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tuning
This commit is contained in:
gatieme
@@ -525,7 +525,170 @@ curr->vruntime - p->vruntime > calc_delta_fair(sysctl_sched_wakeup_granularity,
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#5 `sysctl_sched_child_runs_first`
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#5 `sysctl_sched_tunable_scaling`
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-------
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| 内核参数 | 位置 | 内核默认值 | 描述 |
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|:------------:|:------:|:---------------:|:------:|
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| [`sysctl_sched_tunable_scaling`](http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/fair.c#L68) | `/proc/sys/kernel/sched_tunable_scaling` | 1 | 当内核试图调整sched_min_granularity，sched_latency和sched_wakeup_granularity这三个值的时候所使用的更新方法，0为不调整，1为按照cpu个数以2为底的对数值进行调整，2为按照cpu的个数进行线性比例的调整 |
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##5.1 参数背景
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-------
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内核中有有几个调度参数都是描述的进程运行时间
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* 进程最小运行时间 `sched_min_granularity`
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* 调度周期 `sched_latency`
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* `sysctl_sched_wakeup_granularity` 唤醒后运行时间基数
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```cpp
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// https://elixir.bootlin.com/linux/v4.14.14/source/include/linux/sched/sysctl.h#L27
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enum sched_tunable_scaling {
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SCHED_TUNABLESCALING_NONE,
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SCHED_TUNABLESCALING_LOG,
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SCHED_TUNABLESCALING_LINEAR,
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SCHED_TUNABLESCALING_END,
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};
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// https://elixir.bootlin.com/linux/v4.16-rc1/source/kernel/sched/fair.c#L58
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/*
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* The initial- and re-scaling of tunables is configurable
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*
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* Options are:
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*
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* SCHED_TUNABLESCALING_NONE - unscaled, always *1
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* SCHED_TUNABLESCALING_LOG - scaled logarithmical, *1+ilog(ncpus)
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* SCHED_TUNABLESCALING_LINEAR - scaled linear, *ncpus
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*
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* (default SCHED_TUNABLESCALING_LOG = *(1+ilog(ncpus))
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*/
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enum sched_tunable_scaling sysctl_sched_tunable_scaling = SCHED_TUNABLESCALING_LOG;
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```
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```cpp
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// https://elixir.bootlin.com/linux/v4.16-rc1/source/kernel/sched/fair.c#L153
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/*
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* Increase the granularity value when there are more CPUs,
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* because with more CPUs the 'effective latency' as visible
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* to users decreases. But the relationship is not linear,
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* so pick a second-best guess by going with the log2 of the
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* number of CPUs.
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*
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* This idea comes from the SD scheduler of Con Kolivas:
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*/
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static unsigned int get_update_sysctl_factor(void)
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{
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unsigned int cpus = min_t(unsigned int, num_online_cpus(), 8);
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unsigned int factor;
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switch (sysctl_sched_tunable_scaling) {
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case SCHED_TUNABLESCALING_NONE:
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factor = 1;
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break;
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case SCHED_TUNABLESCALING_LINEAR:
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factor = cpus;
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break;
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case SCHED_TUNABLESCALING_LOG:
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default:
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factor = 1 + ilog2(cpus);
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break;
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}
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return factor;
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}
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static void update_sysctl(void)
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{
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unsigned int factor = get_update_sysctl_factor();
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#define SET_SYSCTL(name) \
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(sysctl_##name = (factor) * normalized_sysctl_##name)
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SET_SYSCTL(sched_min_granularity);
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SET_SYSCTL(sched_latency);
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SET_SYSCTL(sched_wakeup_granularity);
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#undef SET_SYSCTL
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}
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```
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```
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// https://elixir.bootlin.com/linux/v4.14.14/source/kernel/sched/fair.c#L195
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void sched_init_granularity(void)
