Re: [RFC PATCH bpf-next 15/16] tools/bpf: selftests: add dumper progs for bpf_map/task/task_file

Alexei Starovoitov
2020-04-10T03:33:59Z

On Wed, Apr 08, 2020 at 04:25:38PM -0700, Yonghong Song wrote:
> For task/file, the dumper prints out:
> $ cat /sys/kernel/bpfdump/task/file/my1
> tgid gid fd file
> 1 1 0 ffffffff95c97600
> 1 1 1 ffffffff95c97600
> 1 1 2 ffffffff95c97600
> ....
> 1895 1895 255 ffffffff95c8fe00
> 1932 1932 0 ffffffff95c8fe00
> 1932 1932 1 ffffffff95c8fe00
> 1932 1932 2 ffffffff95c8fe00
> 1932 1932 3 ffffffff95c185c0
...
> +SEC("dump//sys/kernel/bpfdump/task/file")
> +int BPF_PROG(dump_tasks, struct task_struct *task, __u32 fd, struct file *file,
> + struct seq_file *seq, u64 seq_num)
> +{
> + static char const banner[] = " tgid gid fd file\n";
> + static char const fmt1[] = "%8d %8d";
> + static char const fmt2[] = " %8d %lx\n";
> +
> + if (seq_num == 0)
> + bpf_seq_printf(seq, banner, sizeof(banner));
> +
> + bpf_seq_printf(seq, fmt1, sizeof(fmt1), task->tgid, task->pid);
> + bpf_seq_printf(seq, fmt2, sizeof(fmt2), fd, (long)file->f_op);
> + return 0;
> +}

I wonder what is the speed of walking all files in all tasks with an empty
program? If it's fast I can imagine a million use cases for such searching bpf
prog. Like finding which task owns particular socket. This could be a massive
feature.

With one redundant spin_lock removed it seems it will be one spin_lock per prog
invocation? May be eventually it can be amortized within seq_file iterating
logic. Would be really awesome if the cost is just refcnt ++/-- per call and
rcu_read_lock.