Re: [PATCH 00/18] [ANNOUNCE] Dynamically created function based events

[Juri Lelli <juri.lelli@redhat.com>]

Hi Steve,

On 03/02/18 16:17, Steven Rostedt wrote:
> On Sat, 3 Feb 2018 12:52:08 -0800
> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> 
> > It's a user space job.
> 
> BTW, I asked around at DevConf.cz, and nobody I talked with (besides
> Arnaldo), have used eBPF. The "path to hello world" is quite high. This
> interface is extremely simple to use, and one doesn't need to install
> LLVM or other tools to interface with it.

Yep. Managed to get this working in less than an hour. :)

With something like

# echo 'replenish_dl_entity(u64 dl_runtime[3] | u64 dl_deadline[4] | u64 dl_period[5] | s64 runtime[8] | u64 deadline[9])' > function_events
# echo 'sched:*' > set_event
# echo replenish_dl_entity >> set_event

you can get something like

--->8---
     [...]
     cpuhog-3556  [002] d..3   727.101815: sched_switch: prev_comm=cpuhog prev_pid=3556 prev_prio=-1 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120                                                                                                         
     <idle>-0     [002] d.s4   727.128139: sched_waking: comm=kworker/2:1 pid=53 prio=120 target_cpu=002                           
     <idle>-0     [002] dNs5   727.128150: sched_wakeup: comm=kworker/2:1 pid=53 prio=120 target_cpu=002                           
     <idle>-0     [002] d..3   727.128184: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=kworker/2:1 next_pid=53 next_prio=120                                                                                                     
kworker/2:1-53    [002] d..3   727.128280: sched_stat_runtime: comm=kworker/2:1 pid=53 runtime=123827 [ns] vruntime=12389788162 [ns]                                                                                                                                       
kworker/2:1-53    [002] d..3   727.128288: sched_switch: prev_comm=kworker/2:1 prev_pid=53 prev_prio=120 prev_state=R+ ==> next_comm=swapper/2 next_pid=0 next_prio=120                                                                                                    
     <idle>-0     [002] d.h5   727.191609: enqueue_task_dl->replenish_dl_entity(dl_runtime=10000000, dl_deadline=100000000, dl_period=100000000, runtime=-218339, deadline=726823680456)                                                                                   
     <idle>-0     [002] d..3   727.191676: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cpuhog next_pid=3556 next_prio=-1
     [...]
--->8---

Which is quite nice already IMHO.

> I used the analogy, that eBPF is like C, and this is like Bash. One is
> much easier to get "Hello World!" out than the other.
> 
> So personally, this is something I know I would use (note, I have
> never used eBPF either). But if I'm the only one to use this
> interface then I'll stop here (and not bother with the function graph
> return interface). If others think this would be helpful, I would ask
> them to speak up now.

First impression is that this is going to be definitely useful if

 - it's possibile to hook at function end (e.g., replenish_dl_entity above
   carries more useful information _after_ it did its job)
 - inside functions? not really sure it's actually going to be needed, but I
   was wondering if it's possible at all :); with tracepoints it's for example
   easy to collect detailed information about which branches has been taken etc.

I'm going to play with this more. Just wanted to give back a quick positive
feedback.

I'm also adding Arm folks to the discussion, as they (and I :) have been
asking to add tracepoints to scheduler code in the past [1].

Best,

- Juri

[1] https://marc.info/?l=linux-kernel&m=149068303518607