Re: [PATCH v2 2/2] pidmap(2)

[Andy Lutomirski <luto@amacapital.net>]

On Sun, Sep 24, 2017 at 1:08 PM, Alexey Dobriyan <adobriyan@gmail.com> wrote:
> From: Tatsiana Brouka <Tatsiana_Brouka@epam.com>
>
> Implement system call for bulk retrieveing of pids in binary form.
>
> Using /proc is slower than necessary: 3 syscalls + another 3 for each thread +
> converting with atoi() + instantiating dentries and inodes.
>
> /proc may be not mounted especially in containers. Natural extension of
> hidepid=2 efforts is to not mount /proc at all.
>
> It could be used by programs like ps, top or CRIU. Speed increase will
> become more drastic once combined with bulk retrieval of process statistics.
>
> Benchmark:
>
>         N=1<<16 times
>         ~130 processes (~250 task_structs) on a regular desktop system
>         opendir + readdir + closedir /proc + the same for every /proc/$PID/task
>         (roughly what htop(1) does) vs pidmap
>
>         /proc 16.80 ± 0.73%
>         pidmap 0.06 ± 0.31%
>
> PIDMAP_* flags are modelled after /proc/task_diag patchset.
>
>
> PIDMAP(2)                  Linux Programmer's Manual                 PIDMAP(2)
>
> NAME
>        pidmap - get allocated PIDs
>
> SYNOPSIS
>        long pidmap(pid_t pid, int *pids, unsigned int count , unsigned int start, int flags);

I think we will seriously regret a syscall that does this.  Djalal is
working on fixing the turd that is hidepid, and this syscall is
basically incompatible with ever fixing hidepids.  I think that, to
make it less regrettable, it needs to take an fd to a proc mount as a
parameter.  This makes me wonder why it's a syscall at all -- why not
just create a new file like /proc/pids?

--Andy