[PATCHSET v2 REPOST sched_ext/for-7.2] sched_ext: Topological CPU IDs and cid-form struct_ops

Tejun Heo
2026-04-24T17:27:23Z

Hello,

Reposting v2 because the original send was not properly threaded -
each patch went out as a standalone top-level message. Content is
unchanged from the original v2.

Original v2: https://lore.kernel.org/r/20260424013220.2923402-1-tj@kernel.org

v2 of https://lore.kernel.org/r/20260421071945.3110084-1-tj@kernel.org

v2:
- Add ext-types.h first patch for early subsystem-wide type defs.
- cid: publish the cid tables with WRITE_ONCE / read with READ_ONCE;
document the visibility contract.
- cid-kfuncs: NULL-guard scx_bpf_this_cid / scx_bpf_task_cid for
TRACING/SYSCALL callers before any SCX sched has enabled.
- cid-struct-ops: use struct_size() for the set_cmask_scratch percpu
alloc; cluster __scx_is_cid_type disable with __scx_enabled disable
in scx_root_disable().
- cid-kfunc-filter: sync per-entry kfunc flags with each kfunc's
primary BTF_ID_FLAGS() declaration (Zhao). pahole intersects flags
across occurrences; omitting them drops the flags globally - the
visible symptom was KF_IMPLICIT_ARGS getting cleared on
scx_bpf_kick_cpu, leaking bpf_prog_aux into vmlinux.h.
- cmask: narrow to the helpers this series actually uses;
cmask_copy_from_kernel contract and runtime guard.

This patchset introduces topological CPU IDs (cids) - dense,
topology-ordered cpu identifiers - and an alternative cid-form struct_ops
type that lets BPF schedulers operate in cid space directly.

Key pieces:

- cid space: scx_cid_init() walks nodes * LLCs * cores * threads and packs
a dense cid mapping. The mapping can be overridden via
scx_bpf_cid_override(). See "Topological CPU IDs" in ext_cid.h for the
model.

- cmask: a base-windowed bitmap over cid space. Kernel and BPF helpers with
identical semantics. Used by scx_qmap for per-task affinity and idle-cid
tracking; meant to be the substrate for sub-sched cid allocation.

- bpf_sched_ext_ops_cid: a parallel struct_ops type whose callbacks take
cids/cmasks instead of cpus/cpumasks. Kernel translates at the boundary
via scx_cpu_arg() / scx_cpu_ret(); the two struct types share offsets up
through @priv (verified by BUILD_BUG_ON) so the union view in scx_sched
works without function-pointer casts. Sub-sched support is tied to
cid-form: validate_ops() rejects cpu-form sub-scheds and cpu-form roots
that expose sub_attach / sub_detach.

- cid-form kfuncs: scx_bpf_kick_cid, scx_bpf_cidperf_{cap,cur,set},
scx_bpf_cid_curr, scx_bpf_task_cid, scx_bpf_this_cid,
scx_bpf_nr_{cids,online_cids}, scx_bpf_cid_to_cpu, scx_bpf_cpu_to_cid.
A cid-form program may not call cpu-only kfuncs (enforced at verifier
load via scx_kfunc_context_filter); the reverse is intentionally
permissive to ease migration.

- scx_qmap port: scx_qmap is converted to cid-form. It uses the cmask-based
idle picker, per-task cid-space cpus_allowed, and cid-form kfuncs
throughout. Sub-sched dispatching via scx_bpf_sub_dispatch() continues to
work.

v2 re-tested on the 16-cpu QEMU: cid-form scx_qmap, cpu-form scx_simple,
cid<->cpu cycling, scx_qmap under stress-ng, hotplug auto-restart, and
sub-sched (root scx_qmap + cgroup-scoped scx_qmap child). Clean.

Based on sched_ext/for-7.2 (c2929bc21dce).

0001-sched_ext-Add-ext_types.h-for-early-subsystem-wide-d.patch
0002-sched_ext-Rename-ops_cpu_valid-to-scx_cpu_valid-and-.patch
0003-sched_ext-Move-scx_exit-scx_error-and-friends-to-ext.patch
0004-sched_ext-Shift-scx_kick_cpu-validity-check-to-scx_b.patch
0005-sched_ext-Relocate-cpu_acquire-cpu_release-to-end-of.patch
0006-sched_ext-Make-scx_enable-take-scx_enable_cmd.patch
0007-sched_ext-Add-topological-CPU-IDs-cids.patch
0008-sched_ext-Add-scx_bpf_cid_override-kfunc.patch
0009-tools-sched_ext-Add-struct_size-helpers-to-common.bp.patch
0010-sched_ext-Add-cmask-a-base-windowed-bitmap-over-cid-.patch
0011-sched_ext-Add-cid-form-kfunc-wrappers-alongside-cpu-.patch
0012-sched_ext-Add-bpf_sched_ext_ops_cid-struct_ops-type.patch
0013-sched_ext-Forbid-cpu-form-kfuncs-from-cid-form-sched.patch
0014-tools-sched_ext-scx_qmap-Restart-on-hotplug-instead-.patch
0015-tools-sched_ext-scx_qmap-Add-cmask-based-idle-tracki.patch
0016-tools-sched_ext-scx_qmap-Port-to-cid-form-struct_ops.patch
0017-sched_ext-Require-cid-form-struct_ops-for-sub-sched-.patch

Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-cid-v2

kernel/sched/build_policy.c | 2 +
kernel/sched/ext.c | 650 +++++++++++++++++++++++++----
kernel/sched/ext_cid.c | 417 ++++++++++++++++++++
kernel/sched/ext_cid.h | 164 ++++++++
kernel/sched/ext_idle.c | 8 +-
kernel/sched/ext_internal.h | 203 +++++++---
kernel/sched/ext_types.h | 104 +++++
tools/sched_ext/include/scx/cid.bpf.h | 597 ++++++++++++++++++++++++++++
tools/sched_ext/include/scx/common.bpf.h | 23 ++
tools/sched_ext/include/scx/compat.bpf.h | 24 ++
tools/sched_ext/scx_qmap.bpf.c | 306 ++++++++-------
tools/sched_ext/scx_qmap.c | 25 +-
tools/sched_ext/scx_qmap.h | 2 +-
13 files changed, 2240 insertions(+), 285 deletions(-)

--
tejun