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

[Kris Van Hees <kris.van.hees@oracle.com>]

On Thu, Dec 18, 2025 at 03:10:32PM -0500, Eugene Loh wrote:
>  Reviewed-by: Eugene Loh <eugene.loh@oracle.com>

Per your reporting on the 2/4 patch, and further testing, the array map needs
to be created with size max_cpuid + 1 to accomodate the CPU with the highest
id (max_cpuid).  I made that change in the patch and it resolves the issues
with tests.

If you agree, I can just make that change in the patch, retain your R-b, and
merge.  Let me know...

> 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 <kris.van.hees@oracle.com>
> > ---
> >   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);
> >   }
> >   /*