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

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Date:   Mon, 6 Jun 2022 12:32:04 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Koutný <mkoutny@...e.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 Mon, Jun 6, 2022 at 5:32 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> 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.

I am not sure if kfuncs have different privilege requirements or if
there is a way to mark a kfunc as privileged. Maybe someone with more
bpf knowledge can help here. But I assume if unprivileged_bpf_disabled
is not set then there is a certain amount of risk/trust that you are
taking anyway?

>
> > 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