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

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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@xxxxxx> 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

# To display the perf.data header info, please use --header/--header-only options.
#
# dso: libdw-0.186.so
#
# Total Lost Samples: 0
#
# Samples: 27K of event 'cycles:u'
# Event count (approx.): 27956766207
#
# Overhead  Shared Object 
# ........  ..............
#
    39.84%  libdw-0.186.so
            |          
            |--25.66%--__libdw_find_attr
            |          |          
            |          |--20.96%--__dwarf_attr_internal (inlined)
            |          |          |          
            |          |          |--10.94%--attr_numeric
            |          |          |          |          
            |          |          |          |--9.96%--die__process_class
            |          |          |          |          __die__process_tag
            |          |          |          |          die__process_unit
            |          |          |          |          die__process
            |          |          |          |          dwarf_cus__create_and_process_cu
            |          |          |          |          dwarf_cus__process_cu_thread
            |          |          |          |          start_thread
            |          |          |          |          __GI___clone (inlined)
            |          |          |          |          
            |          |          |           --0.61%--type__init
            |          |          |                     __die__process_tag
            |          |          |                     die__process_unit
            |          |          |                     die__process
            |          |          |                     dwarf_cus__create_and_process_cu
            |          |          |                     dwarf_cus__process_cu_thread
            |          |          |                     start_thread
            |          |          |                     __GI___clone (inlined)
            |          |          |          
            |          |          |--5.43%--attr_type
            |          |          |          |          
            |          |          |           --4.94%--tag__init
            |          |          |                     |          
            |          |          |                     |--2.60%--die__process_class
            |          |          |                     |          __die__process_tag
            |          |          |                     |          die__process_unit
            |          |          |                     |          die__process
            |          |          |                     |          dwarf_cus__create_and_process_cu
            |          |          |                     |          dwarf_cus__process_cu_thread
            |          |          |                     |          start_thread
            |          |          |                     |          __GI___clone (inlined)
            |          |          |                     |          
            |          |          |                     |--0.99%--__die__process_tag
            |          |          |                     |          |          
            |          |          |                     |           --0.98%--die__process_unit
            |          |          |                     |                     die__process
            |          |          |                     |                     dwarf_cus__create_and_process_cu
            |          |          |                     |                     dwarf_cus__process_cu_thread
            |          |          |                     |                     start_thread
            |          |          |                     |                     __GI___clone (inlined)
            |          |          
            |          |--4.01%--__dwarf_siblingof_internal (inlined)
            |          |          |          
            |          |          |--1.41%--die__process_class
            |          |          |          __die__process_tag
            |          |          |          die__process_unit
            |          |          |          die__process
            |          |          |          dwarf_cus__create_and_process_cu
            |          |          |          dwarf_cus__process_cu_thread
            |          |          |          start_thread
            |          |          |          __GI___clone (inlined)
            |          |          |          
            |          |          |--1.02%--die__process
            |          |          |          dwarf_cus__create_and_process_cu
            |          |          |          dwarf_cus__process_cu_thread
            |          |          |          start_thread
            |          |          |          __GI___clone (inlined)
            |          
            |--2.38%--__libdw_form_val_compute_len
            |          __libdw_find_attr
            |          |          
            |           --1.86%--__dwarf_attr_internal (inlined)
            |                     |          
            |                     |--0.94%--attr_numeric
            |                     |          |          
            |                     |           --0.83%--die__process_class
            |                     |                     __die__process_tag
            |                     |                     die__process_unit
            |                     |                     die__process
            |                     |                     dwarf_cus__create_and_process_cu
            |                     |                     dwarf_cus__process_cu_thread
            |                     |                     start_thread
            |                     |                     __GI___clone (inlined)
            |                     |          
            |                      --0.56%--attr_type
            |                                |          
            |                                 --0.53%--tag__init



#
# (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?

⬢[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?

- Arnaldo



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