[PATCH v3 0/2] sched_ext: lockless peek operation for DSQs

Ryan Newton
2025-10-06T17:04:11Z

This allows sched_ext schedulers an inexpensive operation to peek
at the first element in a queue (DSQ), without creating an iterator
and acquiring the lock on that queue.

Note that manual testing has thus far included a modified version of the
example qmap scheduler that exercises peek, as well as a modified
modified LAVD (from the SCX repo) that exercises peek. The attached test
passes >1000 stress tests when run in concurrent VMs, and when run
sequentially on the host kernel. Presently, tested on the below
workstation and server processors.
- AMD Ryzen Threadripper PRO 7975WX 32-Cores
- AMD EPYC 9D64 88-Core Processor

Initial experiments indicate a substantial speedup (on schbench) when
running an SCX scheduler with per-cpu DSQs and peeking each queue to
retrieve the task with the minimum vruntime across all the CPUs.

---
Changes in v3:
- inline helpers and simplify
- coding style tweaks

Changes in v2:
- make peek() only work for user DSQs and error otherwise
- added a stress test component to the selftest that performs many peeks
- responded to review comments from tj@kernel.org and arighi@nvidia.com
- link: https://lore.kernel.org/lkml/20251003195408.675527-1-rrnewton@gmail.com/

v1 link: https://lore.kernel.org/lkml/20251002025722.3420916-1-rrnewton@gmail.com/

Ryan Newton (2):
sched_ext: Add lockless peek operation for DSQs
sched_ext: Add a selftest for scx_bpf_dsq_peek

include/linux/sched/ext.h | 1 +
kernel/sched/ext.c | 56 +++-
tools/sched_ext/include/scx/common.bpf.h | 1 +
tools/sched_ext/include/scx/compat.bpf.h | 19 ++
tools/testing/selftests/sched_ext/Makefile | 1 +
.../selftests/sched_ext/peek_dsq.bpf.c | 265 ++++++++++++++++++
tools/testing/selftests/sched_ext/peek_dsq.c | 230 +++++++++++++++
7 files changed, 571 insertions(+), 2 deletions(-)
create mode 100644 tools/testing/selftests/sched_ext/peek_dsq.bpf.c
create mode 100644 tools/testing/selftests/sched_ext/peek_dsq.c

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2.51.0