syscall from userspace thread

syscall from userspace thread

[Ran Shalit]

Hello,

I made reading on RT PREEMPT, but didn't find some rules/methods, to
make sure that a real time thread keeps its deadline.
For example:
 Can I use in the userspace thread any syscall , or will it results in
thread preemption ?

Regards,
Ran

Re: syscall from userspace thread

[Sebastian Andrzej Siewior]

On 2017-02-05 00:06:13 [+0200], Ran Shalit wrote:
> Hello,
Hi,

> I made reading on RT PREEMPT, but didn't find some rules/methods, to
> make sure that a real time thread keeps its deadline.
> For example:
>  Can I use in the userspace thread any syscall , or will it results in
> thread preemption ?

You can use any syscall. The less you use the less of the system will
interfere. Since the RT task runs at elevated priority it will be
preferred over other tasks and it will boost everything in order to get
the locks it needs to perform its job.
If you invoke syscalls which can be slow, like write(), it might block
your RT task until the job (writing to disk) is done and the interrupt
is delivered. This gets worse if you also have an application which does
intensive disk-IO. The IO requests from the RT applications don't have a
VIP sticker and will be handled like everything else.

This is one of reason why the cyclictest application has one non-RT
thread which displays the statistics while another does the `RT' work.

> Regards,
> Ran

Sebastian