linux-kernel - Re: [PATCH] tracing/user_events: Add eBPF interface for user

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Date:   Tue, 29 Mar 2022 16:54:38 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Beau Belgrave <beaub@...ux.microsoft.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-trace-devel <linux-trace-devel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] tracing/user_events: Add eBPF interface for user_event
 created events

On Tue, Mar 29, 2022 at 4:11 PM Beau Belgrave <beaub@...ux.microsoft.com> wrote:
>
> On Tue, Mar 29, 2022 at 03:31:31PM -0700, Alexei Starovoitov wrote:
> > On Tue, Mar 29, 2022 at 1:11 PM Beau Belgrave <beaub@...ux.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@...ux.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@...ux.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.
>
> Sorry.
>
> > To take advantage of user_events user space has to be modified
> > and writev syscalls inserted.
>
> Yes, both user_events and lttng require user space modifications to do
> tracing correctly. The syscall overheads are real, and the cost depends
> on the mitigations around spectre/meltdown.
>
> > This is not cheap and I cannot see a production system using this interface.
>
> But you are fine with uprobe costs? uprobes appear to be much more costly
> than a syscall approach on the hardware I've run on.
>
> > All you did is a poor man version of lttng that doesn't rely
> > on such heavy instrumentation.
>
> Well I am a frugal person. :)
>
> This work has solved some critical issues we've been having, and I would
> appreciate a review of the code if possible.

It's a NACK to connect bpf and user_events.
I would remove user_events from the kernel too.