Re: [PATCH 3/3] selftests/sched_ext: Fix consume_immed test reliability

[Andrea Righi]

Hi zhidao,

On Thu, Mar 26, 2026 at 10:28:27AM +0800, zhidao su wrote:
> The consume_immed test was failing because nr_consume_immed_reenq
> stayed 0. Two issues:
> 
> 1. Workers were spread across CPUs, so CPU 0's local DSQ rarely
>    accumulated multiple tasks. Fix: pin all workers to CPU 0 and
>    increase NUM_WORKERS to 8 to ensure USER_DSQ is always backlogged.
> 
> 2. ops.dispatch() called scx_bpf_dsq_move_to_local() only once, so
>    CPU 0's local DSQ would contain exactly 1 task after dispatch.
>    The IMMED slow path requires dsq->nr > 1 at the time of insertion
>    (in dsq_inc_nr()), so a single dispatch call never triggers it.
> 
>    Fix: call scx_bpf_dsq_move_to_local() in a loop (up to 4 times)
>    within a single ops.dispatch() invocation. The second call finds
>    dsq->nr already 1, so dsq->nr increments to 2, triggering
>    schedule_reenq_local() and the IMMED slow path.
> 
> Signed-off-by: zhidao su <suzhidao@xxxxxxxxxx>
> ---
>  .../selftests/sched_ext/consume_immed.bpf.c   | 25 ++++++++++++++-----
>  .../selftests/sched_ext/consume_immed.c       | 14 ++++++++++-
>  2 files changed, 32 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/testing/selftests/sched_ext/consume_immed.bpf.c b/tools/testing/selftests/sched_ext/consume_immed.bpf.c
> index 9c7808f5abe1..e99bea0b2c24 100644
> --- a/tools/testing/selftests/sched_ext/consume_immed.bpf.c
> +++ b/tools/testing/selftests/sched_ext/consume_immed.bpf.c
> @@ -11,9 +11,9 @@
>   *                   explicit SCX_ENQ_IMMED in enq_flags (requires v2 kfunc)
>   *
>   * Worker threads belonging to test_tgid are inserted into USER_DSQ.
> - * ops.dispatch() on CPU 0 consumes from USER_DSQ with SCX_ENQ_IMMED.
> - * With multiple workers competing for CPU 0, dsq->nr > 1 triggers the
> - * IMMED slow path (reenqueue with SCX_TASK_REENQ_IMMED).
> + * ops.dispatch() on CPU 0 consumes multiple tasks from USER_DSQ with
> + * SCX_ENQ_IMMED in a single dispatch call, causing dsq->nr to exceed 1
> + * and triggering the IMMED slow path (reenqueue with SCX_TASK_REENQ_IMMED).
>   *
>   * Requires scx_bpf_dsq_move_to_local___v2() (v7.1+) for enq_flags support.
>   */
> @@ -55,10 +55,23 @@ void BPF_STRUCT_OPS(consume_immed_enqueue, struct task_struct *p,

We should define a custom ops.select_cpu() to make sure all tasks are
bounced to ops.enqueue(), in order to have more inserts into USER_DSQ,
something like:

s32 BPF_STRUCT_OPS(consume_immed_select_cpu, struct task_struct *p,
                   s32 prev_cpu, u64 wake_flags)
{
	return prev_cpu;
}

>  
>  void BPF_STRUCT_OPS(consume_immed_dispatch, s32 cpu, struct task_struct *prev)
>  {
> -	if (cpu == 0)
> -		scx_bpf_dsq_move_to_local(USER_DSQ, SCX_ENQ_IMMED);
> -	else
> +	int i;
> +
> +	if (cpu != 0) {
>  		scx_bpf_dsq_move_to_local(SCX_DSQ_GLOBAL, 0);

Hm.. this should trigger an error, you can't use
scx_bpf_dsq_move_to_local() with SCX_DSQ_GLOBAL.

> +		return;
> +	}
> +
> +	/*
> +	 * Move multiple tasks into CPU 0's local DSQ with SCX_ENQ_IMMED in a
> +	 * single dispatch call. When the second task is inserted (dsq->nr > 1),
> +	 * dsq_inc_nr() triggers the IMMED slow path via schedule_reenq_local(),
> +	 * which calls ops.enqueue() with SCX_ENQ_REENQ | SCX_TASK_REENQ_IMMED.
> +	 */
> +	for (i = 0; i < 4; i++) {
> +		if (!scx_bpf_dsq_move_to_local(USER_DSQ, SCX_ENQ_IMMED))
> +			break;
> +	}

Why 4? Two consecutive scx_bpf_dsq_move_to_local() should be enough, right?

>  }
>  
>  s32 BPF_STRUCT_OPS_SLEEPABLE(consume_immed_init)

We don't see this from the context, but to check if SCX_ENQ_IMMED is
available can we just check if it's != 0, instead of checking the
_scx_bpf_dsq_move_to_local___v2 symbol?

Something like:

if (SCX_ENQ_IMMED == 0) {
	scx_bpf_error("SCX_ENQ_IMMED not available");
	return -EOPNOTSUPP;
}

And remove the same check in consume_immed.c, because we're already
checking it in the BPF part.

Thanks,
-Andrea

> diff --git a/tools/testing/selftests/sched_ext/consume_immed.c b/tools/testing/selftests/sched_ext/consume_immed.c
> index 7f9594cfa9cb..61cbc0fe3663 100644
> --- a/tools/testing/selftests/sched_ext/consume_immed.c
> +++ b/tools/testing/selftests/sched_ext/consume_immed.c
> @@ -12,18 +12,30 @@
>  #include <stdio.h>
>  #include <unistd.h>
>  #include <pthread.h>
> +#include <sched.h>
>  #include <bpf/bpf.h>
>  #include <scx/common.h>
>  #include "consume_immed.bpf.skel.h"
>  #include "scx_test.h"
>  
> -#define NUM_WORKERS		4
> +/*
> + * Use more workers than CPUs, all pinned to CPU 0, so CPU 0's local DSQ
> + * accumulates multiple IMMED tasks at once, reliably triggering the slow path.
> + */
> +#define NUM_WORKERS		8
>  #define TEST_DURATION_SEC	3
>  
>  static volatile bool stop_workers;
>  
>  static void *worker_fn(void *arg)
>  {
> +	cpu_set_t cpuset;
> +
> +	/* Pin to CPU 0 to saturate its local DSQ */
> +	CPU_ZERO(&cpuset);
> +	CPU_SET(0, &cpuset);
> +	pthread_setaffinity_np(pthread_self(), sizeof(cpuset), &cpuset);
> +
>  	while (!stop_workers) {
>  		volatile unsigned long i;
>  
> -- 
> 2.43.0
>