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

[Anton Protopopov <aspsk@isovalent.com>]

On 25/01/14 12:38PM, Nick Zavaritsky wrote:
> 
> > On 9. Jan 2025, at 18:37, Anton Protopopov <aspsk@isovalent.com> wrote:
> > 
> > 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?
> 
> (On behalf of Charalampos Stylianopoulos) we would like to get the
> number of elements in some maps for monitoring purposes. The end goal is
> to get someone paged when a fixed-capacity map is about to start
> rejecting inserts.
> 
> We aim to operate a large number of apps in containers (custom packet
> processing services, telekom). We find it most convenient for an app
> itself to expose metrics concerning the maps it has created.
> 
> We currently use a map iterator and a bunch of bpf_probe_read_kernel. We
> foresee the number of maps in our systems getting significantly higher
> in the near future. Therefore enumerating every map in the system to get
> a number of elements in a particular map doesn't look sustainable.
> 
> How do you feel about introducing bpf_map_sum_elem_count_by_fd kfunc,
> available in syscall programs?

This should work already, something like

    __s64 bpf_map_sum_elem_count(const struct bpf_map *map) __ksym;
    __s64 ret_user;

    struct {
            __uint(type, BPF_MAP_TYPE_HASH);
            __type(key, int);
            __type(value, int);
            __uint(max_entries, 4);
    } your_map SEC(".maps");

    SEC("syscall")
    int sum(void *ctx)
    {
            struct bpf_map *map = (struct bpf_map *)&your_map;

            ret_user = bpf_map_sum_elem_count(map);

            return 0;
    }

    char _license[] SEC("license") = "GPL";

Is this sufficient for your use case?

> > 
> > [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(-)
> >>>>>> 
> >>>>> 
>