Re: [PATCH] sched/fair: avoid vruntime compensation for SCHED_IDLE task

[Vincent Guittot]

On Sun, 23 Aug 2020 at 09:33, Jiang Biao <benbjiang@xxxxxxxxx> wrote:
>
> Hi, Vincent and Peter
>
> On Thu, 20 Aug 2020 at 22:09, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> >
> > On Thu, 20 Aug 2020 at 15:44, <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > > That's been said, not compensating the vruntime for a sched_idle task
> > > > makes sense for me. Even if that will only help for others task in the
> > > > same cfs_rq
> > >
> > > Yeah, but it is worth the extra pointer chasing and branches?
> >
> > For that I let Jiang provides figures to show the worthful
> Using the following configuration for rt-app,
> {
>         "tasks" : {
>                 "task_other" : {
>                         "instance" : 1, //only 1 instance to be easy to observe
>                         "cpus" : [2],
>                         "loop" : 2000,
>                         "policy" : "SCHED_OTHER",
>                         "run" : -1,  //make normal task 100% running
>                         "priority" : 0,
>                         "sleep" : 0
>                 },
>                 "task_idle" : {
>                         "instance" : 1,
>                         "cpus" : [2],
>                         "loop" : 2000,
>                         "policy" : "SCHED_IDLE",
>                         "run" : 1, //only run 1us to avoid
> blocking(always waiting for running), making check_preempt_wakeup
> work(S->R switching)
>                         "timer" : { "ref" : "unique2" , "period" :
> 16000, "mode" : "absolute" }
>                 }
>         },
>         "global" : {
>                 "calibration" : "CPU0",
>                 "default_policy" : "SCHED_OTHER",
>                 "duration" : -1
>         }
> }
> without the patch,
>           <...>-39771 [002] d.h. 42478.177771: sched_wakeup:
> comm=task_idle-1 pid=39772 prio=120 target_cpu=002
>            <...>-39771 [002] d... 42478.190437: sched_switch:
> prev_comm=task_other-0 prev_pid=39771 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=39772 next_prio=120
>            <...>-39771 [002] d.h. 42478.193771: sched_wakeup:
> comm=task_idle-1 pid=39772 prio=120 target_cpu=002
>            <...>-39771 [002] d... 42478.206438: sched_switch:
> prev_comm=task_other-0 prev_pid=39771 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=39772 next_prio=120
>            <...>-39771 [002] d.h. 42478.209771: sched_wakeup:
> comm=task_idle-1 pid=39772 prio=120 target_cpu=002
>            <...>-39771 [002] d... 42478.222438: sched_switch:
> prev_comm=task_other-0 prev_pid=39771 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=39772 next_prio=120
>            <...>-39771 [002] d.h. 42478.225771: sched_wakeup:
> comm=task_idle-1 pid=39772 prio=120 target_cpu=002
>            <...>-39771 [002] d... 42478.238438: sched_switch:
> prev_comm=task_other-0 prev_pid=39771 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=39772 next_prio=120
> *task_idle*  preempts every 12ms because of the compensation.
>
> with the patch,
>    task_other-0-27670 [002] d.h. 136785.278059: sched_wakeup:
> comm=task_idle-1 pid=27671 prio=120 target_cpu=002
>     task_other-0-27670 [002] d... 136785.293623: sched_switch:
> prev_comm=task_other-0 prev_pid=27670 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=27671 next_prio=120
>     task_other-0-27670 [002] d.h. 136785.294059: sched_wakeup:
> comm=task_idle-1 pid=27671 prio=120 target_cpu=002
>     task_other-0-27670 [002] d... 136785.317624: sched_switch:
> prev_comm=task_other-0 prev_pid=27670 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=27671 next_prio=120
>     task_other-0-27670 [002] d.h. 136785.326059: sched_wakeup:
> comm=task_idle-1 pid=27671 prio=120 target_cpu=002
>     task_other-0-27670 [002] d... 136785.341622: sched_switch:
> prev_comm=task_other-0 prev_pid=27670 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=27671 next_prio=120
>     task_other-0-27670 [002] d.h. 136785.342059: sched_wakeup:
> comm=task_idle-1 pid=27671 prio=120 target_cpu=002
>     task_other-0-27670 [002] d... 136785.365623: sched_switch:
> prev_comm=task_other-0 prev_pid=27670 prev_prio=120 prev_state=R ==>
> next_comm=task_idle-1 next_pid=27671 next_prio=120
> *task_idle* preempts every 24 or 16 ms.
>
> This patch could reduce the preempting frequency of task_idle, and
> reduce the interference from SCHED_IDLE task.

For this use case, the preemption is only 1us long. Is it a realistic
problem use case ? your normal threads might be more impacted by tick,
interrupt, timer and others things than this 1us idle thread every
16ms. I mean, the patch moves the impact from 1us every 12ms (0.01%)
to 1us every 24ms (0.005%). Then, If the idle thread starts to run a
bit longer, the period before preempting the normal thread quickly
increases

What is the improvement for an idle thread trying to run 1ms every
16ms as an example ?

Regards,
Vincent
>
> Thx.
> Regards,
> Jiang