Look up all IDDP/XDDP labels in the registry from userspace

Look up all IDDP/XDDP labels in the registry from userspace

[Sam Daniel]

Is it possible to look up from userspace code all of the IDDP/XDDP labels
that exist in the registry without leaving the primary domain?

I have tried a few different ways of reading the contents of
/proc/xenomai/registry/rtipc/iddp. The inotify API causes mode switches (it
relies on select or poll), not to mention that inotify does not work well
with virtual filesystems. And using the C++ filesystem library to read the
contents of the directory also causes mode switches (filesystem
interactions).

Is there a real-time API function that I can use to query these labels?

Re: Look up all IDDP/XDDP labels in the registry from userspace

[Sam Daniel]

On Mon, Jan 25, 2021 at 12:20 PM Sam Daniel <daniel.jacob.samuel@gmail.com>
wrote:

> Is it possible to look up from userspace code all of the IDDP/XDDP labels
> that exist in the registry without leaving the primary domain?
>
> I have tried a few different ways of reading the contents of
> /proc/xenomai/registry/rtipc/iddp. The inotify API causes mode switches (it
> relies on select or poll), not to mention that inotify does not work well
> with virtual filesystems. And using the C++ filesystem library to read the
> contents of the directory also causes mode switches (filesystem
> interactions).
>
> Is there a real-time API function that I can use to query these labels?
>

C++ filesystem library is causing mode switches because of an mmap deep in
its call stack, not because of "filesystem interactions" like I initially
thought.

Re: Look up all IDDP/XDDP labels in the registry from userspace

[Jan Kiszka]

On 25.01.21 22:07, Sam Daniel via Xenomai wrote:
> On Mon, Jan 25, 2021 at 12:20 PM Sam Daniel <daniel.jacob.samuel@gmail.com>
> wrote:
> 
>> Is it possible to look up from userspace code all of the IDDP/XDDP labels
>> that exist in the registry without leaving the primary domain?
>>
>> I have tried a few different ways of reading the contents of
>> /proc/xenomai/registry/rtipc/iddp. The inotify API causes mode switches (it
>> relies on select or poll), not to mention that inotify does not work well
>> with virtual filesystems. And using the C++ filesystem library to read the
>> contents of the directory also causes mode switches (filesystem
>> interactions).
>>
>> Is there a real-time API function that I can use to query these labels?
>>
> 
> C++ filesystem library is causing mode switches because of an mmap deep in
> its call stack, not because of "filesystem interactions" like I initially
> thought.
> 

What is the use case you need label listing from RT context for?

Jan

-- 
Siemens AG, T RDA IOT
Corporate Competence Center Embedded Linux

Re: Look up all IDDP/XDDP labels in the registry from userspace

[Sam Daniel]

On Tue, Jan 26, 2021 at 10:07 AM Jan Kiszka <jan.kiszka@siemens.com> wrote:

> On 25.01.21 22:07, Sam Daniel via Xenomai wrote:
> > On Mon, Jan 25, 2021 at 12:20 PM Sam Daniel <
> daniel.jacob.samuel@gmail.com>
> > wrote:
> >
> >> Is it possible to look up from userspace code all of the IDDP/XDDP
> labels
> >> that exist in the registry without leaving the primary domain?
> >>
> >> I have tried a few different ways of reading the contents of
> >> /proc/xenomai/registry/rtipc/iddp. The inotify API causes mode switches
> (it
> >> relies on select or poll), not to mention that inotify does not work
> well
> >> with virtual filesystems. And using the C++ filesystem library to read
> the
> >> contents of the directory also causes mode switches (filesystem
> >> interactions).
> >>
> >> Is there a real-time API function that I can use to query these labels?
> >>
> >
> > C++ filesystem library is causing mode switches because of an mmap deep
> in
> > its call stack, not because of "filesystem interactions" like I initially
> > thought.
> >
>
> What is the use case you need label listing from RT context for?
>
> Jan
>
> --
> Siemens AG, T RDA IOT
> Corporate Competence Center Embedded Linux
>

My use case is an IDDP-based pub/sub messaging system for real-time threads.

Threads publish and subscribe to a channel ("xyz"). Any number of threads
can publish to any channel. And any number of threads can subscribe to any
channel (and each one should receive every message published to that
channel).

Subscribers bind labeled IDDP sockets for each channel using labels like
so: <channel name>-<thread name>. If three threads subscribe to channel
"xyz" then I would expect /proc/xenomai/registry/rtipc/iddp to show:
xyz-threadA -> 0
xyz-threadB -> 1
xyz-threadC -> 2

For the pub/sub implementation to work, when thread X calls publish("xyz",
&message) the publish method would need to multicast the message to all
three of the IDDP sockets above.

