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Date:   Wed, 30 Mar 2022 14:22:42 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Beau Belgrave <beaub@linux.microsoft.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        linux-trace-devel <linux-trace-devel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH] tracing/user_events: Add eBPF interface for user_event
 created events
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On Tue, 29 Mar 2022 15:31:31 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> On Tue, Mar 29, 2022 at 1:11 PM Beau Belgrave <beaub@linux.microsoft.com> wrote:
> >
> > On Tue, Mar 29, 2022 at 12:50:40PM -0700, Alexei Starovoitov wrote:
> > > On Tue, Mar 29, 2022 at 11:19 AM Beau Belgrave
> > > <beaub@linux.microsoft.com> wrote:
> > > >
> > > > Send user_event data to attached eBPF programs for user_event based perf
> > > > events.
> > > >
> > > > Add BPF_ITER flag to allow user_event data to have a zero copy path into
> > > > eBPF programs if required.
> > > >
> > > > Update documentation to describe new flags and structures for eBPF
> > > > integration.
> > > >
> > > > Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> > >
> > > The commit describes _what_ it does, but says nothing about _why_.
> > > At present I see no use out of bpf and user_events connection.
> > > The whole user_events feature looks redundant to me.
> > > We have uprobes and usdt. It doesn't look to me that
> > > user_events provide anything new that wasn't available earlier.
> >
> > A lot of the why, in general, for user_events is covered in the first
> > change in the series.
> > Link: https://lore.kernel.org/all/20220118204326.2169-1-beaub@linux.microsoft.com/
> >
> > The why was also covered in Linux Plumbers Conference 2021 within the
> > tracing microconference.
> >
> > An example of why we want user_events:
> > Managed code running that emits data out via Open Telemetry.
> > Since it's managed there isn't a stub location to patch, it moves.
> > We watch the Open Telemetry spans in an eBPF program, when a span takes
> > too long we collect stack data and perform other actions.
> > With user_events and perf we can monitor the entire system from the root
> > container without having to have relay agents within each
> > cgroup/namespace taking up resources.
> > We do not need to enter each cgroup mnt space and determine the correct
> > patch location or the right version of each binary for processes that
> > use user_events.
> >
> > An example of why we want eBPF integration:
> > We also have scenarios where we are live decoding the data quickly.
> > Having user_data fed directly to eBPF lets us cast the data coming in to
> > a struct and decode very very quickly to determine if something is
> > wrong.
> > We can take that data quickly and put it into maps to perform further
> > aggregation as required.
> > We have scenarios that have "skid" problems, where we need to grab
> > further data exactly when the process that had the problem was running.
> > eBPF lets us do all of this that we cannot easily do otherwise.
> >
> > Another benefit from user_events is the tracing is much faster than
> > uprobes or others using int 3 traps. This is critical to us to enable on
> > production systems.
> 
> None of it makes sense to me.
> To take advantage of user_events user space has to be modified
> and writev syscalls inserted.

That can be done by introducing new user SDT macros, which currently
expected to use uprobes (thus it just introduces a list of probe
address and semaphore in a section). But we can provide another
implementation for lighter user-events.

> This is not cheap and I cannot see a production system using this interface.

I agree this point. At least this needs to be paired with user-space
library so that the applications can use it. But I also think that
new feature is not always requires an actual production system which
relays on that, since that means such production system must use
out-of-tree custom kernel. That should be avoided from the upstream-first
policy viewpoint. (However, I would like to know the actual use case.)

> All you did is a poor man version of lttng that doesn't rely
> on such heavy instrumentation.

Isn't it reasonable to avoid using heavy instrumentation? :-)

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>