Re: [RFC bpf-next] Hierarchical Cgroup Stats Collection Using BPF

[Tejun Heo <tj@kernel.org>]

Hello,

On Wed, Mar 09, 2022 at 12:27:15PM -0800, Yosry Ahmed wrote:
...
> These problems are already addressed by the rstat aggregation
> mechanism in the kernel, which is primarily used for memcg stats. We

Not that it matters all that much but I don't think the above statement is
true given that sched stats are an integrated part of the rstat
implementation and io was converted before memcg.

> - For every cgroup, we will either use flags to distinguish BPF stats
> updates from normal stats updates, or flush both anyway (memcg stats
> are periodically flushed anyway).

I'd just keep them together. Usually most activities tend to happen
together, so it's cheaper to aggregate all of them in one go in most cases.

> - Provide flags to enable/disable using per-cpu arrays (for stats that
> are not updated frequently), and enable/disable hierarchical
> aggregation (for non-hierarchical stats, they can still make benefit
> of the automatic entries creation & deletion).
> - Provide different hierarchical aggregation operations : SUM, MAX, MIN, etc.
> - Instead of an array as the map value, use a struct, and let the user
> provide an aggregator function in the form of a BPF program.

I'm more partial to the last option. It does make the usage a bit more
compilcated but hopefully it shouldn't be too bad with good examples.

I don't have strong opinions on the bpf side of things but it'd be great to
be able to use rstat from bpf.

Thanks.

-- 
tejun