【WIP】Seastar框架学习

初始化流程

• create_scheduling_group： 创建sg(调度组）
• init_scheduling_group: 初始化每个sg，在每个sg内会创建task队列

主要结构体

task队列

• 固定长度：长度为cores数量；

    boost::container::static_vector, max_scheduling_groups()> _task_queues;
    task_queue_list _active_task_queues;
    task_queue_list _activating_task_queues;
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add_task

    void add_task(task* t) noexcept {
        auto sg = t->group();
        auto* q = _task_queues[sg._id].get();
        bool was_empty = q->_q.empty();
        q->_q.push_back(std::move(t));
#ifdef SEASTAR_SHUFFLE_TASK_QUEUE
        shuffle(q->_q.back(), *q);
#endif
        if (was_empty) {
            activate(*q);
        }
    }
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往task队列里添加任务：

void schedule(task* t) noexcept {
    engine().add_task(t);
}

void schedule_urgent(task* t) noexcept {
    engine().add_urgent_task(t);
}
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用户层api使用：

场景1：yield方法

• yield()函数会将task通过schedule方法添加到task队列中，并返回了future，等待调度执行；

future<>
yield() noexcept {
    memory::scoped_critical_alloc_section _;
    auto tsk = make_task([] {});
    schedule(tsk);
    return tsk->get_future();
}
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场景2：在future中的使用

在future函数中，make_ready()方法会调用schedule函数添加_task到task队列中：

template 
void promise_base::make_ready() noexcept {
    if (_task) {
        if (Urgent == urgent::yes) {
            ::seastar::schedule_urgent(std::exchange(_task, nullptr));
        } else {
            ::seastar::schedule(std::exchange(_task, nullptr));
        }
    }
}
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make_ready是怎么使用的呢？

• future和promise是相关关联的，即使其中一个或者两个都moved；
• 当set_value或者set_exception调用时，future会变得ready，然后continuation会被attach到该future上，该future就被run起来；

    /// \brief Gets the promise's associated future.
    ///
    /// The future and promise will be remember each other, even if either or
    /// both are moved.  When \c set_value() or \c set_exception() are called
    /// on the promise, the future will be become ready, and if a continuation
    /// was attached to the future, it will run.
    future get_future() noexcept;

	template
	template
	SEASTAR_CONCEPT( requires std::invocable )
	void futurize::satisfy_with_result_of(promise_base_with_type&& pr, Func&& func) {
	    using ret_t = decltype(func());
	    if constexpr (std::is_void_v) {
	        func();
	        pr.set_value();
	    } else if constexpr (is_future::value) {
	        func().forward_to(std::move(pr));
	    } else {
	        pr.set_value(func());
	    }
	}
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任务的调度

void
reactor::run_some_tasks() {
    if (!have_more_tasks()) {
        return;
    }
    sched_print("run_some_tasks: start");
    reset_preemption_monitor();
    update_lowres_clocks();

    sched_clock::time_point t_run_completed = now();
    STAP_PROBE(seastar, reactor_run_tasks_start);
    _cpu_stall_detector->start_task_run(t_run_completed);
    do {
        auto t_run_started = t_run_completed;
        insert_activating_task_queues();
        // 从r
        task_queue* tq = pop_active_task_queue(t_run_started);
        sched_print("running tq {} {}", (void*)tq, tq->_name);
        tq->_current = true;
        _last_vruntime = std::max(tq->_vruntime, _last_vruntime);
        // 真正的开始run task
        run_tasks(*tq);
        tq->_current = false;
        t_run_completed = now();
        auto delta = t_run_completed - t_run_started;
        account_runtime(*tq, delta);
        sched_print("run complete ({} {}); time consumed {} usec; final vruntime {} empty {}",
                (void*)tq, tq->_name, delta / 1us, tq->_vruntime, tq->_q.empty());
        tq->_ts = t_run_completed;
        if (!tq->_q.empty()) {
            insert_active_task_queue(tq);
        } else {
            tq->_active = false;
        }
    } while (have_more_tasks() && !need_preempt());
    _cpu_stall_detector->end_task_run(t_run_completed);
    STAP_PROBE(seastar, reactor_run_tasks_end);
    *internal::current_scheduling_group_ptr() = default_scheduling_group(); // Prevent inheritance from last group run
    sched_print("run_some_tasks: end");
}

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