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

Changwoo Min
2025-03-07T03:14:27Z

Hi Andrea,

Thank you for submitting the patch set.

On 25. 3. 7. 03:18, Andrea Righi wrote:
> 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

Typo here. There are two "optimizations".

> 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
>