sched_ext: idle: Introduce scx_bpf_select_cpu_and()

sched_ext: idle: Introduce scx_bpf_select_cpu_and()

Provide a new kfunc, scx_bpf_select_cpu_and(), that can be used to apply
the built-in idle CPU selection policy to a subset of allowed CPU.

This new helper is basically an extension of scx_bpf_select_cpu_dfl().
However, when an idle CPU can't be found, it returns a negative value
instead of @prev_cpu, aligning its behavior more closely with
scx_bpf_pick_idle_cpu().

It also accepts %SCX_PICK_IDLE_* flags, which can be used to enforce
strict selection to @prev_cpu's node (%SCX_PICK_IDLE_IN_NODE), or to
request only a full-idle SMT core (%SCX_PICK_IDLE_CORE), while applying
the built-in selection logic.

With this helper, BPF schedulers can apply the built-in idle CPU
selection policy restricted to any arbitrary subset of CPUs.

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

Possible usage in ops.select_cpu():

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

	cpu = scx_bpf_select_cpu_and(p, prev_cpu, wake_flags, cpus, 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
scx_bpf_select_cpu_and() with 0xff00 as the allowed subset of CPUs:

 $ vng --cpu 16,sockets=4,cores=4,threads=1
 ...
 $ stress-ng -c 16
 ...
 $ htop
 ...
   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%]

With scx_bpf_select_cpu_dfl() tasks would be distributed evenly across
all the available CPUs.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>