netdev - Re: [PATCH 3/3 v4 bpf-next] bpf: optimize task iteration

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Date:   Fri, 18 Dec 2020 13:03:03 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 3/3 v4 bpf-next] bpf: optimize task iteration

On Fri, Dec 18, 2020 at 12:47 PM Jonathan Lemon
<jonathan.lemon@...il.com> wrote:
>
> From: Jonathan Lemon <bsd@...com>
>
> Only obtain the task reference count at the end of the RCU section
> instead of repeatedly obtaining/releasing it when iterating though
> a thread group.
>
> Jump to the correct branch when it is known that the task is NULL.
>
> Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> ---

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>  kernel/bpf/task_iter.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index dc4007f1843b..598a8d7da5bf 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -33,7 +33,7 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
>         pid = find_ge_pid(*tid, ns);
>         if (pid) {
>                 *tid = pid_nr_ns(pid, ns);
> -               task = get_pid_task(pid, PIDTYPE_PID);
> +               task = pid_task(pid, PIDTYPE_PID);
>                 if (!task) {
>                         ++*tid;
>                         goto retry;
> @@ -44,6 +44,7 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
>                         ++*tid;
>                         goto retry;
>                 }
> +               get_task_struct(task);
>         }
>         rcu_read_unlock();
>
> @@ -148,12 +149,12 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info)
>          * it held a reference to the task/files_struct/file.
>          * Otherwise, it does not hold any reference.
>          */
> -again:
>         if (info->task) {
>                 curr_task = info->task;
>                 curr_files = info->files;
>                 curr_fd = info->fd;
>         } else {
> +again:
>                 curr_task = task_seq_get_next(ns, &curr_tid, true);
>                 if (!curr_task) {
>                         info->task = NULL;
> --
> 2.24.1
>