[PATCHSET sched_ext/for-6.15] sched_ext: Enhance built-in idle selection with preferred CPUs

[Andrea Righi <arighi@nvidia.com>]

Many scx schedulers define their own concept of scheduling domains to
represent topology characteristics, such as heterogeneous architectures
(e.g., big.LITTLE, P-cores/E-cores), or to categorize tasks based on
specific properties (e.g., setting the soft-affinity of certain tasks to a
subset of CPUs).

Currently, there is no mechanism to share these domains with the built-in
idle CPU selection policy. As a result, schedulers often implement their
own idle CPU selection policies, which are typically similar to one
another, leading to a lot of code duplication.

To address this, extend the built-in idle CPU selection policy introducing
the concept of preferred CPUs.

With this concept, BPF schedulers can apply the built-in idle CPU selection
policy to a subset of preferred CPUs, allowing them to implement their own
scheduling domains while still using the topology optimizations
optimizations of the built-in policy, preventing code duplication across
different schedulers.

To implement this, introduce a new helper kfunc scx_bpf_select_cpu_pref()
that allows to specify a cpumask of preferred CPUs:

s32 scx_bpf_select_cpu_pref(struct task_struct *p,
			    const struct cpumask *preferred_cpus,
			    s32 prev_cpu, u64 wake_flags, u64 flags);

Moreover, introduce the new idle flag %SCX_PICK_IDLE_IN_PREF that can be
used to enforce selection strictly within the preferred domain.

Example usage
=============

s32 BPF_STRUCT_OPS(foo_select_cpu, struct task_struct *p,
		   s32 prev_cpu, u64 wake_flags)
{
	const struct cpumask *dom = task_domain(p) ?: p->cpus_ptr;
	s32 cpu;

	/*
	 * Pick an idle CPU in the task's domain. If no CPU is found,
	 * extend the search outside the domain.
	 */
	cpu = scx_bpf_select_cpu_pref(p, dom, prev_cpu, wake_flags, 0);
	if (cpu >= 0) {
		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);
		return cpu;
	}

	return prev_cpu;
}

Results
=======

Load distribution on a 4 sockets / 4 cores per socket system, simulated
using virtme-ng, running a modified version of scx_bpfland that uses the
new helper scx_bpf_select_cpu_pref() and 0xff00 as preferred domain:

 $ vng --cpu 16,sockets=4,cores=4,threads=1

Starting 12 CPU hogs to fill the preferred domain:

 $ stress-ng -c 12
 ...
    0[|||||||||||||||||||||||100.0%]   8[||||||||||||||||||||||||100.0%]
    1[|                        1.3%]   9[||||||||||||||||||||||||100.0%]
    2[|||||||||||||||||||||||100.0%]  10[||||||||||||||||||||||||100.0%]
    3[|||||||||||||||||||||||100.0%]  11[||||||||||||||||||||||||100.0%]
    4[|||||||||||||||||||||||100.0%]  12[||||||||||||||||||||||||100.0%]
    5[||                       2.6%]  13[||||||||||||||||||||||||100.0%]
    6[|                        0.6%]  14[||||||||||||||||||||||||100.0%]
    7|                         0.0%]  15[||||||||||||||||||||||||100.0%]

Passing %SCX_PICK_IDLE_IN_PREF to scx_bpf_select_cpu_pref() to enforce
strict selection on the preferred CPUs (with the same workload):

    0[                         0.0%]   8[||||||||||||||||||||||||100.0%]
    1[                         0.0%]   9[||||||||||||||||||||||||100.0%]
    2[                         0.0%]  10[||||||||||||||||||||||||100.0%]
    3[                         0.0%]  11[||||||||||||||||||||||||100.0%]
    4[                         0.0%]  12[||||||||||||||||||||||||100.0%]
    5[                         0.0%]  13[||||||||||||||||||||||||100.0%]
    6[                         0.0%]  14[||||||||||||||||||||||||100.0%]
    7[                         0.0%]  15[||||||||||||||||||||||||100.0%]

Andrea Righi (4):
      sched_ext: idle: Honor idle flags in the built-in idle selection policy
      sched_ext: idle: Introduce the concept of preferred CPUs
      sched_ext: idle: Introduce scx_bpf_select_cpu_pref()
      selftests/sched_ext: Add test for scx_bpf_select_cpu_pref()

 kernel/sched/ext.c                                |   4 +-
 kernel/sched/ext_idle.c                           | 235 ++++++++++++++++++----
 kernel/sched/ext_idle.h                           |   3 +-
 tools/sched_ext/include/scx/common.bpf.h          |   2 +
 tools/sched_ext/include/scx/compat.h              |   1 +
 tools/testing/selftests/sched_ext/Makefile        |   1 +
 tools/testing/selftests/sched_ext/pref_cpus.bpf.c |  95 +++++++++
 tools/testing/selftests/sched_ext/pref_cpus.c     |  58 ++++++
 8 files changed, 354 insertions(+), 45 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/pref_cpus.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/pref_cpus.c