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

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Message-ID: <CAJD7tkaPD94+61hrdr8qTV66RtfWOC+35=nGb9id_N1LrisCPA@mail.gmail.com>
Date:   Fri, 20 May 2022 09:06:45 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Kumar Kartikeya Dwivedi <memxor@...il.com>,
        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 4:16 AM Tejun Heo <tj@...nel.org> wrote:
>
> On Fri, May 20, 2022 at 03:06:07PM +0530, Kumar Kartikeya Dwivedi wrote:
> > With static noinline, the compiler will optimize away the function. With global
> > noinline, it can still optimize away the call site, but will keep the function
> > definition, so attach works. Therefore __weak is needed to ensure call is still
> > emitted. With GCC __attribute__((noipa)) might have been more appropritate, but
> > LLVM doesn't support it, so __weak is the next best thing supported by both with
> > the same side effect.

Thanks a lot for the explanation!

>
> Ah, okay, so it's to prevent compiler from optimizing away call to a noop
> function by telling it that we don't know what the function might eventually
> be. Thanks for the explanation. Yosry, can you please add a comment
> explaining what's going on?

Will add a comment explaining things in the next section.  Thanks for
reviewing this, Tejun!

>
> Thanks.
>
> --
> tejun