Re: [1/2,v2] fdmap(2)

[Alexey Dobriyan <adobriyan@gmail.com>]

On Tue, Oct 10, 2017 at 03:08:06PM -0700, Andrei Vagin wrote:
> On Sun, Sep 24, 2017 at 11:06:20PM +0300, Alexey Dobriyan wrote:
> > From: Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@epam.com>
> > 
> > Implement system call for bulk retrieveing of opened descriptors
> > in binary form.
> > 
> > Some daemons could use it to reliably close file descriptors
> > before starting. Currently they close everything upto some number
> > which formally is not reliable. Other natural users are lsof(1) and CRIU
> > (although lsof does so much in /proc that the effect is thoroughly buried).
> 
> Hello Alexey,
> 
> I am not sure about the idea to add syscalls for all sort of process
> attributes. For example, in CRIU we need file descriptors with their
> properties, which we currently get from /proc/pid/fdinfo/. How can
> this interface be extended to achieve our goal?
> 
> Have you seen the task-diag interface what I sent about a year ago?

Of course, let's discuss /proc/task_diag.

Adding it as /proc file is obviously unnecessary: you do it only
to hook ->read and ->write netlink style
(and BTW you don't need .THIS_MODULE anymore ;-)

Transactional netlink send and recv aren't necessary either.
As I understand it, it comes from old times when netlink was async,
so 2 syscalls were neccesary. Netlink is not async anymore.

Basically you want to do sys_task_diag(2) which accepts set of pids
(maybe) and a mask (see statx()) and returns synchronously result into
a buffer.

> We had a discussion on the previous kernel summit how to rework
> task-diag, so that it can be merged into the upstream kernel.
> Unfortunately, I didn't send a summary for this discussion. But it's
> better now than never. We decided to do something like this:
> 
> 1. Add a new syscall readfile(fname, buf, size), which can be
> used to read small files without opening a file descriptor. It will be
> useful for proc files, configs, etc.

If nothing, it should be done because the number of programmers capable
of writing readfile() in userspace correctly handling all errors and
short reads is very small indeed. Out of curiosity I once booted a kernel
which made all reads short by default. It was fascinating I can tell you.

> 2. bin/text/bin conversion is very slow
>  - 65.47% proc_pid_status
>   - 20.81% render_sigset_t
>    - 18.27% seq_printf
>     + 15.77% seq_vprintf
>   - 10.65% task_mem
>     + 8.78% seq_print
>     + 1.02% hugetlb_rep
>   + 7.40% seq_printf
> so a new interface has to use a binary format and the format of netlink
> messages can be used here. It should be possible to extend a file
> without breaking backward compatibility.

Binary -- yes.
netlink attributes -- maybe.

There is statx() model which is perfect for this usecase:
do not want pagecache of all block devices? sure, no problem.

> 3. There are a lot of objection to use a netlink sockets out of the network
> subsystem. The idea of using a "transaction" file looks weird for many
> people, so we decided to add a few files in /proc/pid/. I see
> minimum two files. One file contains information about a task, it is
> mostly what we have in /proc/pid/status and /proc/pid/stat. Another file
> describes a task memory, it is what we have now in /proc/pid/smaps.
> Here is one more major idea. All attributes in a file has to be equal in
> term of performance, or by other words there should not be attributes,
> which significantly affect a generation time of a whole file.
> 
> If we look at /proc/pid/smaps, we spend a lot of time to get memory
> statistics. This file contains a lot of data and if you read it to get
> VmFlags, the kernel will waste your time by generating a useless data
> for you.

There is a unsolvable problem with /proc/*/stat style files. Anyone
who wants to add new stuff has a desicion to make, whether add new /proc
file or extend existing /proc file.

Adding new /proc file means 3 syscalls currently, it surely will become
better with aforementioned readfileat() but even adding tons of symlinks
like this:

	$ readlink /proc/self/affinity
	0f

would have been better -- readlink doesn't open files.

Adding to existing file means _all_ users have to eat the cost as
read(2) doesn't accept any sort of mask to filter data. Most /proc files
are seqfiles now which most of the time internally generates whole buffer
before shipping data to userspace. cat(1) does 32KB read by default
which is bigger than most of files in /proc and stat'ing /proc files is
useless because they're all 0 length. Reliable rewinding to necessary data
is possible only with memchr() which misses the point.

Basically, those sacred text files the Universe consists of suck.

With statx() model the cost of extending result with new data is very
small -- 1 branch to skip generation of data.

I suggest that anyone who dares to improve the situation with process
statistics and anything /proc related uses it as a model.

Of course, I also suggest to freeze /proc for new stuff to press
the issue but one can only dream.