[lxc-users] Enabling real time support in containers

[Peter Steele]

On 03/31/2017 10:16 AM, Peter Steele wrote:
> As you can see, the sched_setscheduler() call fails with an EPERM 
> error. This same app runs fine on the host.
>
> Ultimately I expect this app to fail when run under my container since 
> I have not given the container any real time bandwidth. I had hoped 
> the option
>
> lxc.cgroup.cpu.rt_runtime_us = 475000
>
> would do the trick but this option is rejected with anything other 
> than "0". So presumably this isn't the correct way to give a container 
> real time bandwidth.
>
> I have more experience with the libvirt-lxc framework and I have been 
> able to enable real time support for containers under libvirt. The 
> approach used in this case involves explicitly setting cgroup 
> parameters, specifically
>
> /sys/fs/cgroup/cpu/machine.slice/cpu.rt_runtime_us
>
> under the host and
>
> /sys/fs/cgroup/cpu/cpu.rt_runtime_us
>
> under the container. For example, I might do something like this:
>
> echo 500000 >/sys/fs/cgroup/cpu/machine.slice/cpu.rt_runtime_us      
> --> on the host
> echo 25000 >/sys/fs/cgroup/cpu/cpu.rt_runtime_us      --> on a container
>
> These do not work for LXC based containers though.
>

The test code I'm running can be simplified to just this simple sequence:

#include <stdio.h>
#include <sched.h>

int main() {
     struct sched_param param;
     param.sched_priority = 50;
     const int myself  =  0; // 0 is the PID of ourself
     if (0 != sched_setscheduler(myself, SCHED_FIFO, &param)) {
         printf("Failure\n");
         return -1;
     }

     printf("Success\n");
     return 0;
}

On a container with RT support enabled, this should print "Success".

Am I correct in assuming LXC *does* provide a means to enable RT 
support? If not, we will need to another approach to this problem.

Peter