On Tue, Mar 29, 2022 at 1:11 PM Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxxxxxxx/ > > 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. This is not cheap and I cannot see a production system using this interface. All you did is a poor man version of lttng that doesn't rely on such heavy instrumentation.
