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

Ryan Newton
2025-10-06T17:35:07Z

Hello Christian,

Sure, the lavd with peek is here:
https://github.com/sched-ext/scx/pull/2675

Beerland with peek is here:
https://github.com/sched-ext/scx/commit/c2a0f185051c06cc1ebae1dc40e5fe2bd3022c1e

The qmap one was not a meaningful change to the scheduler but just
extra peeks thrown in with debug prints.

Cheers,
-Ryan

P.S. Good catch on the two newlines being split into the wrong diff.
Argh, I was playing with Sapling's `sl absorb` and I swear it said it
amended the earlier commit. Apologies for not catching it. FWIW here's
a tip with that correction:
https://github.com/rrnewton/linux/commit/db426f852813e2b6deeae0869d20df1bea647a07

On Mon, Oct 6, 2025 at 1:20 PM Christian Loehle
<christian.loehle@arm.com> wrote:
>
> 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
> >
>