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

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: "Michal Koutný" <mkoutny@suse.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"John Fastabend" <john.fastabend@gmail.com>,
	"KP Singh" <kpsingh@kernel.org>, "Hao Luo" <haoluo@google.com>,
	"Tejun Heo" <tj@kernel.org>, "Zefan Li" <lizefan.x@bytedance.com>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Shuah Khan" <shuah@kernel.org>,
	"Roman Gushchin" <roman.gushchin@linux.dev>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Stanislav Fomichev" <sdf@google.com>,
	"David Rientjes" <rientjes@google.com>,
	"Greg Thelen" <gthelen@google.com>,
	"Shakeel Butt" <shakeelb@google.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
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Subject: Re: [PATCH bpf-next v1 0/5] bpf: rstat: cgroup hierarchical stats
Date: Tue, 7 Jun 2022 01:24:54 +0530	[thread overview]
Message-ID: <20220606195454.byivqaarp6ra7dpc@apollo.legion> (raw)
In-Reply-To: <CAJD7tkZeNhyEL4WtkEMOUeLsLX4x4roMuNCocEhz5yHm7=h4vw@mail.gmail.com>

On Tue, Jun 07, 2022 at 01:02:04AM IST, Yosry Ahmed wrote:
> On Mon, Jun 6, 2022 at 5:32 AM Michal Koutný <mkoutny@suse.com> wrote:
> >
> > On Fri, Jun 03, 2022 at 12:47:19PM -0700, Yosry Ahmed <yosryahmed@google.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?
>

It requires CAP_BPF or CAP_SYS_ADMIN, see verifier.c:add_subprog_or_kfunc.

> >
> > > 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

--
Kartikeya