Re: Subject: [PATCH] sched/fair: prioritize normal task over sched_idle task with vruntime offset

[Peter Zijlstra]

On Fri, Mar 11, 2022 at 03:58:47PM +0800, chenying wrote:
> We add a time offset to the se->vruntime when the idle sched_entity
> is enqueued, so that the idle entity will always be on the right of
> the non-idle in the runqueue. This can allow non-idle tasks to be
> selected and run before the idle.
> 
> A use-case is that sched_idle for background tasks and non-idle
> for foreground. The foreground tasks are latency sensitive and do
> not want to be disturbed by the background. It is well known that
> the idle tasks can be preempted by the non-idle tasks when waking up,
> but will not distinguish between idle and non-idle when pick the next
> entity. This may cause background tasks to disturb the foreground.
> 
> Test results as below:
> 
> ~$ ./loop.sh &
> [1] 764
> ~$ chrt -i 0 ./loop.sh &
> [2] 765
> ~$ taskset -p 04 764
> ~$ taskset -p 04 765
> 
> ~$ top -p 764 -p 765
> top - 13:10:01 up 1 min,  2 users,  load average: 1.30, 0.38, 0.13
> Tasks:   2 total,   2 running,   0 sleeping,   0 stopped,   0 zombie
> %Cpu(s): 12.5 us,  0.0 sy,  0.0 ni, 87.4 id,  0.0 wa,  0.0 hi, 0.0 si,  0.0
> st
> KiB Mem : 16393492 total, 16142256 free,   111028 used,   140208 buff/cache
> KiB Swap:   385836 total,   385836 free,        0 used. 16037992 avail Mem
> 
>   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM TIME+ COMMAND
>   764 chenyin+  20   0   12888   1144   1004 R 100.0  0.0 1:05.12 loop.sh
>   765 chenyin+  20   0   12888   1224   1080 R   0.0  0.0 0:16.21 loop.sh
> 
> The non-idle process (764) can run at 100% and without being disturbed by
> the idle process (765).

Did you just do a very complicated true idle time scheduler, with all
the problems that brings?