Re: [RFC bpf-next 0/3] tools: bpftool: add subcommand to count map entries

[Edward Cree <ecree@solarflare.com>]

On 14/08/2019 10:42, Quentin Monnet wrote:
> 2019-08-13 18:51 UTC-0700 ~ Alexei Starovoitov
> <alexei.starovoitov@gmail.com>
>> The same can be achieved by 'bpftool map dump|grep key|wc -l', no?
> To some extent (with subtleties for some other map types); and we use a
> similar command line as a workaround for now. But because of the rate of
> inserts/deletes in the map, the process often reports a number higher
> than the max number of entries (we observed up to ~750k when max_entries
> is 500k), even is the map is only half-full on average during the count.
> On the worst case (though not frequent), an entry is deleted just before
> we get the next key from it, and iteration starts all over again. This
> is not reliable to determine how much space is left in the map.
>
> I cannot see a solution that would provide a more accurate count from
> user space, when the map is under pressure?
This might be a really dumb suggestion, but: you're wanting to collect a
 summary statistic over an in-kernel data structure in a single syscall,
 because making a series of syscalls to examine every entry is slow and
 racy.  Isn't that exactly a job for an in-kernel virtual machine, and
 could you not supply an eBPF program which the kernel runs on each entry
 in the map, thus supporting people who want to calculate something else
 (mean, min and max, whatever) instead of count?