Re: [PATCH v2 3/4] bpf: allocate the buffers BPF map to fit highest CPU id

  Reviewed-by: Eugene Loh <[email protected]>

On 12/17/25 00:10, Kris Van Hees wrote:
> Even when less than the possible number of CPUs are online, the 'buffers'
> BPF map should be allocated based on the highest possible CPU id because
> probe data is written to the bufer that corresponds to a given CPU id,
> which could be part of non-sequential CPU id configurations.
>
> The observed problem was on a system with 128 possible CPUs (0-127), but
> only CPUs 0-7,100-107 were online.  The 'buffers' BPF map was created with
> space to hold 16 trace data buffers (one for each online CPU), and since it
> is an array map, this caused the trace buffers for CPUs 100-107 to fail to
> allocate buffers, which in turn caused any trace data generated for those
> CPUs never to get reported.  Since 'buffers' is an array map accessed by
> CPU id, enough space must be allocated to accomodate the highest online
> CPU id.
>
> Signed-off-by: Kris Van Hees <[email protected]>
> ---
>   libdtrace/dt_bpf.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
> index 0a57b7d2..6568a572 100644
> --- a/libdtrace/dt_bpf.c
> +++ b/libdtrace/dt_bpf.c
> @@ -755,7 +755,7 @@ gmap_create_buffers(dtrace_hdl_t *dtp)
>   {
>   	return create_gmap(dtp, "buffers", BPF_MAP_TYPE_PERF_EVENT_ARRAY,
>   			   sizeof(uint32_t), sizeof(uint32_t),
> -			   dtp->dt_conf.num_online_cpus);
> +			   dtp->dt_conf.max_cpuid);
>   }
>   
>   /*