[PATCH] perf dsos: Don't block when reading build IDs

[James Clark <james.clark@linaro.org>]

The following command will hang consistently when the GPU is being used
due to non regular files (e.g. /dev/dri/renderD129, /dev/dri/card2)
being opened to read build IDs:

 $ perf record -asdg -e cpu-clock -- true

Change to non blocking reads to avoid the hang here:

  #0  __libc_pread64 (offset=<optimised out>, count=0, buf=0x7fffffffa4a0, fd=237) at ../sysdeps/unix/sysv/linux/pread64.c:25
  #1  __libc_pread64 (fd=237, buf=0x7fffffffa4a0, count=0, offset=0) at ../sysdeps/unix/sysv/linux/pread64.c:23
  #2  ?? () from /lib/x86_64-linux-gnu/libelf.so.1
  #3  read_build_id (filename=0x5555562df333 "/dev/dri/card2", bid=0x7fffffffb680, block=true) at util/symbol-elf.c:880
  #4  filename__read_build_id (filename=0x5555562df333 "/dev/dri/card2", bid=0x7fffffffb680, block=true) at util/symbol-elf.c:924
  #5  dsos__read_build_ids_cb (dso=0x5555562df1d0, data=0x7fffffffb750) at util/dsos.c:84
  #6  __dsos__for_each_dso (dsos=0x55555623de68, cb=0x5555557e7030 <dsos__read_build_ids_cb>, data=0x7fffffffb750) at util/dsos.c:59
  #7  dsos__for_each_dso (dsos=0x55555623de68, cb=0x5555557e7030 <dsos__read_build_ids_cb>, data=0x7fffffffb750) at util/dsos.c:503
  #8  dsos__read_build_ids (dsos=0x55555623de68, with_hits=true) at util/dsos.c:107
  #9  machine__read_build_ids (machine=0x55555623da58, with_hits=true) at util/build-id.c:950
  #10 perf_session__read_build_ids (session=0x55555623d840, with_hits=true) at util/build-id.c:956
  #11 write_build_id (ff=0x7fffffffb958, evlist=0x5555562323d0) at util/header.c:327
  #12 do_write_feat (ff=0x7fffffffb958, type=2, p=0x7fffffffb950, evlist=0x5555562323d0, fc=0x0) at util/header.c:3588
  #13 perf_header__adds_write (header=0x55555623d840, evlist=0x5555562323d0, fd=3, fc=0x0) at util/header.c:3632
  #14 perf_session__do_write_header (session=0x55555623d840, evlist=0x5555562323d0, fd=3, at_exit=true, fc=0x0, write_attrs_after_data=false) at util/header.c:3756
  #15 perf_session__write_header (session=0x55555623d840, evlist=0x5555562323d0, fd=3, at_exit=true) at util/header.c:3796
  #16 record__finish_output (rec=0x5555561838d8 <record>) at builtin-record.c:1899
  #17 __cmd_record (rec=0x5555561838d8 <record>, argc=2, argv=0x7fffffffe320) at builtin-record.c:2967
  #18 cmd_record (argc=2, argv=0x7fffffffe320) at builtin-record.c:4453
  #19 run_builtin (p=0x55555618cbb0 <commands+288>, argc=9, argv=0x7fffffffe320) at perf.c:349
  #20 handle_internal_command (argc=9, argv=0x7fffffffe320) at perf.c:401
  #21 run_argv (argcp=0x7fffffffe16c, argv=0x7fffffffe160) at perf.c:445
  #22 main (argc=9, argv=0x7fffffffe320) at perf.c:553

Fixes: 53b00ff358dc ("perf record: Make --buildid-mmap the default")
Signed-off-by: James Clark <james.clark@linaro.org>
---
I'm not actually sure if this is the right fix for this. Commit
2c369d91d093 ("perf symbol: Add blocking argument to
filename__read_build_id") which added the blocking argument says that it
should be non-blocking reads for event synthesis, but the callstack here
is when writing the header out.

There was also an is_regular_file() check added in commit c21986d33d6b
("perf unwind-libdw: skip non-regular files"), which presumably falls
afoul of the "which unfortunately fails for symlinks" part of the other
linked commit message?

So I can't tell if we should add the is_regular_file() check here too,
or just set it to non-blocking, or it needs some extra state to be
passed all the way down to dsos__read_build_ids_cb() to do different
things.
---
 tools/perf/util/dsos.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/dsos.c b/tools/perf/util/dsos.c
index 64c1d65b0149..5e1c815d7cb0 100644
--- a/tools/perf/util/dsos.c
+++ b/tools/perf/util/dsos.c
@@ -81,13 +81,14 @@ static int dsos__read_build_ids_cb(struct dso *dso, void *data)
 		return 0;
 	}
 	nsinfo__mountns_enter(dso__nsinfo(dso), &nsc);
-	if (filename__read_build_id(dso__long_name(dso), &bid, /*block=*/true) > 0) {
+	/* Don't block in case path isn't for a regular file. */
+	if (filename__read_build_id(dso__long_name(dso), &bid, /*block=*/false) > 0) {
 		dso__set_build_id(dso, &bid);
 		args->have_build_id = true;
 	} else if (errno == ENOENT && dso__nsinfo(dso)) {
 		char *new_name = dso__filename_with_chroot(dso, dso__long_name(dso));
 
-		if (new_name && filename__read_build_id(new_name, &bid, /*block=*/true) > 0) {
+		if (new_name && filename__read_build_id(new_name, &bid, /*block=*/false) > 0) {
 			dso__set_build_id(dso, &bid);
 			args->have_build_id = true;
 		}

---
base-commit: a1d8548c23076c66d96647f5f6f25aa43567f247
change-id: 20251022-james-perf-fix-dso-block-ca1d8437bbc0

Best regards,
-- 
James Clark <james.clark@linaro.org>