Re: working with IIO

Re: working with IIO

[Jonathan Cameron]

"Yuniverg, Michael" <michael.yuniverg@intel.com> wrote:
>Hi Jonathan, guys,
>
>My name is Michael Yuniverg and I'm working for Intel.
>You surely noted that starting Kernel 3.7 some of Intel-supported
>sensors were exposed to user space via iio subsystem.
>
>Me and my colleagues (in CC) are working to expose a new generation of
>Intel-supported sensors and naturally we'd like to  keep using iio.
>However we've seen the limitation of just single user-mode client
>allowed to work with a particular iio device:
>/**
>* iio_chrdev_open() - chrdev file open for buffer access and ioctls
>**/
>static int iio_chrdev_open(struct inode *inode, struct file *filp)
>{
>        struct iio_dev *indio_dev = container_of(inode->i_cdev,
>                                               struct iio_dev, chrdev);
>
>        if (test_and_set_bit(IIO_BUSY_BIT_POS, &indio_dev->flags))
>                return -EBUSY;
>
>        filp->private_data = indio_dev;
>
>        return 0;
>}
>
>This limitation is really painful for our design that is striving to
>achieve better performance moving some of sensors logic to Kernel mode.
>Now please Jonathan, could you explain the ratio behind this
>limitation?

Simplicity and hence speed plus where all this evolved from which was data logging. Technically there is nothing stopping us having a separate buffer per user as we already allow multiple buffers to be fed from the same device for different purposes. This is how the bridge to input functions (not yet in mainline)

What is your application which needs multiple simultaneous buffered users? 

As a quick thought I would not necessarily be against having the ability to request additional chrdevs each with their own buffers. A bit fiddly to retrofit though as not breaking abi would mean we need to keep the existing interfaces as they are.


>And in general - could you share with us your plans of future
>modifications to  iio

Lots though honestly most of the interesting bits are not my ideas or plans but rather those of others.

Personally I am still working through a load of core changes that are not going to change anything fundamental.

Of course feel free to propose any changes you would like!
>
>Thanks in advance,
>Michael
>
>
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RE: working with IIO

[Drubin, Daniel]

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Re: working with IIO

[Lars-Peter Clausen]

On 08/22/2013 01:30 PM, Drubin, Daniel wrote:
> Hi Jonathan,
> 
> I am Daniel and I work with Michael.
> 
> [...]
>>> This limitation is really painful for our design that is striving to
>>> achieve better performance moving some of sensors logic to Kernel mode.
>>> Now please Jonathan, could you explain the ratio behind this
>>> limitation?
>>
>> Simplicity and hence speed plus where all this evolved from which was data
>> logging. Technically there is nothing stopping us having a separate buffer per
>> user as we already allow multiple buffers to be fed from the same device for
>> different purposes. This is how the bridge to input functions (not yet in
>> mainline)
>>
>> What is your application which needs multiple simultaneous buffered users?
> 
> We need multiple higher level clients to be able to configure the same sensor for different rates and receive data at different rates, independently of each other. I.e. device will sense at maximum of configured rates, and clients will poll data as if their configured rates. This is mainly for Android, where there are multiple sensor frameworks independent of each other.
> 
>> As a quick thought I would not necessarily be against having the ability to
>> request additional chrdevs each with their own buffers. A bit fiddly to retrofit
>> though as not breaking abi would mean we need to keep the existing
>> interfaces as they are.
> 
> We actually thought about registering multiple "virtual" sensors for the same physical device in order to stick it into existing IIO, but that has limited use for us (in particular, we will have to only pre-register statically those multiple virtual sensors).
> 
>>> And in general - could you share with us your plans of future
>>> modifications to  iio
> [...]
>> Of course feel free to propose any changes you would like!
> 
> As of now we would like to propose an option for IIO device to allow multiple opens()s. Without that additional option devices will work as today, so that, for example, existing sensor drivers will not have to be modified for reentrancy; but those drivers that need it will signal with that option that they will handle reentrancy themselves.

How about implementing a userspace daemon which does the arbitration between
multiple users? Doing this in kernel space can get tricky, especially if you
want to allow concurrent users with different settings, e.g. sample rate.
This is in part due to the majority of the IIO ABI being stateless and this
doesn't really mix well with concurrent users. Having a daemon will allow
you to implement a stateful API on top of the stateless IIO userspace ABI.
If you go the kernel route though I'm pretty sure you'll run into lots of
problems without an immediate solution.

