netdev - Re: [PATCH bpf-next v1 0/5] bpf: rstat: cgroup hierarchical stats

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Date:   Mon, 6 Jun 2022 14:32:09 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 0/5] bpf: rstat: cgroup hierarchical stats

On Fri, Jun 03, 2022 at 12:47:19PM -0700, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> In short, think of these bpf maps as equivalents to "struct
> memcg_vmstats" and "struct memcg_vmstats_percpu" in the memory
> controller. They are just containers to store the stats in, they do
> not have any subgraph structure and they have no use beyond storing
> percpu and total stats.

Thanks for the explanation.

> I run small microbenchmarks that are not worth posting, they compared
> the latency of bpf stats collection vs. in-kernel code that adds stats
> to struct memcg_vmstats[_percpu] and flushes them accordingly, the
> difference was marginal.

OK, that's a reasonable comparison.

> The main reason for this is to provide data in a similar fashion to
> cgroupfs, in text file per-cgroup. I will include this clearly in the
> next cover message.

Thanks, it'd be great to have that use-case captured there.

> AFAIK loading bpf programs requires a privileged user, so someone has
> to approve such a program. Am I missing something?

A sysctl unprivileged_bpf_disabled somehow stuck in my head. But as I
wrote, this adds a way how to call cgroup_rstat_updated() directly, it's
not reserved for privilged users anyhow.

> bpf_iter_run_prog() is used to run bpf iterator programs, and it grabs
> rcu read lock before doing so. So AFAICT we are good on that front.

Thanks for the clarification.


Michal