Re: [PATCH 1/2] perf: fix synthesizing mmaps for threads

Jiri Olsa
2014-03-05T16:59:12Z

On Wed, Feb 26, 2014 at 10:45:26AM -0500, Don Zickus wrote:
> Currently if a process creates a bunch of threads using pthread_create
> and then perf is run in system_wide mode, the mmaps for those threads
> are not captured with a synthesized mmap event.
>
> The reason is those threads are not visible when walking the /proc/
> directory looking for /proc/<pid>/maps files. Instead they are discovered
> using the /proc/<pid>/tasks file (which the synthesized comm event uses).
>
> This causes problems when a program is trying to map a data address to a
> tid. Because the tid has no maps, the event is dropped. Changing the program
> to look up using the pid instead of the tid, finds the correct maps but creates
> ugly hacks in the program to carry the correct tid around.

hm, 2 hacks comes to my mind ;-)

1) share 'struct thread::mg' among thread group (pid)

2) or lookup the thread group leader if we find out we are
not the leader and dont have the map info (attached)

your change makes the process map info (same info) being duplicated
for all threads (eventhough it's probably not that much bytes wasted)

I think I'd prefer ad 1) ... the patch for ad 2) assumes there's
always thread group leader (which might not be the case always?)
also 'thread->pid_' handling seems troubled

I dont have code solution for 1), maybe you've already cover that
and considered it hacky.. just throwing ideas ;-)

jirka

diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
index b0f3ca8..c428186 100644
--- a/tools/perf/util/event.c
+++ b/tools/perf/util/event.c
@@ -654,9 +654,17 @@ void thread__find_addr_map(struct thread *thread,
enum map_type type, u64 addr,
struct addr_location *al)
{
- struct map_groups *mg = &thread->mg;
+ struct map_groups *mg;
bool load_map = false;

+ if (thread->tid != thread->pid_) {
+ thread = machine__findnew_thread(machine, thread->pid_, thread->pid_);
+ if (!thread)
+ return;
+ }
+
+ mg = &thread->mg;
+
al->machine = machine;
al->thread = thread;
al->addr = addr;