- Lars

RE: working with IIO

[Drubin, Daniel]

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cGllcy4K

Re: working with IIO

[Lars-Peter Clausen]

On 08/22/2013 03:39 PM, Drubin, Daniel wrote:
> [...]
>>> As of now we would like to propose an option for IIO device to allow
>> multiple opens()s. Without that additional option devices will work as today,
>> so that, for example, existing sensor drivers will not have to be modified for
>> reentrancy; but those drivers that need it will signal with that option that
>> they will handle reentrancy themselves.
>>
>> How about implementing a userspace daemon which does the arbitration
>> between multiple users? Doing this in kernel space can get tricky, especially if
>> you want to allow concurrent users with different settings, e.g. sample rate.
>> This is in part due to the majority of the IIO ABI being stateless and this
>> doesn't really mix well with concurrent users. Having a daemon will allow you
>> to implement a stateful API on top of the stateless IIO userspace ABI.
>> If you go the kernel route though I'm pretty sure you'll run into lots of
>> problems without an immediate solution.
> 
> That's the direction in which we are currently advancing. Not because we are afraid of kernel-mode problems - after all they are very similar to what kernel-mode filesystem driver faces when serving multiple processes accessing the same FS, just less complicated (e.g. no writers); but mainly because we want to use existing IIO as framework.
> 

The problem is that the IIO ABI is stateless, so either you need to add some
very crude hacks on-top of it to allow this or you'd have to throw  away the
current ABI and develop a IIOv2. The userspace daemon is in my opinion
preferable to both cases.

> The main drawback that we see in user-mode daemon is performance. Consider sequence of events between the daemon and the caller process:
> 
> - Caller process invokes some sort of RPC via socket/pipe/message queue [system call, context switch]
> - Daemon receives request message [system call]
> - Daemon pushes sample data through IPC [system call, data copy, context switch]
> - Caller pops data off IPC [system call, data copy]
> 
> I.e. there are 4 system calls, 2 context switches and 2 data copies added solely for the purpose of arbitration for EACH client, even for sensors not currently shared.

If done right the overhead should hopefully be negligible. E.g. right now we
do not have mmap support for IIO, but this is something that will be
implemented sooner or later (probably sooner than later). I think we should
take a look at how ALSA does these things. There we also have no in-kernel
multiplexing or mixing and things are handled by a userspace daemon.

> 
> BTW (not directly related), I've read somewhere that on some system IIO did up to 200M samples per second. Is it true? If yes, how such data rate was achieved?

Yes, but not continuous streaming. This is implemented by sampling at 200
MHz, put data into a buffer, stop sampling and then let userspace read the
buffer and after that start again.

- Lars

RE: working with IIO

[Drubin, Daniel]

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Re: working with IIO

[Lars-Peter Clausen]

On 08/22/2013 04:45 PM, Drubin, Daniel wrote:
> 
> 
>> -----Original Message-----
>> From: Lars-Peter Clausen [mailto:lars@metafoo.de]
>> Sent: Thursday, August 22, 2013 5:17 PM
>> To: Drubin, Daniel
>> Cc: Jonathan Cameron; Yuniverg, Michael; linux-iio@vger.kernel.org;
>> Haimovich, Yoav
>> Subject: Re: working with IIO
>>
>> On 08/22/2013 03:39 PM, Drubin, Daniel wrote:
>>> [...]
>>>>> As of now we would like to propose an option for IIO device to allow
>>>> multiple opens()s. Without that additional option devices will work
>>>> as today, so that, for example, existing sensor drivers will not have
>>>> to be modified for reentrancy; but those drivers that need it will
>>>> signal with that option that they will handle reentrancy themselves.
>>>>
>>>> How about implementing a userspace daemon which does the arbitration
>>>> between multiple users? Doing this in kernel space can get tricky,
>>>> especially if you want to allow concurrent users with different settings,
>> e.g. sample rate.
>>>> This is in part due to the majority of the IIO ABI being stateless
>>>> and this doesn't really mix well with concurrent users. Having a
>>>> daemon will allow you to implement a stateful API on top of the stateless
>> IIO userspace ABI.
>>>> If you go the kernel route though I'm pretty sure you'll run into
>>>> lots of problems without an immediate solution.
>>>
>>> That's the direction in which we are currently advancing. Not because we
>> are afraid of kernel-mode problems - after all they are very similar to what
>> kernel-mode filesystem driver faces when serving multiple processes
>> accessing the same FS, just less complicated (e.g. no writers); but mainly
>> because we want to use existing IIO as framework.
>>>
>>
>> The problem is that the IIO ABI is stateless, so either you need to add some
>> very crude hacks on-top of it to allow this or you'd have to throw  away the
>> current ABI and develop a IIOv2. The userspace daemon is in my opinion
>> preferable to both cases.
> 
> From practical POV we don't have much choice (timeline), since we have to reuse driver that is bound to IIO. From principle standpoint I somehow fail to see a problem. It seems to me that all state handling that an IIO driver needs to do is to keep associations of PIDs to sensor rates, configure sensor to the highest rate in the list and replicate shared data at rates requested by the clients. When a file descriptor is closed (due to process termination or another reasons), the actual sensor is re-configured with next-highest rate among the open FDs.

