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Message-ID: <bfe873a2-4dbb-8ad0-5089-61ccf6a10eeb@arm.com>
Date: Fri, 4 Mar 2022 09:11:23 +0000
From: James Clark <james.clark@....com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: acme@...nel.org, linux-perf-users@...r.kernel.org,
coresight@...ts.linaro.org, Denis Nikitin <denik@...omium.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] perf: Set build-id using build-id header on new mmap
records
On 03/03/2022 11:36, Jiri Olsa wrote:
> On Wed, Mar 02, 2022 at 04:20:00PM +0000, James Clark wrote:
>>
>>
>> On 27/02/2022 22:50, Jiri Olsa wrote:
>>> On Thu, Feb 24, 2022 at 05:19:55PM +0000, James Clark wrote:
>>>> MMAP records that occur after the build-id header is parsed do not have
>>>> their build-id set even if the filename matches an entry from the
>>>> header. Set the build-id on these dsos as long as the MMAP record
>>>> doesn't have its own build-id set.
>>>>
>>>> This fixes an issue with off target analysis where the local version of
>>>> a dso is loaded rather than one from ~/.debug via a build-id.
>>>
>>> nice catch :)
>>>
>>>>
>>>> Reported-by: Denis Nikitin <denik@...omium.org>
>>>> Signed-off-by: James Clark <james.clark@....com>
>>>> ---
>>>> tools/perf/util/dso.h | 1 +
>>>> tools/perf/util/header.c | 1 +
>>>> tools/perf/util/map.c | 16 ++++++++++++++--
>>>> 3 files changed, 16 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
>>>> index 011da3924fc1..3a9fd4d389b5 100644
>>>> --- a/tools/perf/util/dso.h
>>>> +++ b/tools/perf/util/dso.h
>>>> @@ -167,6 +167,7 @@ struct dso {
>>>> enum dso_load_errno load_errno;
>>>> u8 adjust_symbols:1;
>>>> u8 has_build_id:1;
>>>> + u8 header_build_id:1;
>>>> u8 has_srcline:1;
>>>> u8 hit:1;
>>>> u8 annotate_warned:1;
>>>> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
>>>> index 6da12e522edc..571d73d4f976 100644
>>>> --- a/tools/perf/util/header.c
>>>> +++ b/tools/perf/util/header.c
>>>> @@ -2200,6 +2200,7 @@ static int __event_process_build_id(struct perf_record_header_build_id *bev,
>>>>
>>>> build_id__init(&bid, bev->data, size);
>>>> dso__set_build_id(dso, &bid);
>>>> + dso->header_build_id = 1;
>>>>
>>>> if (dso_space != DSO_SPACE__USER) {
>>>> struct kmod_path m = { .name = NULL, };
>>>> diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
>>>> index 1803d3887afe..4ae91e491e23 100644
>>>> --- a/tools/perf/util/map.c
>>>> +++ b/tools/perf/util/map.c
>>>> @@ -127,7 +127,7 @@ struct map *map__new(struct machine *machine, u64 start, u64 len,
>>>>
>>>> if (map != NULL) {
>>>> char newfilename[PATH_MAX];
>>>> - struct dso *dso;
>>>> + struct dso *dso, *header_bid_dso;
>>>> int anon, no_dso, vdso, android;
>>>>
>>>> android = is_android_lib(filename);
>>>> @@ -185,7 +185,19 @@ struct map *map__new(struct machine *machine, u64 start, u64 len,
>>>>
>>>> if (build_id__is_defined(bid))
>>>> dso__set_build_id(dso, bid);
>>>> -
>>>> + else {
>>>
>>> nit please add { } to the if clause as well
>>>
>>>> + /*
>>>> + * If the mmap event had no build ID, search for an existing dso from the
>>>> + * build ID header by name. Otherwise only the dso loaded at the time of
>>>> + * reading the header will have the build ID set and all future mmaps will
>>>> + * have it missing.
>>>> + */
>>>> + header_bid_dso = __dsos__find(&machine->dsos, filename, false);
>>>
>>> is this 'perf top' safe? I think dso should be added in the
>>> same thread, but please check and add comment why we don't
>>> need locking in here
>>
>> Seems like there are multiple synthesize_threads_workers using the same machine->dsos object so
>> I think locking is needed.
>>
>> At first I thought of doing this:
>>
>> diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
>> index 4ae91e491e23..b87b81e3d41c 100644
>> --- a/tools/perf/util/map.c
>> +++ b/tools/perf/util/map.c
>> @@ -192,7 +192,9 @@ struct map *map__new(struct machine *machine, u64 start, u64 len,
>> * reading the header will have the build ID set and all future mmaps will
>> * have it missing.
>> */
>> + down_read(&machine->dsos.lock);
>> header_bid_dso = __dsos__find(&machine->dsos, filename, false);
>> + up_read(&machine->dsos.lock);
>> if (header_bid_dso && header_bid_dso->header_build_id) {
>> dso__set_build_id(dso, &header_bid_dso->bid);
>> dso->header_build_id = 1;
>>
>> But then I was wondering why it doesn't need a write lock all the way from machine__findnew_dso_id() to
>> dso__put()? At the moment there are writes to the dso like dso__set_loaded(), dso->nsinfo = nsi and
>> dso__set_build_id(), so another thread could find the dso in a partially constructed state.
>
> I think that's fine, machine__findnew_dso_id takes the dso ref so
> the real 'release' is when the machine object goes down at dsos__purge
>
>>
>> Not sure if this is an issue currently without my patch, but at least with it they would have to be found
>> with header_build_id already set to 1 otherwise it will mess things up.
>
> as for the partial changes I think it's also fine, because it happens
> at separate places.. we'd need to investigate specific example to see
> if there's a problem
>
>>
>> Extending the write lock outside of machine__findnew_dso_id() is difficult because it already
>> releases it before it returns. Does it need to be changed so that machine__findnew_dso_id() takes all the
>> arguments needed to construct it inside the lock?
>
> the change above should do it.. as for top threads I don't think
> it actually matters because there's events processing and hists
> processing, and I did not find them clashing in this.. but there's
> some setup session code that touches this, so it's better to be safe
Ok yes I think I agree. The header flag isn't important for perf top so having it all
constructed in place isn't necessary.
I've submitted v2 with this change.
Thanks for the review.
>
> thanks,
> jirka
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