Message-ID: <CAP-5=fV-g1CeJCoEnNnz_eFsks01JyUR3Fxz_H0X_rdTzOGuAw@mail.gmail.com>
Date: Tue, 28 Oct 2025 08:07:38 -0700
From: Ian Rogers <irogers@...gle.com>
To: James Clark <james.clark@...aro.org>
Cc: Namhyung Kim <namhyung@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf dsos: Don't block when reading build IDs

On Tue, Oct 28, 2025 at 3:33 AM James Clark <james.clark@...aro.org> wrote:
>
> On 26/10/2025 4:52 am, Ian Rogers wrote:
> > On Sat, Oct 25, 2025 at 4:59 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >>
> >> Hello,
> >>
> >> On Fri, Oct 24, 2025 at 11:26:18AM -0700, Ian Rogers wrote:
> >>> On Wed, Oct 22, 2025 at 8:46 AM James Clark <james.clark@...aro.org> wrote:
> >>>>
> >>>> 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@...aro.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.
> >>>
> >>> The fix lgtm but I worry about making all the build ID reading
> >>> non-blocking meaning build IDs getting lost.
> >>
> >> I'm not sure what non-blocking means for regular file system operations
> >> on a block device.  But it may have some impact on regular files on a
> >> network filesystem.
>
> It depends on the filesystem, but I think the assurances given by
> O_NONBLOCK are very weak anyway. It can be practically ignored or do
> different things in different situations. Especially as we don't handle
> EAGAIN or do anything fancy we shouldn't use it.
>
> For our case it looks like we should always do blocking reads but make
> sure to not open a non-regular file.
>
> >
> > Agreed. Prior to using blocking we tried to imply from the file type
> > from stat, but that skipped things like symlinks :-/
> >
>
> Am I missing something here? is_regular_file() uses stat() which returns
> info about the target file, rather than the symlink, so they wouldn't be
> skipped. lstat() is the one that returns info about the link file.
>
> I tested is_regular_file() with links, links to links, pipes, files and
> devices and it works as expected and would avoid the need to use O_NONBLOCK.

I don't mind we back out the use of the is_block parameter and use
is_regular_file. In the code before the change to use is_block the
callers would call is_regular_file and then call
filename__read_build_id. This seemed error prone as some callers were
doing the is_regular_file check and others not. I think I favor just
getting rid of the argument but putting the is_regular_file check in
filename__read_build_id. I'd switched to using O_NONBLOCK rather than
is_regular_file not just because of symlinks but just because it
wasn't clear that anything non-regular may still have a build id.

There's the separate issue of why are we writing out buildids in the
features when the buildid-mmaps contain the same data. It seems
Namhyung has a patch just about optimizing that case :-)

Thanks,
Ian

> James
>
> >>> It seems that generating
> >>> the build ID feature table is unnecessary if we have build ID mmap
> >>> events, including synthesized ones that will have read the build IDs.
> >>> I wonder if a better "fix" is:
> >>> ```
> >>> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> >>> index cb52aea9607d..15f75c70eb76 100644
> >>> --- a/tools/perf/builtin-record.c
> >>> +++ b/tools/perf/builtin-record.c
> >>> @@ -1842,7 +1842,7 @@ static void record__init_features(struct record *rec)
> >>>         for (feat = HEADER_FIRST_FEATURE; feat < HEADER_LAST_FEATURE; feat++)
> >>>                 perf_header__set_feat(&session->header, feat);
> >>>
> >>> -       if (rec->no_buildid)
> >>> +       if (rec->no_buildid || rec->buildid_mmap)
> >>>                 perf_header__clear_feat(&session->header, HEADER_BUILD_ID);
> >>>
> >>>         if (!have_tracepoints(&rec->evlist->core.entries))
> >>> ```
> >>> that should disable the build ID feature table generation when build
> >>> ID mmaps are in use (the default). Having the build IDs twice in the
> >>> data file feels redundant. Or we could do your fix or both, wdyt?
> >>
> >> I'm ok to remove the header feature but I think it should update
> >> build-ID cache even with this change.
> >
> > Yeah, dropping the feature writing also impacts updating the build ID
> > cache because:
> > https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/header.c?h=perf-tools-next#n338
> > It is kind of strange that writing a header feature does this. What if
> > I want to write a header with build IDs but not update the cache? It'd
> > make more sense, I think, for perf_session__cache_build_ids to be
> > called explicitly. There is also the global no_buildid_cache
> > controlling behavior:
> > https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/build-id.c?h=perf-tools-next#n43
> > But that's kind of a hack as we may have >1 session such as with TPEBS.
> >
> >> I'm also curious if the device has samples actually.  It should be
> >> checked by dso->hit.
> >
> > In this case the header writing is happening after the samples have
> > been processed, but it looks like marking of samples doesn't consider
> > data addresses:
> > https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/build-id.c?h=perf-tools-next#n55
> > just sample->ip and the callchain. If the marking was updated for data
> > pages then just writing build IDs for marked dsos would make sense.
> > There could be callers not marking dsos so they'd need altering, or
> > two versions of the code.
> >
> > Thanks,
> > Ian
> >
> >> Thanks,
> >> Namhyung
> >>
> >>>
> >>> Thanks,
> >>> Ian
> >>>
> >>>> ---
> >>>>   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@...aro.org>
> >>>>
>

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