But you can't track the configured rate per PID with the current API. That's
why I keep saying that the API is stateless. You can not track state per
application without inventing a new API.

- Lars

Re: working with IIO

[Jonathan Cameron]

On 22/08/13 15:52, Lars-Peter Clausen wrote:
> On 08/22/2013 04:45 PM, Drubin, Daniel wrote:
>>
>>
>>> -----Original Message-----
>>> From: Lars-Peter Clausen [mailto:lars@metafoo.de]
>>> Sent: Thursday, August 22, 2013 5:17 PM
>>> To: Drubin, Daniel
>>> Cc: Jonathan Cameron; Yuniverg, Michael; linux-iio@vger.kernel.org;
>>> Haimovich, Yoav
>>> Subject: Re: working with IIO
>>>
>>> On 08/22/2013 03:39 PM, Drubin, Daniel wrote:
>>>> [...]
>>>>>> As of now we would like to propose an option for IIO device to allow
>>>>> multiple opens()s. Without that additional option devices will work
>>>>> as today, so that, for example, existing sensor drivers will not have
>>>>> to be modified for reentrancy; but those drivers that need it will
>>>>> signal with that option that they will handle reentrancy themselves.
>>>>>
>>>>> How about implementing a userspace daemon which does the arbitration
>>>>> between multiple users? Doing this in kernel space can get tricky,
>>>>> especially if you want to allow concurrent users with different settings,
>>> e.g. sample rate.
>>>>> This is in part due to the majority of the IIO ABI being stateless
>>>>> and this doesn't really mix well with concurrent users. Having a
>>>>> daemon will allow you to implement a stateful API on top of the stateless
>>> IIO userspace ABI.
>>>>> If you go the kernel route though I'm pretty sure you'll run into
>>>>> lots of problems without an immediate solution.
>>>>
>>>> That's the direction in which we are currently advancing. Not because we
>>> are afraid of kernel-mode problems - after all they are very similar to what
>>> kernel-mode filesystem driver faces when serving multiple processes
>>> accessing the same FS, just less complicated (e.g. no writers); but mainly
>>> because we want to use existing IIO as framework.
>>>>
>>>
>>> The problem is that the IIO ABI is stateless, so either you need to add some
>>> very crude hacks on-top of it to allow this or you'd have to throw  away the
>>> current ABI and develop a IIOv2. The userspace daemon is in my opinion
>>> preferable to both cases.
>>
>>  From practical POV we don't have much choice (timeline), since we have to reuse driver that is bound to IIO. From principle standpoint I somehow fail to see a problem. It seems to me that all state handling that an IIO driver needs to do is to keep associations of PIDs to sensor rates, configure sensor to the highest rate in the list and replicate shared data at rates requested by the clients. When a file descriptor is closed (due to process termination or another reasons), the actual sensor is re-configured with next-highest rate among the open FDs.
>
> But you can't track the configured rate per PID with the current API. That's
> why I keep saying that the API is stateless. You can not track state per
> application without inventing a new API.

The fastest way of doing this that I can see would be to allow 
initialization of multiple buffers per device (not actually that
hard to do given we have the multiple stream demux there for other 
reasons) and allow subsampling of the datastream in the demux.  Note 
that there are no assumptions that the triggering will be fixed
frequency, hence it could only be done as a filter on whatever comes up 
rather than explicit frequency requests.