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{
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update_sysctl();
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}
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// https://elixir.bootlin.com/linux/v4.14.14/source/kernel/sched/fair.c#L9024
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static void rq_online_fair(struct rq *rq)
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{
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update_sysctl();
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update_runtime_enabled(rq);
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}
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// https://elixir.bootlin.com/linux/v4.14.14/source/kernel/sched/fair.c#L9051
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static void rq_offline_fair(struct rq *rq)
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{
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update_sysctl();
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/* Ensure any throttled groups are reachable by pick_next_task */
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unthrottle_offline_cfs_rqs(rq);
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}
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```
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```cpp
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// https://elixir.bootlin.com/linux/v4.16-rc1/source/kernel/sched/fair.c#L154
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/**************************************************************
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* Scheduling class statistics methods:
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*/
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int sched_proc_update_handler(struct ctl_table *table, int write,
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void __user *buffer, size_t *lenp,
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loff_t *ppos)
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{
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int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
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unsigned int factor = get_update_sysctl_factor();
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if (ret || !write)
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return ret;
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sched_nr_latency = DIV_ROUND_UP(sysctl_sched_latency,
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sysctl_sched_min_granularity);
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#define WRT_SYSCTL(name) \
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(normalized_sysctl_##name = sysctl_##name / (factor))
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WRT_SYSCTL(sched_min_granularity);
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WRT_SYSCTL(sched_latency);
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WRT_SYSCTL(sched_wakeup_granularity);
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#undef WRT_SYSCTL
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return 0;
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}
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```
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##5.2 参数详解
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-------
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##5.3 接口详解
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-------
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#6 `sysctl_sched_child_runs_first`
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-------
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@@ -534,7 +697,7 @@ curr->vruntime - p->vruntime > calc_delta_fair(sysctl_sched_wakeup_granularity,
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| [`sysctl_sched_child_runs_first`](http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/fair.c#L87) | `/proc/sys/kernel/sched_child_runs_first` | 0 | 该变量表示在创建子进程的时候是否让子进程抢占父进程，即使父进程的vruntime小于子进程，这个会减少公平性，但是可以降低write_on_copy，具体要根据系统的应用情况来考量使用哪种方式（见task_fork_fair过程） |
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##5.1 参数背景
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##6.1 参数背景
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-------
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@@ -544,7 +707,7 @@ curr->vruntime - p->vruntime > calc_delta_fair(sysctl_sched_wakeup_granularity,
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该变量表示在创建子进程的时候是否让子进程抢占父进程，即使父进程的 `vruntime` 小于子进程，这个会减少公平性，但是可以降低 `write_on_copy`，具体要根据系统的应用情况来考量使用哪种方式（见 `task_fork_fair` 过程）
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##5.2 `sysctl_sched_child_runs_first` 保证子进程先运行
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##6.2 `sysctl_sched_child_runs_first` 保证子进程先运行
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-------
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@@ -618,13 +781,285 @@ static void task_fork_fair(struct task_struct *p)
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如果参数 `sysctl_sched_child_runs_first` 被设置, 同时 `curr` 进程的虚拟运行时间比创建的子进程 `se`, 就交换它们的虚拟运行时间. 通过这种方式来保证子进程的虚拟运行时间比父进程的小, 从而保证子进程优先运行.
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##5.3 `sysctl_sched_child_runs_first` 接口
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##6.3 `sysctl_sched_child_runs_first` 接口
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-------
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#6
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#7 `sysctl_sched_migration_cost`
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-------
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#2 调度器特性 features
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| 内核参数 | 位置 | 内核默认值 | 描述 |
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|:------------:|:------:|:---------------:|:------:|
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| [`sysctl_sched_migration_cost`](http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/fair.c#L101) | `/proc/sys/kernel/sched_migration_cost` | `500000ns` | 该变量用来判断一个进程是否还是 `hot`，如果进程的运行时间 (`now - p->se.exec_start`) 小于它，那么内核认为它的 `code` 还在 `cache` 里，所以该进程还是 `hot`，那么在迁移的时候就不会考虑它 |
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##7.1 参数背景
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-------
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`CPU` 负载平衡有两种方式 : `pull` 和 `push`, 即
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* 空闲 `CPU` 从其他忙的 `CPU` 队列中拉一个进程到当前 `CPU` 队列, 通过 `idle_balance` 完成;
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* 或者忙的CPU队列将一个进程推送到空闲的 `CPU` 队列中.
