * makes @cpu no longer available for executing sched_ext tasks. As we
+	 * don't want the tasks in @cpu's local dsq to sit there until @cpu
+	 * becomes available again, re-enqueue them into the global dsq. See
+	 * %SCX_ENQ_REENQ handling in qmap_enqueue().
+	 */
+	cnt = scx_bpf_reenqueue_local();
+	if (cnt)
+		__sync_fetch_and_add(&nr_reenqueued, cnt);
+}
+
 s32 BPF_STRUCT_OPS(qmap_init_task, struct task_struct *p,
 		   struct scx_init_task_args *args)
 {
@@ -339,6 +373,7 @@ SCX_OPS_DEFINE(qmap_ops,
 	       .enqueue			= (void *)qmap_enqueue,
 	       .dequeue			= (void *)qmap_dequeue,
 	       .dispatch		= (void *)qmap_dispatch,
+	       .cpu_release		= (void *)qmap_cpu_release,
 	       .init_task		= (void *)qmap_init_task,
 	       .dump			= (void *)qmap_dump,
 	       .dump_cpu		= (void *)qmap_dump_cpu,
diff --git a/tools/sched_ext/scx_qmap.c b/tools/sched_ext/scx_qmap.c
index 594147a710a8..2a97421afe9a 100644
--- a/tools/sched_ext/scx_qmap.c
+++ b/tools/sched_ext/scx_qmap.c
@@ -112,9 +112,9 @@ int main(int argc, char **argv)
 		long nr_enqueued = skel->bss->nr_enqueued;
 		long nr_dispatched = skel->bss->nr_dispatched;
 
-		printf("stats  : enq=%lu dsp=%lu delta=%ld deq=%"PRIu64"\n",
+		printf("stats  : enq=%lu dsp=%lu delta=%ld reenq=%"PRIu64" deq=%"PRIu64"\n",
 		       nr_enqueued, nr_dispatched, nr_enqueued - nr_dispatched,
-		       skel->bss->nr_dequeued);
+		       skel->bss->nr_reenqueued, skel->bss->nr_dequeued);
 		fflush(stdout);
 		sleep(1);
 	}