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

Andrea Righi
2025-03-06T18:26:07Z

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