Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <20220201205534.1962784-1-haoluo@google.com> <20220201205534.1962784-6-haoluo@google.com>
 <20220203180414.blk6ou3ccmod2qck@ast-mbp.dhcp.thefacebook.com>
 <CA+khW7jkJbvQrTx4oPJAoBZ0EOCtr3C2PKbrzhxj-7euBK8ojg@mail.gmail.com>
 <CAADnVQLZZ3SM2CDxnzgOnDgRtGU7+6wT9u5v4oFas5MnZF6DsQ@mail.gmail.com>
 <CA+khW7i+TScwPZ6-rcFKiXtxMm8hiZYJGH-wYb=7jBvDWg8pJQ@mail.gmail.com>
 <CAADnVQ+-29CS7nSXghKMgZjKte84L0nRDegUE0ObFm3d7E=eWw@mail.gmail.com>
 <CA+khW7iWd5MzZW_mCfgqHESi8okjNRiRMr0TM=CQzLkMsa_a5g@mail.gmail.com>
 <CAADnVQJcTAgcbwrOWO8EnbTdAcQ91HQmtpn7aKJGwHc=mEpJ1g@mail.gmail.com> <CA+khW7i46Rg8q=8goXdmuJuZ+NOuZ5AP6fSbSVzyqcU3C5iX4A@mail.gmail.com>
In-Reply-To: <CA+khW7i46Rg8q=8goXdmuJuZ+NOuZ5AP6fSbSVzyqcU3C5iX4A@mail.gmail.com>
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Mon, 14 Feb 2022 11:24:50 -0800
Message-ID: <CAADnVQK+Eh9qCuoBWZ1cRQ4h+fS5J+zy+GePEGXijZ_BD_5Q3w@mail.gmail.com>
Subject: Re: [PATCH RFC bpf-next v2 5/5] selftests/bpf: test for pinning for
 cgroup_view link
To:     Hao Luo <haoluo@google.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Joe Burton <jevburton.kernel@gmail.com>,
        Stanislav Fomichev <sdf@google.com>, bpf <bpf@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Mon, Feb 14, 2022 at 10:29 AM Hao Luo <haoluo@google.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?
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.