linux-kernel - Re: [PATCH bpf-next v1 2/5] cgroup: bpf: add cgroup_rstat_updated() and cgroup_rstat

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Date:   Fri, 20 May 2022 02:13:03 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Tejun Heo <tj@...nel.org>
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>,
        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 2/5] cgroup: bpf: add cgroup_rstat_updated()
 and cgroup_rstat_flush() kfuncs

On Fri, May 20, 2022 at 12:24 AM Tejun Heo <tj@...nel.org> wrote:
>
> On Fri, May 20, 2022 at 01:21:30AM +0000, Yosry Ahmed wrote:
> > Add cgroup_rstat_updated() and cgroup_rstat_flush() kfuncs to bpf
> > tracing programs. bpf programs that make use of rstat can use these
> > functions to inform rstat when they update stats for a cgroup, and when
> > they need to flush the stats.
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
>
> Do patch 1 and 2 need to be separate? Also, can you explain and comment why
> it's __weak?

I will squash them in the next version.

As for the declaration, I took the __weak annotation from Alexei's
reply to the previous version. I thought it had something to do with
how fentry progs attach to functions with BTF and all.
When I try the same code with a static noinline declaration instead,
fentry attachment fails to find the BTF type ID of bpf_rstat_flush.
When I try it with just noinline (without __weak), the fentry program
attaches, but is never invoked. I tried looking at the attach code but
I couldn't figure out why this happens.

In retrospect, I should have given this more thought. It would be
great if Alexei could shed some light on this.

>
> Thanks.


>
> --
> tejun