Implicit casts from per-CPU integers into regular integers

[ajor]

Example usage:

tracepoint:syscalls:sys_enter_read { @c = count(); }
interval:s:1 { if (@c > 100) { exit() } }

Current behaviour:

ERROR: Type mismatch for '>': comparing 'count' with 'int64'
interval:s:1 { if (@c > 100) { exit() } }
                  ~~~~~~~~~

Per-CPU maps help to improve the performance of writes, but it is difficult to read their values. Currently they can only be aggregated and read in userspace, so people often fall back to using regular, non-per-cpu maps instead.

It'd be useful if we provided a mechanism to read the aggregated values of per-cpu maps from within BPF.

The bpf_map_lookup_percpu_elem helper can be used to look up a Per-CPU map's value from a different CPU.

A cast from a per-CPU integer to a regular integer could be done with this pseudo-code:

@percpu = count();
$int = 0;
$int += bpf_map_lookup_percpu_elem(@percpu, 0, 0); // CPU 0
$int += bpf_map_lookup_percpu_elem(@percpu, 0, 1); // CPU 1
$int += bpf_map_lookup_percpu_elem(@percpu, 0, 2); // CPU 2
$int += bpf_map_lookup_percpu_elem(@percpu, 0, 3); // CPU 3
...

[jordalgo]

We can use a const volatile variable for nr_cpus and update it from userspace before program load then use bpf_for(i, 0, nr_cpus) { to iterate.

Note: Open-coded iterators didn't make it in until kernel 6.4