On Tue, 29 Mar 2022 15:31:31 -0700 Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > 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. 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@xxxxxxxxxx>
