Re: [PATCH/RFC] perf buildid: Cache kernel DSO created when reading buildid header table

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Sep 25, 2015 at 12:01:12AM +0900, Namhyung Kim escreveu:
> Hi Arnaldo,
> 
> On Thu, Sep 24, 2015 at 10:42:28AM -0300, Arnaldo Carvalho de Melo wrote:
> > Namhyung,
> > 
> > 	Can you take a look and perhaps give me your Acked-by?
> 
> The dso->kernel and event->cpumode is always confusing for modules..

Right, my idea is to delete dso->kernel, to remove this confusion.

Please see my tmp.perf/core branch, there are a few patches there using
alternative methods in places previously using the botched dso->kernel.

> In this case, it seems that mmap events set kernel cpumode but
> build-id events don't.

in the previous sentence you think build-id events don't set
perf_event_attr.misc bits related to cpumode...

> So kernel cpumode in a bulid-id event
> indicates that it is a kernel (vmlinux) dso, right?

While here you ask if it does?

All these are synthesized, it is a matter of looking at the routines
synthesizing them :-)

> I'll test this tomorrow..

Thanks, over time there were areas where multiple people touched and
added different semantics that are biting now, I'm trying to, while
fixing a bug, the one Wang Nan reported, clarify those things.

It is taking more time than I antecipated, as I'm stumbling in what look
like other bugs in the perf-probe codebase :-/

- Arnaldo
 
> Thanks,
> Namhyung
> 
> 
> > 
> > From: Arnaldo Carvalho de Melo <acme@redhat.com>
> > 
> > No need to traverse it all again using the filename extension to
> > disambiguate kernel from kernel modules, just cache it when
> > reading the buildid table.
> > 
> > This also fixes a refcount bug in the error path, i.e. in the error path
> > for the __machine__create_kernel_maps() call we do a dso__put(), which
> > is ok and expected if that dso came from machine__findnew_dso(), but
> > wasn't when we found it by traversing the machine dso list without
> > grabbing a dso refcount (dso__get()) before dropping the list lock.
> > 
> > Fixes: b837a8bdc489 ("perf tools: Fix build-id matching on vmlinux")
> > Link: http://lkml.kernel.org/n/tip-48fh3cf6pvs4zs2fj4nhc4b5@git.kernel.org
> > Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> > ---
> >  tools/perf/util/header.c  |  7 +++++++
> >  tools/perf/util/machine.c | 34 +++-------------------------------
> >  tools/perf/util/machine.h |  1 +
> >  3 files changed, 11 insertions(+), 31 deletions(-)
> > 
> > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > index 43838003c1a1..6bc92ae1e99a 100644
> > --- a/tools/perf/util/header.c
> > +++ b/tools/perf/util/header.c
> > @@ -1260,6 +1260,13 @@ static int __event_process_build_id(struct build_id_event *bev,
> >  
> >  		if (!is_kernel_module(filename, cpumode))
> >  			dso->kernel = dso_type;
> > +		/*
> > +		 * We don't need to grab a reference as long as
> > +		 * the kernel dso is in the machine dso list.
> > +		 */
> > +		if (cpumode == PERF_RECORD_MISC_KERNEL ||
> > +		    cpumode == PERF_RECORD_MISC_GUEST_KERNEL)
> > +			machine->kernel_dso = dso;
> >  
> >  		build_id__sprintf(dso->build_id, sizeof(dso->build_id),
> >  				  sbuild_id);
> > diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> > index d07a38678e14..f6e689b2c83c 100644
> > --- a/tools/perf/util/machine.c
> > +++ b/tools/perf/util/machine.c
> > @@ -62,6 +62,7 @@ int machine__init(struct machine *machine, const char *root_dir, pid_t pid)
> >  	}
> >  
> >  	machine->current_tid = NULL;
> > +	machine->kernel_dso = NULL;
> >  
> >  	return 0;
> >  }
> > @@ -1180,39 +1181,10 @@ static int machine__process_kernel_mmap_event(struct machine *machine,
> >  		 * Should be there already, from the build-id table in
> >  		 * the header.
> >  		 */
> > -		struct dso *kernel = NULL;
> > -		struct dso *dso;
> > +		struct dso *kernel;
> >  
> >  		pthread_rwlock_rdlock(&machine->dsos.lock);
> > -
> > -		list_for_each_entry(dso, &machine->dsos.head, node) {
> > -
> > -			/*
> > -			 * The cpumode passed to is_kernel_module is not the
> > -			 * cpumode of *this* event. If we insist on passing
> > -			 * correct cpumode to is_kernel_module, we should
> > -			 * record the cpumode when we adding this dso to the
> > -			 * linked list.
> > -			 *
> > -			 * However we don't really need passing correct
> > -			 * cpumode.  We know the correct cpumode must be kernel
> > -			 * mode (if not, we should not link it onto kernel_dsos
> > -			 * list).
> > -			 *
> > -			 * Therefore, we pass PERF_RECORD_MISC_CPUMODE_UNKNOWN.
> > -			 * is_kernel_module() treats it as a kernel cpumode.
> > -			 */
> > -
> > -			if (!dso->kernel ||
> > -			    is_kernel_module(dso->long_name,
> > -					     PERF_RECORD_MISC_CPUMODE_UNKNOWN))
> > -				continue;
> > -
> > -
> > -			kernel = dso;
> > -			break;
> > -		}
> > -
> > +		kernel = dso__get(machine->kernel_dso);
> >  		pthread_rwlock_unlock(&machine->dsos.lock);
> >  
> >  		if (kernel == NULL)
> > diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
> > index 9dfc4281f940..d7b044a1046f 100644
> > --- a/tools/perf/util/machine.h
> > +++ b/tools/perf/util/machine.h
> > @@ -38,6 +38,7 @@ struct machine {
> >  	struct dsos	  dsos;
> >  	struct map_groups kmaps;
> >  	struct map	  *vmlinux_maps[MAP__NR_TYPES];
> > +	struct dso	  *kernel_dso;
> >  	u64		  kernel_start;
> >  	symbol_filter_t	  symbol_filter;
> >  	pid_t		  *current_tid;
> > -- 
> > 2.1.0
> >