linux-kernel - Re: [PATCH V3 bpf-next 1/2] bpf: add bpf_task_get

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Date: Wed, 20 Mar 2024 16:05:38 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: Jose Fernandez <josef@...flix.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Tycho Andersen <tycho@...ho.pizza>
Subject: Re: [PATCH V3 bpf-next 1/2] bpf: add bpf_task_get_cgroup kfunc

On Wed, Mar 20, 2024 at 7:41 AM Tejun Heo <tj@...nel.org> wrote:
>
> On Mon, Mar 18, 2024 at 11:03:01PM -0600, Jose Fernandez wrote:
> > This patch enhances the BPF helpers by adding a kfunc to retrieve the
> > cgroup v2 of a task, addressing a previous limitation where only
> > bpf_task_get_cgroup1 was available for cgroup v1. The new kfunc is
> > particularly useful for scenarios where obtaining the cgroup ID of a
> > task other than the "current" one is necessary, which the existing
> > bpf_get_current_cgroup_id helper cannot accommodate. A specific use
> > case at Netflix involved the sched_switch tracepoint, where we had to
> > get the cgroup IDs of both the prev and next tasks.
> >
> > The bpf_task_get_cgroup kfunc acquires and returns a reference to a
> > task's default cgroup, ensuring thread-safe access by correctly
> > implementing RCU read locking and unlocking. It leverages the existing
> > cgroup.h helper, and cgroup_tryget to safely acquire a reference to it.
> >
> > Signed-off-by: Jose Fernandez <josef@...flix.com>
> > Reviewed-by: Tycho Andersen <tycho@...ho.pizza>
> > Acked-by: Yonghong Song <yonghong.song@...ux.dev>
> > Acked-by: Stanislav Fomichev <sdf@...gle.com>
>
> Acked-by: Tejun Heo <tj@...nel.org>
>
> but some questions below
>
> > +__bpf_kfunc struct cgroup *bpf_task_get_cgroup(struct task_struct *task)
> > +{
> > +     struct cgroup *cgrp;
> > +
> > +     rcu_read_lock();
> > +     cgrp = task_dfl_cgroup(task);
> > +     if (!cgroup_tryget(cgrp))
> > +             cgrp = NULL;
> > +     rcu_read_unlock();
> > +     return cgrp;
> > +}
>
> So, as this is a lot easier in cgroup2, the above can probably be written
> directly in BPF (untested and not sure the necessary annotations are in
> place, so please take it with a big grain of salt):
>
>         bpf_rcu_read_lock();
>         cgrp = task->cgroups->dfl_cgrp;
>         cgrp = bpf_cgroup_from_id(cgrp->kn.id);
>         bpf_rcu_read_unlock();
>
> If all you need is ID, it's even simpler:
>
>         bpf_rcu_read_lock();
>         cgrp_id = task->cgroups->dfl_cgrp->kn.id;
>         bpf_rcu_read_unlock();

Good point.
Looks like this kfunc isn't really needed.

We even have the following in one of the selftests:
        /* simulate bpf_get_current_cgroup_id() helper */
        bpf_rcu_read_lock();
        cgroups = task->cgroups;
        if (!cgroups)
                goto unlock;
        cgroup_id = cgroups->dfl_cgrp->kn->id;
unlock:
        bpf_rcu_read_unlock();


> In the first example, it's not great that we go from task pointer to cgroup
> pointer to ID and then back to acquired cgroup pointer. I wonder whether
> what we really want is to support something like the following:
>
>         bpf_rcu_read_lock();
>         cgrp = bpf_cgroup_tryget(task->cgroups->dfl_cgrp);
>         bpf_rcu_read_unlock();

this one should work already.
that's existing bpf_cgroup_acquire().

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