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

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Message-ID: <cbc8d5bc-4ae7-4b61-81f4-9629857736f3@arm.com>
Date: Mon, 6 Oct 2025 18:20:40 +0100
From: Christian Loehle <christian.loehle@....com>
To: Ryan Newton <rrnewton@...il.com>, linux-kernel@...r.kernel.org
Cc: sched-ext@...ts.linux.dev, tj@...nel.org, arighi@...dia.com,
 newton@...a.com
Subject: Re: [PATCH v3 0/2] sched_ext: lockless peek operation for DSQs

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@...nel.org and arighi@...dia.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
>