Re: HASH_OF_MAPS inner map allocation from BPF

[Borna Cafuk <borna.cafuk@sartura.hr>]

On Mon, Sep 7, 2020 at 3:33 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Borna Cafuk <borna.cafuk@sartura.hr> writes:
>
> > On Sat, Sep 5, 2020 at 12:47 AM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> >>
> >> On Fri, Sep 4, 2020 at 7:57 AM Borna Cafuk <borna.cafuk@sartura.hr> wrote:
> >> >
> >> > Hello everyone,
> >> >
> >> > Judging by [0], the inner maps in BPF_MAP_TYPE_HASH_OF_MAPS can only be created
> >> > from the userspace. This seems quite limiting in regard to what can be done
> >> > with them.
> >> >
> >> > Are there any plans to allow for creating the inner maps from BPF programs?
> >> >
> >> > [0] https://stackoverflow.com/a/63391528
> >>
> >> Did you ask that question or your use case is different?
> >> Creating a new map for map_in_map from bpf prog can be implemented.
> >> bpf_map_update_elem() is doing memory allocation for map elements.
> >> In such a case calling this helper on map_in_map can, in theory, create a new
> >> inner map and insert it into the outer map.
> >
> > No, it wasn't me who asked that question, but it seemed close enough to
> > my issue. My use case calls for modifying the syscount example from BCC[1].
> >
> > The idea is to have an outer map where the keys are PIDs, and inner maps where
> > the keys are system call numbers. This would enable tracking the number of
> > syscalls made by each process and the makeup of those calls for all processes
> > simultaneously.
> >
> > [1] https://github.com/iovisor/bcc/blob/master/libbpf-tools/syscount.bpf.c
>
> Well, if you just want to count, map-in-map seems a bit overkill? You
> could just do:
>
> struct {
>   u32 pid;
>   u32 syscall;
> } map_key;
>
> and use that?
>
> -Toke
>

I have considered that, but maps in maps seem better for when I need to get the
data about a single process's syscalls: It requires reading only one of the
inner maps in its entirety. If I have a composite key like that, I don't see
any way, other than:
 * either iterating through all the possible keys for a process
   (i.e. over all syscalls) and looking them up in the map, or
 * iterating over all entries in the map and filtering them.

Looking at it again, the first option does not seem _that_ bad, but just
iterating over one (inner) map would be easier to fit into our use-case.