linux-kernel - Re: [PATCH] fix share rt runtime with offline rq

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Date:   Mon, 23 Dec 2019 11:40:30 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     chenying <chen.ying153@....com.cn>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, linux-kernel@...r.kernel.org,
        xue.zhihong@....com.cn, wang.yi59@....com.cn,
        jiang.xuexin@....com.cn
Subject: Re: [PATCH] fix share rt runtime with offline rq

On Sat, 21 Dec 2019 10:20:12 +0800
chenying <chen.ying153@....com.cn> wrote:

> In my environment,cpu0-11 are online, cpu12-15 are offline, CPU2 is isolated,
> sched_rt_runtime_us is 950000,and then bind a rt process with dead loop to CPU2.
> We can see that CPU usage on CPU2 reaches 100%,but only one cpu is isolated,
> so it can be inferred that CPU2 shares the rt runtime of offline cpu.
> 
> / # cat /sys/devices/system/cpu/online
> 0-11
> / # cat /sys/devices/system/cpu/offline
> 12-15
> / # cat /sys/devices/system/cpu/isolated
> 2
> / # cat /proc/sys/kernel/sched_rt_runtime_us
> 950000
> / # chrt -p 357
> pid 357's current scheduling policy: SCHED_FIFO
> pid 357's current scheduling priority: 1

I'm guessing that you took the cpus offline via the kernel command line
parameter. Because when I tried this with just:

 # echo 0 > /sys/devices/system/cpu/cpu${cpu}/online

I could not reproduce it. But when I booted with maxcpus=X set, I could.


> 
> top - 15:52:12 up 4 min,  0 users,  load average: 0.92, 0.41, 0.16
> Tasks: 201 total,   2 running, 199 sleeping,   0 stopped,   0 zombie
> %Cpu0  :  0.3 us,  0.3 sy,  0.0 ni, 99.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu1  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu2  :100.0 us,  0.0 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu3  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu4  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu5  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu6  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu7  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu8  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu9  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> %Cpu10 :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> 
>   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
>   357 root      -2   0    4044    172    136 R 100.0  0.0   2:32.99 deadloop
>   366 root      20   0   22060   2404   2128 R   0.7  0.0   0:00.06 top
>     1 root      20   0    2624     20      0 S   0.0  0.0   0:05.93 init
>     2 root      20   0       0      0      0 S   0.0  0.0   0:00.00 kthreadd
>     3 root      20   0       0      0      0 S   0.0  0.0   0:00.00 ksoftirqd/0
>     4 root      20   0       0      0      0 S   0.0  0.0   0:00.00 kworker/0:0
> 
> Signed-off-by: chenying <chen.ying153@....com.cn>
> ---
>  kernel/sched/rt.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index a532558..d20dc86 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -648,8 +648,12 @@ static void do_balance_runtime(struct rt_rq *rt_rq)
>  	rt_period = ktime_to_ns(rt_b->rt_period);
>  	for_each_cpu(i, rd->span) {
>  		struct rt_rq *iter = sched_rt_period_rt_rq(rt_b, i);
> +		struct rq *rq = rq_of_rt_rq(iter);
>  		s64 diff;
>  
> +		if (!rq->online)
> +			continue;
> +

I think this might be papering over the real issue. Perhaps
rq_offline_rt() needs to be called for CPUs not being brought online?

-- Steve


>  		if (iter == rt_rq)
>  			continue;
>