Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature

[Daniel Bristot de Oliveira]

On 11/07/2016 09:16 PM, Steven Rostedt wrote:
> I'm confused? Are you saying that RR tasks don't get throttled in the
> current code? That sounds like a bug to me.

If the RT_RUNTIME_SHARING is enabled, the CPU in which the RR tasks are
running (and pinned) will borrow RT runtime from another CPU, allowing
the RR tasks to run forever. For example:

[root@kiron debug]# cat /proc/sched_debug | grep rt_runtime
  .rt_runtime                    : 950.000000
  .rt_runtime                    : 950.000000
  .rt_runtime                    : 950.000000
  .rt_runtime                    : 950.000000
[root@kiron debug]# echo RT_RUNTIME_SHARE > sched_features
[root@kiron debug]# taskset -c 2 chrt -r 5 /home/bristot/f &
[1] 23908
[root@kiron debug]# taskset -c 2 chrt -r 5 /home/bristot/f &
[2] 23915
[root@kiron debug]# cat /proc/sched_debug | grep rt_runtime
  .rt_runtime                    : 900.000000
  .rt_runtime                    : 950.000000
  .rt_runtime                    : 1000.000000
  .rt_runtime                    : 950.000000

You see? the rt_runtime of the CPU 2 was borrowed time from CPU 0.

It is not a BUG but a feature (no jokes haha). With RT_RUNTIME_SHARE,
the rt_runtime is such a global runtime. It works fine for tasks that
can migrate... but that is not the case for per-cpu kworkers.

-- Daniel