Re: uprobe overhead when specifying a pid

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Nov 13, 2024 at 11:33:01PM +0000, Sebastião Santos Boavida Amaro wrote:
> Hi,
> I am using:
> libbpf-cargo = "0.24.6"
> libbpf-rs = "0.24.6"
> libbpf-sys = "1.4.3"
> On kernel 6.8.0-47-generic.
> I contacted the libbpf-rs guys, and they told me this belonged here.
> I am attaching 252 uprobes to a system, these symbols are not regularly
> called (90ish times over 9 minutes), however, when I specify a pid the
> throughput drops 3 times from 12k ops/sec to 4k ops/sec. When I do not
> specify a PID, and simply pass -1 the throughput remains the same (as it
> should, since 90 times is not significant to affect overhead I would say).
> It looks as if we are switching from userspace to kernel space without
> triggering the uprobe.
> Do not know if this is a known issue, it does not look like an intended
> behavior.

hi,
thanks for the report, I cc-ed some other folks and trace list

I'm not aware about such slowdown, I think with pid filter in place
there should be less work to do

could you please provide more details?
  - do you know which uprobe interface you are using
    uprobe over perf event or uprobe_multi (likely uprobe_multi,
    because you said above you attach 250 probes)
  - more details on the workload, like is the threads/processes,
    how many and I guess you trigger bpf program
  - do you filter out single pid or more
  - could you profile the workload with perf

thanks,
jirka