Does v4l handle multiple video streams to userspace at different
frame rates?  That is what we are talking about here effectively.

The few times this has come up before we have always concluded that
it is better done in userspace, but there is some infrastructure in
place now that would make it 'not truly horrible' to do some of the
work in kernel.

Jonathan

RE: working with IIO

[Drubin, Daniel]

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RE: working with IIO

[Drubin, Daniel]

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Re: working with IIO

[Lars-Peter Clausen]

On 08/22/2013 05:16 PM, Drubin, Daniel wrote:
> [...]
>>>  From practical POV we don't have much choice (timeline), since we have to
>> reuse driver that is bound to IIO. From principle standpoint I somehow fail to
>> see a problem. It seems to me that all state handling that an IIO driver needs
>> to do is to keep associations of PIDs to sensor rates, configure sensor to the
>> highest rate in the list and replicate shared data at rates requested by the
>> clients. When a file descriptor is closed (due to process termination or
>> another reasons), the actual sensor is re-configured with next-highest rate
>> among the open FDs.
>>
>> But you can't track the configured rate per PID with the current API. That's
>> why I keep saying that the API is stateless. You can not track state per
>> application without inventing a new API.
>
> Why can't I during keep a list of PIDs that currently use a sensor and record current->pid together with "default" rate during the first sampling request that doesn't have a matching PID, and in write_raw() handler that updates rate match that current->pid against list of recorded PIDs? I didn't see a possibility that sensor driver's handler may get called in a different context than IIO core fops handler.

So each time a process writes to a IIO sysfs file you want to record which 
value that application wrote. So when I run `for i in `seq 0 100000`; do echo 
$i > sampling_frequency; done` I'd end up with a list with one million entries 
which will stay in the list forever.

- Lars

RE: working with IIO

[Drubin, Daniel]

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Re: working with IIO

[Lars-Peter Clausen]

On 08/22/2013 05:48 PM, Drubin, Daniel wrote:
>
>
>> -----Original Message-----
>> From: Lars-Peter Clausen [mailto:lars@metafoo.de]
>> Sent: Thursday, August 22, 2013 6:42 PM
>> To: Drubin, Daniel
>> Cc: Jonathan Cameron; Yuniverg, Michael; linux-iio@vger.kernel.org;
>> Haimovich, Yoav
>> Subject: Re: working with IIO
>>
>> On 08/22/2013 05:16 PM, Drubin, Daniel wrote:
>>> [...]
>>>>>   From practical POV we don't have much choice (timeline), since we
>>>>> have to
>>>> reuse driver that is bound to IIO. From principle standpoint I
>>>> somehow fail to see a problem. It seems to me that all state handling
>>>> that an IIO driver needs to do is to keep associations of PIDs to
>>>> sensor rates, configure sensor to the highest rate in the list and
>>>> replicate shared data at rates requested by the clients. When a file
>>>> descriptor is closed (due to process termination or another reasons),
>>>> the actual sensor is re-configured with next-highest rate among the open
>> FDs.
>>>>
>>>> But you can't track the configured rate per PID with the current API.
>>>> That's why I keep saying that the API is stateless. You can not track
>>>> state per application without inventing a new API.
>>>
>>> Why can't I during keep a list of PIDs that currently use a sensor and record
>> current->pid together with "default" rate during the first sampling request
>> that doesn't have a matching PID, and in write_raw() handler that updates
>> rate match that current->pid against list of recorded PIDs? I didn't see a
>> possibility that sensor driver's handler may get called in a different context
>> than IIO core fops handler.
>>
>> So each time a process writes to a IIO sysfs file you want to record which
>> value that application wrote. So when I run `for i in `seq 0 100000`; do echo $i
>>> sampling_frequency; done` I'd end up with a list with one million entries
>> which will stay in the list forever.
>
> No, there is only one entry per PID. Next value that the same process writes will replace the previous one, not create a new entry. An entry will be create only if the write request arrived from a PID currently not in list.
>

Assume that echo is a /bin/echo, not a shell built-in command.