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`idle_balance` 干的则是 `pull` 的事情, `idle_balance` 会 `for_each_domain(this_cpu, sd)` 则是遍历当前 `CPU`所在的调度域, 维持调度域内的核间平衡.
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但是负载平衡有一个矛盾就是 : 负载平衡的频度和 `CPU cache` 的命中率是矛盾的, `CPU` 调度域就是将各个 `CPU` 分成层次不同的组, 低层次搞定的平衡就绝不上升到高层次处理, 避免影响 `cache` 的命中率.
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* 调度器通过 `sysctl_sched_migration_cost` 这个 `proc` 阈值判断进程来判断进程是否是 `cache hot` 的, 如果是 `hot` 的则不会进行负载均衡迁移.
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* 同时通过该阈值控制当前 `CPU` 是否可以 `pull`. `sysctl_sched_migration_cost` 对应 `proc` 控制文件是 `/proc/sys/kernel/sched_migration_cost`, 开关代表如果 `CPU` 队列空闲了 `500000ns = 500us`(`sysctl_sched_migration_cost` 默认值) 以上, 则进行 `pull`, 否则则返回.
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##7.2 `sysctl_sched_migration_cost` 实现详解
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-------
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###7.2.1 判断进程是否是 `cache-hot`
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-------
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调度器通过 [`task_hot`](http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/fair.c#L6596) 函数检查一个进程是否是 `cache-hot` 的. 如果该进程还是 `cache-hot`，那么在迁移的时候就不会考虑它.
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* 如果 `sysctl_sched_migration_cost` 被设置为 `-1`, 则 `task_hot` 永远返回 `1`, 即认为进程永远是 `cache-hot` 的.
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* 如果 `sysctl_sched_migration_cost` 被设置为 `0`, 则禁用了该配置, `task_hot` 永远返回 `0`.
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* 如果进程的运行时间 (`now - p->se.exec_start`) 小于 `sysctl_sched_migration_cost`，那么内核认为它的 `code` 还在 `cache` 里.
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```cpp
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// http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/fair.c#L6596
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/*
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* Is this task likely cache-hot:
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*/
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static int task_hot(struct task_struct *p, struct lb_env *env)
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{
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// ......
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if (sysctl_sched_migration_cost == -1)
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return 1;
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if (sysctl_sched_migration_cost == 0)
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return 0;
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delta = rq_clock_task(env->src_rq) - p->se.exec_start;
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return delta < (s64)sysctl_sched_migration_cost;
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}
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```
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注意
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`task_hot(cahce-hot)` 只是禁止进程迁移的充分条件, 而不是必要条件.
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具体的信息请参见 [`can_migrate_task`](https://elixir.bootlin.com/linux/latest/source/kernel/sched/fair.c#L7063) 函数.
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```cpp
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// (https://elixir.bootlin.com/linux/latest/source/kernel/sched/fair.c#L7063
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/*
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* can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
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*/
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static
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int can_migrate_task(struct task_struct *p, struct lb_env *env)
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{
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int tsk_cache_hot;
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lockdep_assert_held(&env->src_rq->lock);
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/*
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* We do not migrate tasks that are:
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* 1) throttled_lb_pair, or
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* 2) cannot be migrated to this CPU due to cpus_allowed, or
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* 3) running (obviously), or
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* 4) are cache-hot on their current CPU.
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*/
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if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
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return 0;
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if (!cpumask_test_cpu(env->dst_cpu, &p->cpus_allowed)) {
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// ......
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return 0;
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}
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/* Record that we found atleast one task that could run on dst_cpu */
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env->flags &= ~LBF_ALL_PINNED;
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if (task_running(env->src_rq, p)) {
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// ......
