On Thu, Jun 17, 2021 at 01:29:45PM -0700, Andrii Nakryiko wrote: > On Sat, Jun 5, 2021 at 4:12 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > hi, > > saga continues.. ;-) previous post is in here [1] > > > > After another discussion with Steven, he mentioned that if we fix > > the ftrace graph problem with direct functions, he'd be open to > > add batch interface for direct ftrace functions. > > > > He already had prove of concept fix for that, which I took and broke > > up into several changes. I added the ftrace direct batch interface > > and bpf new interface on top of that. > > > > It's not so many patches after all, so I thought having them all > > together will help the review, because they are all connected. > > However I can break this up into separate patchsets if necessary. > > > > This patchset contains: > > > > 1) patches (1-4) that fix the ftrace graph tracing over the function > > with direct trampolines attached > > 2) patches (5-8) that add batch interface for ftrace direct function > > register/unregister/modify > > 3) patches (9-19) that add support to attach BPF program to multiple > > functions > > > > In nutshell: > > > > Ad 1) moves the graph tracing setup before the direct trampoline > > prepares the stack, so they don't clash > > > > Ad 2) uses ftrace_ops interface to register direct function with > > all functions in ftrace_ops filter. > > > > Ad 3) creates special program and trampoline type to allow attachment > > of multiple functions to single program. > > > > There're more detailed desriptions in related changelogs. > > > > I have working bpftrace multi attachment code on top this. I briefly > > checked retsnoop and I think it could use the new API as well. > > Ok, so I had a bit of time and enthusiasm to try that with retsnoop. > The ugly code is at [0] if you'd like to see what kind of changes I > needed to make to use this (it won't work if you check it out because > it needs your libbpf changes synced into submodule, which I only did > locally). But here are some learnings from that experiment both to > emphasize how important it is to make this work and how restrictive > are some of the current limitations. > > First, good news. Using this mass-attach API to attach to almost 1000 > kernel functions goes from > > Plain fentry/fexit: > =================== > real 0m27.321s > user 0m0.352s > sys 0m20.919s > > to > > Mass-attach fentry/fexit: > ========================= > real 0m2.728s > user 0m0.329s > sys 0m2.380s I did not meassured the bpftrace speedup, because the new code attached instantly ;-) > > It's a 10x speed up. And a good chunk of those 2.7 seconds is in some > preparatory steps not related to fentry/fexit stuff. > > It's not exactly apples-to-apples, though, because the limitations you > have right now prevents attaching both fentry and fexit programs to > the same set of kernel functions. This makes it pretty useless for a hum, you could do link_update with fexit program on the link fd, like in the selftest, right? > lot of cases, in particular for retsnoop. So I haven't really tested > retsnoop end-to-end, I only verified that I do see fentries triggered, > but can't have matching fexits. So the speed-up might be smaller due > to additional fexit mass-attach (once that is allowed), but it's still > a massive difference. So we absolutely need to get this optimization > in. > > Few more thoughts, if you'd like to plan some more work ahead ;) > > 1. We need similar mass-attach functionality for kprobe/kretprobe, as > there are use cases where kprobe are more useful than fentry (e.g., >6 > args funcs, or funcs with input arguments that are not supported by > BPF verifier, like struct-by-value). It's not clear how to best > represent this, given currently we attach kprobe through perf_event, > but we'll need to think about this for sure. I'm fighting with the '2 trampolines concept' at the moment, but the mass attach for kprobes seems interesting ;-) will check > > 2. To make mass-attach fentry/fexit useful for practical purposes, it > would be really great to have an ability to fetch traced function's > IP. I.e., if we fentry/fexit func kern_func_abc, bpf_get_func_ip() > would return IP of that functions that matches the one in > /proc/kallsyms. Right now I do very brittle hacks to do that. so I hoped that we could store ip always in ctx-8 and have the bpf_get_func_ip helper to access that, but the BPF_PROG macro does not pass ctx value to the program, just args we could perhaps somehow store the ctx in BPF_PROG before calling the bpf program, but I did not get to try that yet > > So all-in-all, super excited about this, but I hope all those issues > are addressed to make retsnoop possible and fast. > > [0] https://github.com/anakryiko/retsnoop/commit/8a07bc4d8c47d025f755c108f92f0583e3fda6d8 thanks for checking on this, jirka
