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

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Date:   Thu, 3 Mar 2022 14:06:02 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Yonghong Song <yhs@...com>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        KP Singh <kpsingh@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Joe Burton <jevburton.kernel@...il.com>,
        Tejun Heo <tj@...nel.org>, Josh Don <joshdon@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 4/9] bpf: Introduce sleepable tracepoints

On Thu, Mar 3, 2022 at 12:04 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Mar 3, 2022 at 12:02 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Thu, Mar 3, 2022 at 11:43 AM Hao Luo <haoluo@...gle.com> wrote:
> > >
> > > On Wed, Mar 2, 2022 at 6:29 PM Alexei Starovoitov
> > > <alexei.starovoitov@...il.com> wrote:
> > > >
> > > > On Wed, Mar 2, 2022 at 5:09 PM Yonghong Song <yhs@...com> wrote:
> > > > >
> > > > >
> > > > >
> > > > > On 3/2/22 1:30 PM, Alexei Starovoitov wrote:
> > > > > > On Wed, Mar 2, 2022 at 1:23 PM Yonghong Song <yhs@...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.
>
> To add to the above... The only downside of sleepable nop func
> comparing to tp is the lack of static_branch.
> So this nop call will always be there.
> For map tracing and for cgroup mkdir/rmdir the few nanosecond
> overhead of calling an empty function isn't even measurable.

The overhead should be fine, I think. mkdir/rmdir won't be frequent
operations. Thanks for the explanation. Let me give it a try and
report back how it works.