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return 0;
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}
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/*
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* Aggressive migration if:
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* 1) destination numa is preferred
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* 2) task is cache cold, or
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* 3) too many balance attempts have failed.
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*/
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tsk_cache_hot = migrate_degrades_locality(p, env);
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if (tsk_cache_hot == -1)
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tsk_cache_hot = task_hot(p, env);
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if (tsk_cache_hot <= 0 ||
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env->sd->nr_balance_failed > env->sd->cache_nice_tries) {
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if (tsk_cache_hot == 1) {
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schedstat_inc(env->sd->lb_hot_gained[env->idle]);
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schedstat_inc(p->se.statistics.nr_forced_migrations);
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}
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return 1;
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}
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schedstat_inc(p->se.statistics.nr_failed_migrations_hot);
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return 0;
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}
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```
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###7.2.2 `idle_balance` 是否需要 `pull`
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-------
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当一个 `CPU` 进入 `idle` 状态时, 将通过 `idle_balance` 从其他繁忙的 `CPU` 上 `pull` 一个进程下来.
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如果 `CPU` 的运行队列 `rq` 空闲时间超过 `sysctl_sched_migration_cost` 以上, 才会从其他 `CPU` 运行队列上 `pull`, 否则则返回, 不会进行 `pull` 操作.
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|
||||
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||||
```cpp
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// http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/fair.c#L8374
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/*
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* idle_balance is called by schedule() if this_cpu is about to become
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* idle. Attempts to pull tasks from other CPUs.
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*/
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static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
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{
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// ......
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/*
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* This is OK, because current is on_cpu, which avoids it being picked
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* for load-balance and preemption/IRQs are still disabled avoiding
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* further scheduler activity on it and we're being very careful to
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* re-start the picking loop.
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*/
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rq_unpin_lock(this_rq, rf);
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if (this_rq->avg_idle < sysctl_sched_migration_cost ||
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!this_rq->rd->overload) {
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rcu_read_lock();
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sd = rcu_dereference_check_sched_domain(this_rq->sd);
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if (sd)
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update_next_balance(sd, &next_balance);
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rcu_read_unlock();
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goto out;
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}
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||||
// ......
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out:
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/* Move the next balance forward */
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if (time_after(this_rq->next_balance, next_balance))
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this_rq->next_balance = next_balance;
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/* Is there a task of a high priority class? */
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if (this_rq->nr_running != this_rq->cfs.h_nr_running)
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pulled_task = -1;
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if (pulled_task)
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this_rq->idle_stamp = 0;
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||||
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rq_repin_lock(this_rq, rf);
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return pulled_task;
|
||||
}
|
||||
```
|
||||
|
||||
|
||||
##7.3 `sysctl_sched_migration_cost` 接口详解
|
||||
-------
|
||||
|
||||
|
||||
|
||||
#8 sysctl_sched_rt_period 与 sysctl_sched_rt_runtime
|
||||
-------
|
||||
|
||||
| 内核参数 | 位置 | 内核默认值 | 描述 |
|
||||
|:------------:|:------:|:---------------:|:------:|
|
||||
| [`sysctl_sched_rt_period`](http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/core.c#L76) | `/proc/sys/kernel/sched_rt_period_us` | `1000000us` | 该参数与下面的sysctl_sched_rt_runtime一起决定了实时进程在以sysctl_sched_rt_period为周期的时间内，实时进程最多能够运行的总的时间不能超过sysctl_sched_rt_runtime（代码见sched_rt_global_constraints |
|
||||
| 内核参数 | 位置 | 内核默认值 | 描述 |
|
||||
|:------------:|:------:|:---------------:|:------:|
|
||||
| [`sysctl_sched_rt_runtime`](http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/core.c#L84) | `/proc/sys/kernel/sched_rt_runtime_us` | `950000us` | 见上 `sysctl_sched_rt_period` 变量的解释 |
|
||||
|
||||
|
||||
|
||||
```cpp
|
||||
// https://elixir.bootlin.com/linux/v4.14.14/source/kernel/sched/core.c#L72
|
||||
/*
|
||||
* period over which we measure -rt task CPU usage in us.