I would like to look up all labels in the registry from within the publish
method to check if any subscribers have gone away or if any new subscribers
have appeared and then adjust the multicast of the message accordingly.

Re: Look up all IDDP/XDDP labels in the registry from userspace

[Jan Kiszka]

On 26.01.21 20:44, Sam Daniel wrote:
> On Tue, Jan 26, 2021 at 10:07 AM Jan Kiszka <jan.kiszka@siemens.com
> <mailto:jan.kiszka@siemens.com>> wrote:
> 
>     On 25.01.21 22:07, Sam Daniel via Xenomai wrote:
>     > On Mon, Jan 25, 2021 at 12:20 PM Sam Daniel
>     <daniel.jacob.samuel@gmail.com <mailto:daniel.jacob.samuel@gmail.com>>
>     > wrote:
>     >
>     >> Is it possible to look up from userspace code all of the
>     IDDP/XDDP labels
>     >> that exist in the registry without leaving the primary domain?
>     >>
>     >> I have tried a few different ways of reading the contents of
>     >> /proc/xenomai/registry/rtipc/iddp. The inotify API causes mode
>     switches (it
>     >> relies on select or poll), not to mention that inotify does not
>     work well
>     >> with virtual filesystems. And using the C++ filesystem library to
>     read the
>     >> contents of the directory also causes mode switches (filesystem
>     >> interactions).
>     >>
>     >> Is there a real-time API function that I can use to query these
>     labels?
>     >>
>     >
>     > C++ filesystem library is causing mode switches because of an mmap
>     deep in
>     > its call stack, not because of "filesystem interactions" like I
>     initially
>     > thought.
>     >
> 
>     What is the use case you need label listing from RT context for?
> 
>     Jan
> 
>     -- 
>     Siemens AG, T RDA IOT
>     Corporate Competence Center Embedded Linux
> 
> 
> My use case is an IDDP-based pub/sub messaging system for real-time threads.
> 
> Threads publish and subscribe to a channel ("xyz"). Any number of
> threads can publish to any channel. And any number of threads can
> subscribe to any channel (and each one should receive every message
> published to that channel).
> 
> Subscribers bind labeled IDDP sockets for each channel using labels like
> so: <channel name>-<thread name>. If three threads subscribe to channel
> "xyz" then I would expect /proc/xenomai/registry/rtipc/iddp to show:
> xyz-threadA -> 0
> xyz-threadB -> 1
> xyz-threadC -> 2
> 
> For the pub/sub implementation to work, when thread X calls
> publish("xyz", &message) the publish method would need to multicast the
> message to all three of the IDDP sockets above.
> 
> I would like to look up all labels in the registry from within the
> publish method to check if any subscribers have gone away or if any new
> subscribers have appeared and then adjust the multicast of the message
> accordingly.

RTIPC is not designed for such use cases - no multicast support as you
may have noticed already. Polling for labels to appear or disappear
would be a poor man's workaround. Still, you could make it "work": poll
with a non-RT thread for registry changes, update a lock-protected list
of subscribers, and use that list from the publish method in RT context.

That reminds of that simple (but rather domain specific) IPC driver we
once had, Sebastian. ;)

Jan

-- 
Siemens AG, T RDA IOT
Corporate Competence Center Embedded Linux

Re: Look up all IDDP/XDDP labels in the registry from userspace

[Sebastian.Smolorz]

