Currently, task_file iterator iterates all files from all tasks.
This may potentially visit a lot of duplicated files if there are
many tasks sharing the same files, e.g., typical pthreads
where these pthreads and the main thread are sharing the same files.

This patch changed task_file iterator to skip a particular task
if that task shares the same files as its group_leader (the task
having the same tgid and also task->tgid == task->pid).
This will preserve the same result, visiting all files from all
tasks, and will reduce runtime cost significantl, e.g., if there are
a lot of pthreads and the process has a lot of open files.

Suggested-by: Andrii Nakryiko <redacted>
Signed-off-by: Yonghong Song <redacted>
---
  kernel/bpf/task_iter.c | 14 +++++++++-----
  1 file changed, 9 insertions(+), 5 deletions(-)

It would be good if somebody familar with sched code can help check
whether I missed anything or not (e.g., locks, etc.)
for the code change
   task->files == task->group_leader->files

Note the change in this patch might have conflicts with
e60572b8d4c3 ("bpf: Avoid visit same object multiple times")
which is merged into bpf/net sometimes back.

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 232df29793e9..0c5c96bb6964 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c

@@ -22,7 +22,8 @@ struct bpf_iter_seq_task_info {
  };
  
  static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
-					     u32 *tid)
+					     u32 *tid,
+					     bool skip_if_dup_files)
  {
  	struct task_struct *task = NULL;
  	struct pid *pid;

@@ -32,7 +33,10 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
  	pid = idr_get_next(&ns->idr, tid);
  	if (pid) {
  		task = get_pid_task(pid, PIDTYPE_PID);
-		if (!task) {
+		if (!task ||
+		    (skip_if_dup_files &&
+		     task->tgid != task->pid &&
+		     task->files == task->group_leader->files)) {