|
||||
* default: 1s
|
||||
*/
|
||||
unsigned int sysctl_sched_rt_period = 1000000;
|
||||
|
||||
|
||||
// https://elixir.bootlin.com/linux/v4.14.14/source/kernel/sched/core.c#L80
|
||||
/*
|
||||
* part of the period that we allow rt tasks to run in us.
|
||||
* default: 0.95s
|
||||
*/
|
||||
int sysctl_sched_rt_runtime = 950000;
|
||||
```
|
||||
|
||||
|
||||
#9 sysctl_sched_rt_runtime
|
||||
-------
|
||||
|
||||
```cpp
|
||||
// https://elixir.bootlin.com/linux/v4.14.14/source/kernel/sched/sched.h#L1264
|
||||
static inline u64 global_rt_period(void)
|
||||
{
|
||||
return (u64)sysctl_sched_rt_period * NSEC_PER_USEC;
|
||||
}
|
||||
|
||||
static inline u64 global_rt_runtime(void)
|
||||
{
|
||||
if (sysctl_sched_rt_runtime < 0)
|
||||
return RUNTIME_INF;
|
||||
|
||||
return (u64)sysctl_sched_rt_runtime * NSEC_PER_USEC;
|
||||
}
|
||||
```
|
||||
|
||||
|
||||
#10
|
||||
-------
|
||||
|
||||
|
||||
```cpp
|
||||
// https://elixir.bootlin.com/linux/v4.14.14/source/kernel/sched/rt.c#L12
|
||||
int sched_rr_timeslice = RR_TIMESLICE;
|
||||
int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
|
||||
```
|
||||
|
||||
|
||||
#10 sysctl_sched_compat_yield
|
||||
-------
|
||||
|
||||
|
||||
| 内核参数 | 位置 | 内核默认值 | 描述 |
|
||||
|:------------:|:------:|:---------------:|:------:|
|
||||
| [`sysctl_sched_compat_yield`]() | `/proc/sys/kernel/sched_compat_yield` | 0 | 该参数可以让 `sched_yield()` 系统调用更加有效，让它使用更少的cpu，对于那些依赖sched_yield来获得更好性能的应用可以考虑设置它为1 |
|
||||
|
||||
|
||||
|
||||
#11 调度器特性 features
|
||||
-------
|
||||
|
||||
|
||||
@@ -662,8 +1097,12 @@ const_debug unsigned int sysctl_sched_features =
|
||||
|
||||
#undef SCHED_FEAT
|
||||
```
|
||||
|
||||
|
||||
首先定义了一个 `SCHED_FEAT` 宏, 然后定义并初始化 `sysctl_sched_features` 的时候通过包含 `features.h` 完成. 该头文件的定义在 [`kernel/sched/features.h, v4.14.14`](http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/features.h).
|
||||
|
||||
|
||||
|
||||
```cpp
|
||||
// http://elixir.free-electrons.com/linux/v4.14.14/source/kernel/sched/features.h
|
||||
SCHED_FEAT(GENTLE_FAIR_SLEEPERS, true)
|
||||
@@ -733,6 +1172,7 @@ extern struct static_key sched_feat_keys[__SCHED_FEAT_NR];
|
||||
##2.2 调度器特性 sched_features
|
||||
-------
|
||||
|
||||
|
||||
每个特性对应 `sysctl_sched_features` 中的一位, 通过 `__SCHED_FEAT_XXX` 宏定义
|
||||
|
||||
|
||||
@@ -776,3 +1216,4 @@ extern struct static_key sched_feat_keys[__SCHED_FEAT_NR];
|
||||
|
||||
* 基于本文修改后的作品务必以相同的许可发布. 如有任何疑问，请与我联系.
|
||||
|
||||
`
|
||||
Reference in New Issue
Block a user