linux - 安装 libpam-systemd 后的 sched_setscheduler 错误 (EPERM)

Question

只是尝试使用 SCHED_FIFO 策略将调度优先级设置为最大。在我安装 libpam-systemd 后，这段代码开始抛出错误。

编辑:归结为下面的两个问题，尽可能把它们说清楚。

追踪:

sched_setscheduler(564, SCHED_FIFO, { 99 }) = -1 EPERM (Operation not permitted)

param.sched_priority = sched_get_priority_max(SCHED_FIFO);
printf("priority %d \n", param.sched_priority);

ret1 = sched_setscheduler(getpid(), SCHED_FIFO, &param);
printf("sched_setscheduler ret %d \n", ret1);
if(ret1 == -1){
    perror("sched_setscheduler");
    goto fail;
}

问题:为什么 root 用户应该获得 EPERM？

还尝试在 /etc/security/limits.conf 中进行以下设置，但没有成功。

root   soft   rtprio       99    
root   hard   rtprio       99

来自 KENREL 配置文件

#CONFIG_SCHED_AUTOGROUP is not set
CONFIG_CGROUP_SCHED=y
CONFIG_FAIR_GROUP_SCHED=y
CONFIG_CFS_BANDWIDTH=y
CONFIG_RT_GROUP_SCHED=y
CONFIG_GENERIC_SCHED_CLOCK=y
CONFIG_CGROUPS=y
CONFIG_CGROUP_DEBUG=y
CONFIG_CGROUP_FREEZER=y

更新文件 linux/kernel/sched/core.c

中的内核代码错误

#ifdef CONFIG_RT_GROUP_SCHED
    /*
     * Do not allow realtime tasks into groups that have no runtime
     * assigned.
     */
    if (rt_bandwidth_enabled() && rt_policy(policy) &&
            task_group(p)->rt_bandwidth.rt_runtime == 0 &&
            !task_group_is_autogroup(task_group(p))) {
        task_rq_unlock(rq, p, &flags);
        return -EPERM;
    }
#endif

通过案例:没有libpam-systemd，我得到以下值

[   36.278241] rt_bandwidth_enabled(): 1 
[   36.281977] rt_policy(policy) : 1 
[   36.285367] task_group_is_autogroup(task_group(p)) : 0 
[   36.289883] task_group(p)->rt_bandwidth.rt_runtime : 950000000 

失败案例:使用libpam-systemd，我得到以下值

[ 2096.713855] rt_bandwidth_enabled(): 1 
[ 2096.717871] rt_policy(policy) : 1 
[ 2096.721408] task_group_is_autogroup(task_group(p)) : 0 
[ 2096.726180] task_group(p)->rt_bandwidth.rt_runtime: 0

问题:为什么 libpam-systemd 应该修改 root 用户的 rt 运行时带宽？

来自 proc 的调度程序 RT 运行时值

root@jarvis:/home/jarvis# cat /proc/sys/kernel/sched_rt_runtime_us 
950000
root@jarvis:/home/jarvis# cat /proc/sys/kernel/sched_rt_period_us
1000000

非常感谢任何帮助。

Answer 1

systemd 创建一个单独的 cgroup对于每个用户和每项服务。检查 /proc/self/cgroup 以查看差异。注意这里有 known problems使用 RT 调度:

    We recommend to turn off Real-Time group scheduling in the
    kernel when using systemd. RT group scheduling effectively
    makes RT scheduling unavailable for most userspace, since it
    requires explicit assignment of RT budgets to each unit whose
    processes making use of RT. As there's no sensible way to
    assign these budgets automatically this cannot really be
    fixed, and it's best to disable group scheduling hence.
       CONFIG_RT_GROUP_SCHED=n