Re: [PATCH v3 2/2] sched_ext: Add a selftest for scx_bpf_dsq_peek

Christian Loehle
2025-10-06T17:17:54Z

On 10/6/25 18:04, Ryan Newton wrote:
> From: Ryan Newton <newton@meta.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@meta.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]