Re: [RFC bpf-next 0/4] bpf: Speed up symbol resolving in kprobe multi link

[Jiri Olsa]

On Fri, Apr 08, 2022 at 04:29:22PM -0700, Alexei Starovoitov wrote:
> On Thu, Apr 07, 2022 at 02:52:20PM +0200, Jiri Olsa wrote:
> > hi,
> > sending additional fix for symbol resolving in kprobe multi link
> > requested by Alexei and Andrii [1].
> > 
> > This speeds up bpftrace kprobe attachment, when using pure symbols
> > (3344 symbols) to attach:
> > 
> > Before:
> > 
> >   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
> >   ...
> >   6.5681 +- 0.0225 seconds time elapsed  ( +-  0.34% )
> > 
> > After:
> > 
> >   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
> >   ...
> >   0.5661 +- 0.0275 seconds time elapsed  ( +-  4.85% )
> > 
> > 
> > There are 2 reasons I'm sending this as RFC though..
> > 
> >   - I added test that meassures attachment speed on all possible functions
> >     from available_filter_functions, which is 48712 functions on my setup.
> >     The attach/detach speed for that is under 2 seconds and the test will
> >     fail if it's bigger than that.. which might fail on different setups
> >     or loaded machine.. I'm not sure what's the best solution yet, separate
> >     bench application perhaps?
> 
> are you saying there is a bug in the code that you're still debugging?
> or just worried about time?

just the time, I can make the test fail (cross the 2 seconds limit)
when the machine is loaded, like with running kernel build

but I couldn't reproduce this with just paralel test_progs run

> 
> I think it's better for it to be a part of selftest.
> CI will take extra 2 seconds to run.
> That's fine. It's a good stress test.

ok, great

thanks,
jirka

> 
> >   - copy_user_syms function potentially allocates lot of memory (~6MB in my
> >     tests with attaching ~48k functions). I haven't seen this to fail yet,
> >     but it might need to be changed to allocate memory gradually if needed,
> >     do we care? ;-)
> 
> replied in the other email.
> 
> Thanks for working on this!