Re: [PATCH 6/7] sched_ext: Make qmap dump operation non-destructive

[Andrea Righi]

On Sun, Sep 21, 2025 at 03:32:45PM -1000, Tejun Heo wrote:
> The qmap dump operation was destructively consuming queue entries while
> displaying them. As dump can be triggered anytime, this can easily lead to
> stalls. Add a temporary dump_store queue and modify the dump logic to pop
> entries, display them, and then restore them back to the original queue.
> This allows dump operations to be performed without affecting the
> scheduler's queue state.
> 
> Note that if racing against new enqueues during dump, ordering can get
> mixed up, but this is acceptable for debugging purposes.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Makes sense.

Acked-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> ---
>  tools/sched_ext/scx_qmap.bpf.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
> index 69d877501cb7..cd50a94326e3 100644
> --- a/tools/sched_ext/scx_qmap.bpf.c
> +++ b/tools/sched_ext/scx_qmap.bpf.c
> @@ -56,7 +56,8 @@ struct qmap {
>    queue1 SEC(".maps"),
>    queue2 SEC(".maps"),
>    queue3 SEC(".maps"),
> -  queue4 SEC(".maps");
> +  queue4 SEC(".maps"),
> +  dump_store SEC(".maps");
>  
>  struct {
>  	__uint(type, BPF_MAP_TYPE_ARRAY_OF_MAPS);
> @@ -578,11 +579,26 @@ void BPF_STRUCT_OPS(qmap_dump, struct scx_dump_ctx *dctx)
>  			return;
>  
>  		scx_bpf_dump("QMAP FIFO[%d]:", i);
> +
> +		/*
> +		 * Dump can be invoked anytime and there is no way to iterate in
> +		 * a non-destructive way. Pop and store in dump_store and then
> +		 * restore afterwards. If racing against new enqueues, ordering
> +		 * can get mixed up.
> +		 */
>  		bpf_repeat(4096) {
>  			if (bpf_map_pop_elem(fifo, &pid))
>  				break;
> +			bpf_map_push_elem(&dump_store, &pid, 0);
>  			scx_bpf_dump(" %d", pid);
>  		}
> +
> +		bpf_repeat(4096) {
> +			if (bpf_map_pop_elem(&dump_store, &pid))
> +				break;
> +			bpf_map_push_elem(fifo, &pid, 0);
> +		}
> +
>  		scx_bpf_dump("\n");
>  	}
>  }
> -- 
> 2.51.0
>