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

On Sun, Sep 24, 2017 at 1:08 PM, Alexey Dobriyan [off-list ref] wrote:

From: Tatsiana Brouka <redacted>

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