Re: [PATCH] tracing/user_events: Add eBPF interface for user_event created events

[Beau Belgrave <beaub@linux.microsoft.com>]

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@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.

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.

Thanks,
-Beau