linux-kernel - Re: [PATCH v3 2/2] sched_ext: Add a selftest for scx_bpf_dsq

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Message-ID: <54151879-f2bd-4e87-b5b5-5e916ee1e743@arm.com>
Date: Mon, 6 Oct 2025 18:17:50 +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 2/2] sched_ext: Add a selftest for scx_bpf_dsq_peek

On 10/6/25 18:04, Ryan Newton wrote:
> From: Ryan Newton <newton@...a.com>
> 
> Perform the most basic unit test: make sure an empty queue peeks as
> empty, and when we put one element in the queue, make sure peek returns
> that element.
> 
> However, even this simple test is a little complicated by the different
> behavior of scx_bpf_dsq_insert in different calling contexts:
>  - insert is for direct dispatch in enqueue
>  - insert is delayed when called from select_cpu
> 
> In this case we split the insert and the peek that verifies the
> result between enqueue/dispatch. As a second phase, we stress test by
> performing many peeks on an array of user DSQs.
> 
> Note: An alternative would be to call `scx_bpf_dsq_move_to_local` on an
> empty queue, which in turn calls `flush_dispatch_buf`, in order to flush
> the buffered insert. Unfortunately, this is not viable within the
> enqueue path, as it attempts a voluntary context switch within an RCU
> read-side critical section.
> 
> Signed-off-by: Ryan Newton <newton@...a.com>
> ---
>  kernel/sched/ext.c                            |   2 +
>  tools/testing/selftests/sched_ext/Makefile    |   1 +
>  .../selftests/sched_ext/peek_dsq.bpf.c        | 265 ++++++++++++++++++
>  tools/testing/selftests/sched_ext/peek_dsq.c  | 230 +++++++++++++++
>  4 files changed, 498 insertions(+)
>  create mode 100644 tools/testing/selftests/sched_ext/peek_dsq.bpf.c
>  create mode 100644 tools/testing/selftests/sched_ext/peek_dsq.c
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 6d3537e65001..ec7e791cd4c8 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -6120,6 +6120,7 @@ __bpf_kfunc struct task_struct *scx_bpf_dsq_peek(u64 dsq_id)
>  	sch = rcu_dereference(scx_root);
>  	if (unlikely(!sch))
>  		return NULL;
> +

Accidental whitespace

>  	if (unlikely(dsq_id & SCX_DSQ_FLAG_BUILTIN)) {
>  		scx_error(sch, "peek disallowed on builtin DSQ 0x%llx", dsq_id);
>  		return NULL;
> @@ -6130,6 +6131,7 @@ __bpf_kfunc struct task_struct *scx_bpf_dsq_peek(u64 dsq_id)
>  		scx_error(sch, "peek on non-existent DSQ 0x%llx", dsq_id);
>  		return NULL;
>  	}
> +

Accidental whitespace

>  	return rcu_dereference(dsq->first_task);
>  }
>  [snip]