Re: [PATCH dwarves v4 3/4] pahole: Use per-thread btf instances to avoid mutex locking.

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Jan 28, 2022 at 11:52 AM Arnaldo Carvalho de Melo
<arnaldo.melo@gmail.com> wrote:
>
> Em Thu, Jan 27, 2022 at 05:12:02PM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Wed, Jan 26, 2022 at 11:58:27AM -0800, Andrii Nakryiko escreveu:
> > > On Wed, Jan 26, 2022 at 11:21 AM Kui-Feng Lee <kuifeng@fb.com> wrote:
> > > > Create an instance of btf for each worker thread, and add type info to
> > > > the local btf instance in the steal-function of pahole without mutex
> > > > acquiring.  Once finished with all worker threads, merge all
> > > > per-thread btf instances to the primary btf instance.
>
> > > There are still unnecessary casts and missing {} in the else branch,
> > > but I'll let Arnaldo decide or fix it up.
>
> So its just one unneeded cast as thr_data here is just a 'void *':
>
> diff --git a/pahole.c b/pahole.c
> index 8c0a982f05c9ae3d..39e18804100dbfda 100644
> --- a/pahole.c
> +++ b/pahole.c
> @@ -2924,7 +2924,7 @@ static enum load_steal_kind pahole_stealer(struct cu *cu,
>                  * avoids copying the data collected by the first thread.
>                  */
>                 if (thr_data) {
> -                       struct thread_data *thread = (struct thread_data *)thr_data;
> +                       struct thread_data *thread = thr_data;
>
>                         if (thread->encoder == NULL) {
>                                 thread->encoder =
>
>
> This other is needed as it is a "void **":
>
> @@ -2832,7 +2832,7 @@ static int pahole_thread_exit(struct conf_load *conf, void *thr_data)
>  static int pahole_threads_collect(struct conf_load *conf, int nr_threads, void **thr_data,
>                                   int error)
>  {
> -       struct thread_data **threads = (struct thread_data **)thr_data;
> +       struct thread_data **threads = thr_data;
>         int i;
>         int err = 0;
>
>
> Removing it:
>
> /var/home/acme/git/pahole/pahole.c: In function ‘pahole_threads_collect’:
> /var/home/acme/git/pahole/pahole.c:2835:40: warning: initialization of ‘struct thread_data **’ from incompatible pointer type ‘void **’ [-Wincompatible-pointer-types]
>  2835 |         struct thread_data **threads = thr_data;
>       |                                        ^~~~~~~~
>
>
> And I did some more profiling, now the focus should go to elfutils:
>
> ⬢[acme@toolbox pahole]$ perf report --no-children -s dso --call-graph none 2> /dev/null | head -20
> # To display the perf.data header info, please use --header/--header-only options.
> #
> #
> # Total Lost Samples: 0
> #
> # Samples: 27K of event 'cycles:u'
> # Event count (approx.): 27956766207
> #
> # Overhead  Shared Object
> # ........  ...................
> #
>     46.70%  libdwarves.so.1.0.0
>     39.84%  libdw-0.186.so
>      9.70%  libc-2.33.so
>      2.14%  libpthread-2.33.so
>      1.47%  [unknown]
>      0.09%  ld-2.33.so
>      0.06%  libelf-0.186.so
>      0.00%  libcrypto.so.1.1.1l
>      0.00%  libk5crypto.so.3.1
> ⬢[acme@toolbox pahole]$
>
> $ perf report -g graph,0.5,2 --stdio --no-children -s dso --dso libdw-0.186.so
>

[...]

>
> #
> # (Tip: If you have debuginfo enabled, try: perf report -s sym,srcline)
> #
>
> This find_attr thing needs improvements, its a linear search AFAIK, some
> hashtable could do wonders, I guess.
>
> Mark, was this considered at some point?

This would be a great improvement, yes!

But strange that you didn't see any BTF-related functions, are you
sure you profiled the entire DWARF to BTF conversion process? BTF
encoding is not dominant, but noticeable anyways (e.g., adding unique
strings to BTF is relatively expensive still).

>
> ⬢[acme@toolbox pahole]$ rpm -q elfutils-libs
> elfutils-libs-0.186-1.fc34.x86_64
>
> Andrii https://github.com/libbpf/libbpf/actions/workflows/pahole.yml is
> in failure mode for 3 days, and only yesterday I pushed these changes,
> seems unrelated to pahole:
>
> Tests exit status: 1
> Test Results:
>              bpftool: PASS
>           test_progs: FAIL (returned 1)
>  test_progs-no_alu32: FAIL (returned 1)
>        test_verifier: PASS
>             shutdown: CLEAN
> Error: Process completed with exit code 1.
>
> Can you please check?

Yes, it's not related to pahole. This is the BPF selftests issue which
I already fixed last week, but didn't get a chance to sync everything
to Github repo before leaving for a short vacation. I'll do another
sync tonight and it should be all green again.

>
> - Arnaldo