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Date:   Sun, 13 Mar 2022 13:37:37 +0800
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Subject: Re: [External] Re: Subject: [PATCH] sched/fair: prioritize normal
 task over sched_idle task with vruntime offset
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, mgorman@suse.de, bristot@redhat.com,
        bsegall@google.com, linux-kernel@vger.kernel.org,
        duanxiongchun@bytedance.com, zhouchengming@bytedance.com,
        songmuchun@bytedance.com, zhengqi.arch@bytedance.com,
        zhoufeng.zf@bytedance.com, ligang.bdlg@bytedance.com
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 <20220312120309.GB6235@worktop.programming.kicks-ass.net>
From:   chenying <chenying.kernel@bytedance.com>
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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.