linux-kernel - Re: [PATCH RFC bpf-next v2 5/5] selftests/bpf: test for pinning for cgroup

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Date:   Mon, 14 Feb 2022 12:23:29 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Joe Burton <jevburton.kernel@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC bpf-next v2 5/5] selftests/bpf: test for pinning for
 cgroup_view link

On Mon, Feb 14, 2022 at 11:25 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, Feb 14, 2022 at 10:29 AM Hao Luo <haoluo@...gle.com> wrote:
> > Hi Alexei,
> >
> > Actually, I found this almost worked, except that the tracepoints
> > cgroup_mkdir and cgroup_rmdir are not sleepable. They are inside a
> > spinlock's critical section with irq off. I guess one solution is to
> > offload the sleepable part of the bpf prog into a thread context. We
> > may create a dedicated kernel thread or use workqueue for this. Do you
> > have any advice?
>
> Are you referring to spin_lock in TRACE_CGROUP_PATH
> that protects global trace_cgroup_path[] buffer?

Yes, that's the spin_lock I am talking about.

> That is fixable.
> Do you actually need the string path returned by cgroup_path() in bpf prog?
> Maybe prog can call cgroup_path() by itself when necessary.
> Parsing strings isn't great anyway. The bpf prog probably needs the last
> part of the dir only. So cgrp->kn->name would do it?
> The TRACE_CGROUP_PATH wasn't designed to be turned on 24/7.
> That global spin_lock is not great for production use.
> No need to delegate sleepable bpf to thread context.
> Let's refactor that tracepoint a bit.

No, we don't need cgroup_path(). We are going to name the directories
by cgrp->kn->id. I can add a fast version for cgroup_xxx tracepoints,
which don't require the full path and can be turned on 24/7.