Re: [PATCH bpf-next v2 1/2] bpf: avoid iterating duplicated files for task_file iterator

[Josef Bacik <josef@toxicpanda.com>]

On 8/28/20 1:38 AM, Yonghong Song wrote:
> 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 <andriin@fb.com>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>   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)) {

This is fine, we're not deref'ing files, if we were you'd need 
get_files_struct().  You can deref task->group_leader here because you got the 
task so this is safe.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef