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

[Anton Protopopov <aspsk@isovalent.com>]

On 25/01/07 12:10PM, Charalampos Stylianopoulos wrote:
> (sorry for double posting, this time in plain text)
> Thanks a lot for the feedback!
> 
> So, to double check, the suggestion is to only extend the libbpf API
> with a new helper that does pretty much what get_cur_elements() does
> in tools/testing/selftests/bpf/map_tests/map_percpu_stats.c ?

What is your use case for getting the number of elements in a
particular map? Will it work for you to just use a variant of
get_cur_elements() from selftests vs. adding new API to libbpf?

[Also, please try not to top-post, see https://www.idallen.com/topposting.html]

> > On Tue, 7 Jan 2025 at 08:44, Anton Protopopov <aspsk@isovalent.com> wrote:
> >>
> >> 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(-)
> >> > >
> >> >