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

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Mar 13, 2022 at 01:37:37PM +0800, chenying wrote:
> 在 2022/3/12 20:03, Peter Zijlstra 写道:
> > 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?
> 
> When colocating CPU-intensive jobs with latency-sensitive services can
> improve CPU utilization but it is difficult to meet the stringent
> tail-latency requirements of latency-sensitive services. We use a true idle
> time scheduler for CPU-intensive jobs to minimize the impact on
> latency-sensitive services.

Hard NAK on any true idle-time scheduler until you make the whole kernel
immune to lock holder starvation issues.

And as said; this is a terrible way to do a true idle-time scheduler.