Re: [PATCH 01/13] kdbus: add documentation

[Greg Kroah-Hartman]

On Tue, Feb 03, 2015 at 08:47:51PM -0600, Eric W. Biederman wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
> 
> > On Tue, Feb 3, 2015 at 2:09 AM, Daniel Mack <daniel@xxxxxxxxxx> wrote:
> >> Hi Andy,
> >>
> >> On 02/02/2015 09:12 PM, Andy Lutomirski wrote:
> >>> On Feb 2, 2015 1:34 AM, "Daniel Mack" <daniel@xxxxxxxxxx> wrote:
> >>
> >>>> That's right, but again - if an application wants to gather this kind of
> >>>> information about tasks it interacts with, it can do so today by looking
> >>>> at /proc or similar sources. Desktop machines do exactly that already,
> >>>> and the kernel code executed in such cases very much resembles that in
> >>>> metadata.c, and is certainly not cheaper. kdbus just makes such
> >>>> information more accessible when requested. Which information is
> >>>> collected is defined by bit-masks on both the sender and the receiver
> >>>> connection, and most applications will effectively only use a very
> >>>> limited set by default if they go through one of the more high-level
> >>>> libraries.
> >>>
> >>> I should rephrase a bit.  Kdbus doesn't require use of send-time
> >>> metadata.  It does, however, strongly encourage it, and it sounds like
> >>
> >> On the kernel level, kdbus just *offers* that, just like sockets offer
> >> SO_PASSCRED. On the userland level, kdbus helps applications get that
> >> information race-free, easier and faster than they would otherwise.
> >>
> >>> systemd and other major users will use send-time metadata.  Once that
> >>> happens, it's ABI (even if it's purely in userspace), and changing it
> >>> is asking for security holes to pop up.  So you'll be mostly stuck
> >>> with it.
> >>
> >> We know we can't break the ABI. At most, we could deprecate item types
> >> and introduce new ones, but we want to avoid that by all means of
> >> course. However, I fail to see how that is related to send time
> >> metadata, or even to kdbus in general, as all ABIs have to be kept stable.
> >
> > I should have said it differently.  ABI is the wrong term -- it's more
> > of a protocol issue.
> >
> > It looks like, with the current code, the kernel will provide
> > (optional) send-time metadata, and the sd-bus library will use it.
> > The result will be that the communication protocol between clients and
> > udev, systemd, systemd-logind, g-s-d, etc, will likely involve
> > send-time metadata.  This may end up being a bottleneck.
> 
> A quick note on a couple of things I have seen in this conversation.
> 
> - The reason for kdbus is performance.

No, that's not the only reason for kdbus, don't focus only on this.  I
set out a long list of things for why we created kdbus, speed was only
one of the things.  Security is also one, and the ability to gather
these attributes in an atomic and secure way is very important as
userspace wants this.

> - pipes rather than unix domain sockets are likely the standard to meet.
>   If you can't equal unix domain sockets for simple things you are
>   likely leaving a lot of stops in.  Last I looked pipes in general were
>   notiably faster than unix domain sockets.
> 
>   The performance numbers I saw posted up-thread were horrible.  I have
>   seen faster numbers across a network of machines.  If your ping-pong
>   latency isn't measured in nano-seconds you are probably doing
>   something wrong.

It all depends on what you are passing on that "ping-pong", a real
D-Bus connection has real data and meta data that has to be sent.
Trying to make a fake benchmark number isn't going to show anything.

> - syscalls remove overhead.  So since performance is kdbus's reason for existence
>   let's remove some ridiculous stops, and get a fast path into the kernel.

Again, not the only reason, see my first post in this thread for
details.

thanks,

greg k-h
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