Re: [PATCH bpf-next v1 4/9] bpf: Introduce sleepable tracepoints

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Hao Luo <haoluo@google.com>
Cc: Yonghong Song <yhs@fb.com>, 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>,
	KP Singh <kpsingh@kernel.org>, Shakeel Butt <shakeelb@google.com>,
	Joe Burton <jevburton.kernel@gmail.com>,
	Tejun Heo <tj@kernel.org>, Josh Don <joshdon@google.com>,
	Stanislav Fomichev <sdf@google.com>, bpf <bpf@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next v1 4/9] bpf: Introduce sleepable tracepoints
Date: Thu, 3 Mar 2022 12:02:13 -0800	[thread overview]
Message-ID: <CAADnVQ+wsp1+4DvrJjw_CAZDatsaQKKz-ZZADdTqSfUAqhv3SA@mail.gmail.com> (raw)
In-Reply-To: <CA+khW7gyOGgqJjyuSjJMJ8+iQmozZ6VhSJ7exZF0gGLOeS5gog@mail.gmail.com>

On Thu, Mar 3, 2022 at 11:43 AM Hao Luo <haoluo@google.com> wrote:
>
> On Wed, Mar 2, 2022 at 6:29 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Wed, Mar 2, 2022 at 5:09 PM Yonghong Song <yhs@fb.com> wrote:
> > >
> > >
> > >
> > > On 3/2/22 1:30 PM, Alexei Starovoitov wrote:
> > > > On Wed, Mar 2, 2022 at 1:23 PM Yonghong Song <yhs@fb.com> wrote:
> > > >>
> > > >>
> > > >>
> > > >> On 2/25/22 3:43 PM, Hao Luo wrote:
> > > >>> Add a new type of bpf tracepoints: sleepable tracepoints, which allows
> > > >>> the handler to make calls that may sleep. With sleepable tracepoints, a
> > > >>> set of syscall helpers (which may sleep) may also be called from
> > > >>> sleepable tracepoints.
> > > >>
> > > >> There are some old discussions on sleepable tracepoints, maybe
> > > >> worthwhile to take a look.
> > > >>
> > > >> https://lore.kernel.org/bpf/20210218222125.46565-5-mjeanson@efficios.com/T/
> > > >
> > > > Right. It's very much related, but obsolete too.
> > > > We don't need any of that for sleeptable _raw_ tps.
> > > > I prefer to stay with "sleepable" name as well to
> > > > match the rest of the bpf sleepable code.
> > > > In all cases it's faultable.
> > >
> > > sounds good to me. Agree that for the bpf user case, Hao's
> > > implementation should be enough.
> >
> > Just remembered that we can also do trivial noinline __weak
> > nop function and mark it sleepable on the verifier side.
> > That's what we were planning to do to trace map update/delete ops
> > in Joe Burton's series.
> > Then we don't need to extend tp infra.
> > I'm fine whichever way. I see pros and cons in both options.
>
> Joe is also cc'ed in this patchset, I will sync up with him on the
> status of trace map work.
>
> Alexei, do we have potentially other variants of tp? We can make the
> current u16 sleepable a flag, so we can reuse this flag later when we
> have another type of tracepoints.

When we added the ability to attach to kernel functions and mark them
as allow_error_inject the usefulness of tracepoints and even
writeable tracepoints was deminissed.
If we do sleepable tracepoint, I suspect, it may be the last extension
in that area.
I guess I'm convincing myself that noinline weak nop func
is better here. Just like it's better for Joe's map tracing.