Message-ID: <643e2956-2ddb-45a8-9c8f-c77a73bac4d2@linaro.org>
Date: Tue, 28 Oct 2025 15:26:45 +0000
From: James Clark <james.clark@...aro.org>
To: Ian Rogers <irogers@...gle.com>
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 28/10/2025 3:07 pm, Ian Rogers wrote:
> 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

That makes sense to me. Should I make that change in addition to 
Namhyung's patch?

> 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.

That I'm not sure about either. Can we assume that no non-regular files 
have build IDs as we're not handling EAGAIN anyway? Or at least if they 
do it's not currently supported.

> 
> 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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