Hi Jan,

Jan Kiszka wrote:
> On 26.01.21 20:44, Sam Daniel wrote:
> > On Tue, Jan 26, 2021 at 10:07 AM Jan Kiszka <jan.kiszka@siemens.com
> > <mailto:jan.kiszka@siemens.com>> wrote:
> > 
> >     On 25.01.21 22:07, Sam Daniel via Xenomai wrote:
> >     > On Mon, Jan 25, 2021 at 12:20 PM Sam Daniel
> >     <daniel.jacob.samuel@gmail.com <mailto:daniel.jacob.samuel@gmail.com>>
> >     > wrote:
> >     >
> >     >> Is it possible to look up from userspace code all of the
> >     IDDP/XDDP labels
> >     >> that exist in the registry without leaving the primary domain?
> >     >>
> >     >> I have tried a few different ways of reading the contents of
> >     >> /proc/xenomai/registry/rtipc/iddp. The inotify API causes mode
> >     switches (it
> >     >> relies on select or poll), not to mention that inotify does not
> >     work well
> >     >> with virtual filesystems. And using the C++ filesystem library to
> >     read the
> >     >> contents of the directory also causes mode switches (filesystem
> >     >> interactions).
> >     >>
> >     >> Is there a real-time API function that I can use to query these
> >     labels?
> >     >>
> >     >
> >     > C++ filesystem library is causing mode switches because of an mmap
> >     deep in
> >     > its call stack, not because of "filesystem interactions" like I
> >     initially
> >     > thought.
> >     >
> > 
> >     What is the use case you need label listing from RT context for?
> > 
> >     Jan
> > 
> >     -- 
> >     Siemens AG, T RDA IOT
> >     Corporate Competence Center Embedded Linux
> > 
> > 
> > My use case is an IDDP-based pub/sub messaging system for real-time threads.
> > 
> > Threads publish and subscribe to a channel ("xyz"). Any number of
> > threads can publish to any channel. And any number of threads can
> > subscribe to any channel (and each one should receive every message
> > published to that channel).
> > 
> > Subscribers bind labeled IDDP sockets for each channel using labels like
> > so: <channel name>-<thread name>. If three threads subscribe to channel
> > "xyz" then I would expect /proc/xenomai/registry/rtipc/iddp to show:
> > xyz-threadA -> 0
> > xyz-threadB -> 1
> > xyz-threadC -> 2
> > 
> > For the pub/sub implementation to work, when thread X calls
> > publish("xyz", &message) the publish method would need to multicast the
> > message to all three of the IDDP sockets above.
> > 
> > I would like to look up all labels in the registry from within the
> > publish method to check if any subscribers have gone away or if any new
> > subscribers have appeared and then adjust the multicast of the message
> > accordingly.
> 
> RTIPC is not designed for such use cases - no multicast support as you
> may have noticed already. Polling for labels to appear or disappear
> would be a poor man's workaround. Still, you could make it "work": poll
> with a non-RT thread for registry changes, update a lock-protected list
> of subscribers, and use that list from the publish method in RT context.
> 
> That reminds of that simple (but rather domain specific) IPC driver we
> once had, Sebastian. ;)

Yeah, but that driver communicates over mailboxes rather than over
topics.

FWIW, it also is used in a really cool product. ;-)

— 
Sebastian 

Re: Look up all IDDP/XDDP labels in the registry from userspace

[Philippe Gerum]

Sam Daniel via Xenomai <xenomai@xenomai.org> writes:

> On Tue, Jan 26, 2021 at 10:07 AM Jan Kiszka <jan.kiszka@siemens.com> wrote:
>
>> On 25.01.21 22:07, Sam Daniel via Xenomai wrote:
>> > On Mon, Jan 25, 2021 at 12:20 PM Sam Daniel <
>> daniel.jacob.samuel@gmail.com>
>> > wrote:
>> >
>> >> Is it possible to look up from userspace code all of the IDDP/XDDP
>> labels
>> >> that exist in the registry without leaving the primary domain?
>> >>
>> >> I have tried a few different ways of reading the contents of
>> >> /proc/xenomai/registry/rtipc/iddp. The inotify API causes mode switches
>> (it
>> >> relies on select or poll), not to mention that inotify does not work
>> well
>> >> with virtual filesystems. And using the C++ filesystem library to read
>> the
>> >> contents of the directory also causes mode switches (filesystem
>> >> interactions).
>> >>
>> >> Is there a real-time API function that I can use to query these labels?
>> >>
>> >
>> > C++ filesystem library is causing mode switches because of an mmap deep
>> in
>> > its call stack, not because of "filesystem interactions" like I initially
>> > thought.
>> >
>>
>> What is the use case you need label listing from RT context for?
>>
>> Jan
>>
>> --
>> Siemens AG, T RDA IOT
>> Corporate Competence Center Embedded Linux
>>
>
> My use case is an IDDP-based pub/sub messaging system for real-time threads.
>
> Threads publish and subscribe to a channel ("xyz"). Any number of threads
> can publish to any channel. And any number of threads can subscribe to any
> channel (and each one should receive every message published to that
> channel).
>
> Subscribers bind labeled IDDP sockets for each channel using labels like
> so: <channel name>-<thread name>. If three threads subscribe to channel
> "xyz" then I would expect /proc/xenomai/registry/rtipc/iddp to show:
> xyz-threadA -> 0
> xyz-threadB -> 1
> xyz-threadC -> 2
>
> For the pub/sub implementation to work, when thread X calls publish("xyz",
> &message) the publish method would need to multicast the message to all
> three of the IDDP sockets above.
>

This is pretty much what the Observable feature does in the EVL core
which we are going to use for Xenomai 4:
https://evlproject.org/core/user-api/observable/

Adding this feature natively to the existing RTIPC driver may be a
better option than emulating it over IDDP.

-- 
Philippe.