Re: [PATCH bpf-next 0/4] expose number of map entries to userspace

[Anton Protopopov <aspsk@isovalent.com>]

On 25/01/06 05:19PM, Daniel Borkmann wrote:
> On 1/6/25 3:53 PM, Charalampos Stylianopoulos wrote:
> > This patch series provides an easy way for userspace applications to
> > query the number of entries currently present in a map.
> > 
> > Currently, the number of entries in a map is accessible only from kernel space
> > and eBPF programs. A userspace program that wants to track map utilization has to
> > create and attach an eBPF program solely for that purpose.
> > 
> > This series makes the number of entries in a map easily accessible, by extending the
> > main bpf syscall with a new command. The command supports only maps that already
> > track utilization, namely hash maps, LPM maps and queue/stack maps.
> 
> An earlier attempt to directly expose it to user space can be found here [0], which
> eventually led to [1] to only expose it via kfunc for BPF programs in order to avoid
> extending UAPI.
> 
> Perhaps instead add a small libbpf helper (e.g. bpf_map__current_entries to complement
> bpf_map__max_entries) which does all the work to extract that info via [1] underneath?

One small thingy here is that bpf_map_sum_elem_count() is only
available from the map iterator. Which means that to get the
bpf_map_sum_elem_count() for one map only, one have to iterate
through the whole set of maps (and filter out all but one).

I wanted to follow up my series by either adding the result of
calling bpf_map_sum_elem_count() to map_info as u32 or to add
possibility to provide a map_fd/map_id when creating an iterator
(so that it is only called for one map). But so far I haven't
a real use case for getting the number of elements for one map only.

> Thanks,
> Daniel
> 
>   [0] https://lore.kernel.org/bpf/20230531110511.64612-1-aspsk@isovalent.com/
>   [1] https://lore.kernel.org/bpf/20230705160139.19967-1-aspsk@isovalent.com/
>       https://lore.kernel.org/bpf/20230719092952.41202-1-aspsk@isovalent.com/
> 
> > Charalampos Stylianopoulos (4):
> >    bpf: Add map_num_entries map op
> >    bpf: Add bpf command to get number of map entries
> >    libbpf: Add support for MAP_GET_NUM_ENTRIES command
> >    selftests/bpf: Add tests for bpf_map_get_num_entries
> > 
> >   include/linux/bpf.h                           |  3 ++
> >   include/linux/bpf_local_storage.h             |  1 +
> >   include/uapi/linux/bpf.h                      | 17 +++++++++
> >   kernel/bpf/devmap.c                           | 14 ++++++++
> >   kernel/bpf/hashtab.c                          | 10 ++++++
> >   kernel/bpf/lpm_trie.c                         |  8 +++++
> >   kernel/bpf/queue_stack_maps.c                 | 11 +++++-
> >   kernel/bpf/syscall.c                          | 32 +++++++++++++++++
> >   tools/include/uapi/linux/bpf.h                | 17 +++++++++
> >   tools/lib/bpf/bpf.c                           | 16 +++++++++
> >   tools/lib/bpf/bpf.h                           |  2 ++
> >   tools/lib/bpf/libbpf.map                      |  1 +
> >   .../bpf/map_tests/lpm_trie_map_basic_ops.c    |  5 +++
> >   tools/testing/selftests/bpf/test_maps.c       | 35 +++++++++++++++++++
> >   14 files changed, 171 insertions(+), 1 deletion(-)
> > 
>