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

Christian Loehle
2025-10-06T17:20:44Z

On 10/6/25 18:04, Ryan Newton wrote:
> 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

Is the adapted qmap and lavd available somewhere?

>
> 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
>