Re: [PATCH 1/1] perf: Set build-id using build-id header on new mmap records

[Jiri Olsa]

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@xxxxxxxxxxxx>
> >> Signed-off-by: James Clark <james.clark@xxxxxxx>
> >> ---
> >>  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

thanks,
jirka