- Lars

RE: working with IIO

[Jonathan Cameron]

"Drubin, Daniel" <daniel.drubin@intel.com> wrote:
>[...]
>> Does v4l handle multiple video streams to userspace at different
>frame
>> rates?  That is what we are talking about here effectively.
>
>I actually mentioned v4l in context of mmap().
Fair enough. Indeed mmap support is a great feature as is splice support for other uses.
Honestly I was originally waiting for the tracing guys to produce the magic lockless ring buffer that does all of this.  Been a long time since I picked up on anything about that though!
 Not for replicating the
>same mmap()ed data for multiple clients of course, but IIRC yes, v4l
>allows multiple channels to be opened. E.g. for peeking full video,
>preview and teletext or for switching between video sources on the same
>grabber. Making the whole major node singleton is a bit harsh
>restriction IMHO.
Sure to that but here equivalent is opening main stream an pulling out different frame rates. Equivalent of your example is a multiple sample rate hardware device. Those are handled using multiple instances of iio_Dev.
>
>Best regards,
>Daniel
>---------------------------------------------------------------------
>Intel Israel (74) Limited
>
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RE: working with IIO

[Drubin, Daniel]

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RE: working with IIO

[Drubin, Daniel]

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Re: working with IIO

[Lars-Peter Clausen]

On 08/22/2013 06:26 PM, Drubin, Daniel wrote:
> 
> 
>> -----Original Message-----
>> From: Lars-Peter Clausen [mailto:lars@metafoo.de]
>> Sent: Thursday, August 22, 2013 7:00 PM
>> To: Drubin, Daniel
>> Cc: Jonathan Cameron; Yuniverg, Michael; linux-iio@vger.kernel.org;
>> Haimovich, Yoav
>> Subject: Re: working with IIO
>>
>> On 08/22/2013 05:48 PM, Drubin, Daniel wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Lars-Peter Clausen [mailto:lars@metafoo.de]
>>>> Sent: Thursday, August 22, 2013 6:42 PM
>>>> To: Drubin, Daniel
>>>> Cc: Jonathan Cameron; Yuniverg, Michael; linux-iio@vger.kernel.org;
>>>> Haimovich, Yoav
>>>> Subject: Re: working with IIO
>>>>
>>>> On 08/22/2013 05:16 PM, Drubin, Daniel wrote:
>>>>> [...]
>>>>>>>   From practical POV we don't have much choice (timeline), since
>>>>>>> we have to
>>>>>> reuse driver that is bound to IIO. From principle standpoint I
>>>>>> somehow fail to see a problem. It seems to me that all state
>>>>>> handling that an IIO driver needs to do is to keep associations of
>>>>>> PIDs to sensor rates, configure sensor to the highest rate in the
>>>>>> list and replicate shared data at rates requested by the clients.
>>>>>> When a file descriptor is closed (due to process termination or
>>>>>> another reasons), the actual sensor is re-configured with
>>>>>> next-highest rate among the open
>>>> FDs.
>>>>>>
>>>>>> But you can't track the configured rate per PID with the current API.
>>>>>> That's why I keep saying that the API is stateless. You can not
>>>>>> track state per application without inventing a new API.
>>>>>
>>>>> Why can't I during keep a list of PIDs that currently use a sensor
>>>>> and record
>>>> current->pid together with "default" rate during the first sampling
>>>> current->request
>>>> that doesn't have a matching PID, and in write_raw() handler that
>>>> updates rate match that current->pid against list of recorded PIDs? I
>>>> didn't see a possibility that sensor driver's handler may get called
>>>> in a different context than IIO core fops handler.
>>>>
>>>> So each time a process writes to a IIO sysfs file you want to record
>>>> which value that application wrote. So when I run `for i in `seq 0
>>>> 100000`; do echo $i
>>>>> sampling_frequency; done` I'd end up with a list with one million
>>>>> entries
>>>> which will stay in the list forever.
>>>
>>> No, there is only one entry per PID. Next value that the same process
>> writes will replace the previous one, not create a new entry. An entry will be
>> create only if the write request arrived from a PID currently not in list.
>>>
>>
>> Assume that echo is a /bin/echo, not a shell built-in command.
> 
> Then indeed a new entry will be created 100000 times. But before creating a new instance of /bin/echo, the previous one will terminate closing all file descriptors. A device driver would miss this event and thus an ability to remove PID from list only if the framework for some reason chose to ban it from knowing.

Which file descriptor? That of the sample_frequency sysfs file? Please try
to think this through. This approach is in my opinion a bad idea, if it is
to ever work at all, its implementation will be really ugly and its
semantics will probably be bogus. If you are on a tight deadline don't try
run down a dead-end first, but rather try to take the proper road (userspace
daemon).

