Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sat, 12 Mar 2022 13:03:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     chenying <chenying.kernel@bytedance.com>
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
Subject: Re: Subject: [PATCH] sched/fair: prioritize normal task over
 sched_idle task with vruntime offset
Message-ID: <20220312120309.GB6235@worktop.programming.kicks-ass.net>
References: <f87a8c0d-527d-a9bc-9653-ff955e0e95b4@bytedance.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <f87a8c0d-527d-a9bc-9653-ff955e0e95b4@bytedance.com>
Precedence: bulk

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?