- Lars

RE: working with IIO

[Jonathan Cameron]

"Drubin, Daniel" <daniel.drubin@intel.com> wrote:
>[...]
>>  Not for replicating the
>> >same mmap()ed data for multiple clients of course, but IIRC yes, v4l
>> >allows multiple channels to be opened. E.g. for peeking full video,
>> >preview and teletext or for switching between video sources on the
>same
>> >grabber. Making the whole major node singleton is a bit harsh
>> >restriction IMHO.
>> Sure to that but here equivalent is opening main stream an pulling
>out
>> different frame rates. Equivalent of your example is a multiple
>sample rate
>> hardware device. Those are handled using multiple instances of
>iio_Dev.
>
>We actually considered creating a separate iio_dev per "virtual" sensor
>(pair of {sensor, rate}). The problem is, they can pop-up dynamically
>and even if we opened a backdoor interface of silently creating new
>chardevs, we can't eat up all major numbers in the system. And we can
>get dangerously close to that :-(

Not a problem as can create anonymous ones like we already do for events.

Now how you control which channels go where is harder as normally we do this sysfs and I would be very dubious about changing that.  It could obviously be done but I can't think of a clean way of doing it.

Just how many readers are we talking?
>
>Best regards,
>Daniel
>
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>Intel Israel (74) Limited
>
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RE: working with IIO

[Jonathan Cameron]

Jonathan Cameron <jic23@kernel.org> wrote:
>
>
>"Drubin, Daniel" <daniel.drubin@intel.com> wrote:
>>[...]
>>>  Not for replicating the
>>> >same mmap()ed data for multiple clients of course, but IIRC yes,
>v4l
>>> >allows multiple channels to be opened. E.g. for peeking full video,
>>> >preview and teletext or for switching between video sources on the
>>same
>>> >grabber. Making the whole major node singleton is a bit harsh
>>> >restriction IMHO.
>>> Sure to that but here equivalent is opening main stream an pulling
>>out
>>> different frame rates. Equivalent of your example is a multiple
>>sample rate
>>> hardware device. Those are handled using multiple instances of
>>iio_Dev.
>>
>>We actually considered creating a separate iio_dev per "virtual"
>sensor
>>(pair of {sensor, rate}). The problem is, they can pop-up dynamically
>>and even if we opened a backdoor interface of silently creating new
>>chardevs, we can't eat up all major numbers in the system. And we can
>>get dangerously close to that :-(
>
>Not a problem as can create anonymous ones like we already do for
>events.
>
>Now how you control which channels go where is harder as normally we do
>this sysfs and I would be very dubious about changing that.  It could
>obviously be done but I can't think of a clean way of doing it.
>
>Just how many readers are we talking?

Note I agree with Lars that this probably better done in user space. I just find the idea of doing it in kernel space interesting!
>>
>>Best regards,
>>Daniel
>>
>>---------------------------------------------------------------------
>>Intel Israel (74) Limited
>>
>>This e-mail and any attachments may contain confidential material for
>>the sole use of the intended recipient(s). Any review or distribution
>>by others is strictly prohibited. If you are not the intended
>>recipient, please contact the sender and delete all copies.
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Re: working with IIO

[Alexander Holler]

Am 22.08.2013 18:26, schrieb Drubin, Daniel:

>>>> So each time a process writes to a IIO sysfs file you want to record
>>>> which value that application wrote. So when I run `for i in `seq 0
>>>> 100000`; do echo $i
>>>>> sampling_frequency; done` I'd end up with a list with one million
>>>>> entries
>>>> which will stay in the list forever.
>>>
>>> No, there is only one entry per PID. Next value that the same process
>> writes will replace the previous one, not create a new entry. An entry will be
>> create only if the write request arrived from a PID currently not in list.
>>>
>>
>> Assume that echo is a /bin/echo, not a shell built-in command.
> 
> Then indeed a new entry will be created 100000 times. But before creating a new instance of /bin/echo, the previous one will terminate closing all file descriptors. A device driver would miss this event and thus an ability to remove PID from list only if the framework for some reason chose to ban it from knowing.

Try

sysctl kernel.pid_max

So the list would be likely smaller and reused PIDs will end up with a
funny behaviour. ;)

Regards,

Alexander Holler