Index/include / boost / corosio / native / detail / epoll / epoll_scheduler.hpp

include/boost/corosio/native/detail/epoll/epoll_scheduler.hpp

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include/boost/corosio/native/detail/epoll/epoll_scheduler.hpp
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1 //
2 // Copyright (c) 2026 Steve Gerbino
3 //
4 // Distributed under the Boost Software License, Version 1.0. (See accompanying
5 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 //
7 // Official repository: https://github.com/cppalliance/corosio
8 //
9
10 #ifndef BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
11 #define BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
12
13 #include <boost/corosio/detail/platform.hpp>
14
15 #if BOOST_COROSIO_HAS_EPOLL
16
17 #include <boost/corosio/detail/config.hpp>
18 #include <boost/capy/ex/execution_context.hpp>
19
20 #include <boost/corosio/native/native_scheduler.hpp>
21 #include <boost/corosio/detail/scheduler_op.hpp>
22
23 #include <boost/corosio/native/detail/epoll/epoll_op.hpp>
24 #include <boost/corosio/detail/timer_service.hpp>
25 #include <boost/corosio/detail/make_err.hpp>
26 #include <boost/corosio/native/detail/posix/posix_resolver_service.hpp>
27 #include <boost/corosio/native/detail/posix/posix_signal_service.hpp>
28
29 #include <boost/corosio/detail/except.hpp>
30 #include <boost/corosio/detail/thread_local_ptr.hpp>
31
32 #include <atomic>
33 #include <chrono>
34 #include <condition_variable>
35 #include <cstddef>
36 #include <cstdint>
37 #include <limits>
38 #include <mutex>
39 #include <utility>
40
41 #include <errno.h>
42 #include <fcntl.h>
43 #include <sys/epoll.h>
44 #include <sys/eventfd.h>
45 #include <sys/socket.h>
46 #include <sys/timerfd.h>
47 #include <unistd.h>
48
49 namespace boost::corosio::detail {
50
51 struct epoll_op;
52 struct descriptor_state;
53 namespace epoll {
54 struct BOOST_COROSIO_SYMBOL_VISIBLE scheduler_context;
55 } // namespace epoll
56
57 /** Linux scheduler using epoll for I/O multiplexing.
58
59 This scheduler implements the scheduler interface using Linux epoll
60 for efficient I/O event notification. It uses a single reactor model
61 where one thread runs epoll_wait while other threads
62 wait on a condition variable for handler work. This design provides:
63
64 - Handler parallelism: N posted handlers can execute on N threads
65 - No thundering herd: condition_variable wakes exactly one thread
66 - IOCP parity: Behavior matches Windows I/O completion port semantics
67
68 When threads call run(), they first try to execute queued handlers.
69 If the queue is empty and no reactor is running, one thread becomes
70 the reactor and runs epoll_wait. Other threads wait on a condition
71 variable until handlers are available.
72
73 @par Thread Safety
74 All public member functions are thread-safe.
75 */
76 class BOOST_COROSIO_DECL epoll_scheduler final
77 : public native_scheduler
78 , public capy::execution_context::service
79 {
80 public:
81 using key_type = scheduler;
82
83 /** Construct the scheduler.
84
85 Creates an epoll instance, eventfd for reactor interruption,
86 and timerfd for kernel-managed timer expiry.
87
88 @param ctx Reference to the owning execution_context.
89 @param concurrency_hint Hint for expected thread count (unused).
90 */
91 epoll_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
92
93 /// Destroy the scheduler.
94 ~epoll_scheduler() override;
95
96 epoll_scheduler(epoll_scheduler const&) = delete;
97 epoll_scheduler& operator=(epoll_scheduler const&) = delete;
98
99 void shutdown() override;
100 void post(std::coroutine_handle<> h) const override;
101 void post(scheduler_op* h) const override;
102 bool running_in_this_thread() const noexcept override;
103 void stop() override;
104 bool stopped() const noexcept override;
105 void restart() override;
106 std::size_t run() override;
107 std::size_t run_one() override;
108 std::size_t wait_one(long usec) override;
109 std::size_t poll() override;
110 std::size_t poll_one() override;
111
112 /** Return the epoll file descriptor.
113
114 Used by socket services to register file descriptors
115 for I/O event notification.
116
117 @return The epoll file descriptor.
118 */
119 int epoll_fd() const noexcept
120 {
121 return epoll_fd_;
122 }
123
124 /** Reset the thread's inline completion budget.
125
126 Called at the start of each posted completion handler to
127 grant a fresh budget for speculative inline completions.
128 */
129 void reset_inline_budget() const noexcept;
130
131 /** Consume one unit of inline budget if available.
132
133 @return True if budget was available and consumed.
134 */
135 bool try_consume_inline_budget() const noexcept;
136
137 /** Register a descriptor for persistent monitoring.
138
139 The fd is registered once and stays registered until explicitly
140 deregistered. Events are dispatched via descriptor_state which
141 tracks pending read/write/connect operations.
142
143 @param fd The file descriptor to register.
144 @param desc Pointer to descriptor data (stored in epoll_event.data.ptr).
145 */
146 void register_descriptor(int fd, descriptor_state* desc) const;
147
148 /** Deregister a persistently registered descriptor.
149
150 @param fd The file descriptor to deregister.
151 */
152 void deregister_descriptor(int fd) const;
153
154 void work_started() noexcept override;
155 void work_finished() noexcept override;
156
157 /** Offset a forthcoming work_finished from work_cleanup.
158
159 Called by descriptor_state when all I/O returned EAGAIN and no
160 handler will be executed. Must be called from a scheduler thread.
161 */
162 void compensating_work_started() const noexcept;
163
164 /** Drain work from thread context's private queue to global queue.
165
166 Called by thread_context_guard destructor when a thread exits run().
167 Transfers pending work to the global queue under mutex protection.
168
169 @param queue The private queue to drain.
170 @param count Item count for wakeup decisions (wakes other threads if positive).
171 */
172 void drain_thread_queue(op_queue& queue, long count) const;
173
174 /** Post completed operations for deferred invocation.
175
176 If called from a thread running this scheduler, operations go to
177 the thread's private queue (fast path). Otherwise, operations are
178 added to the global queue under mutex and a waiter is signaled.
179
180 @par Preconditions
181 work_started() must have been called for each operation.
182
183 @param ops Queue of operations to post.
184 */
185 void post_deferred_completions(op_queue& ops) const;
186
187 private:
188 struct work_cleanup
189 {
190 epoll_scheduler* scheduler;
191 std::unique_lock<std::mutex>* lock;
192 epoll::scheduler_context* ctx;
193 ~work_cleanup();
194 };
195
196 struct task_cleanup
197 {
198 epoll_scheduler const* scheduler;
199 std::unique_lock<std::mutex>* lock;
200 epoll::scheduler_context* ctx;
201 ~task_cleanup();
202 };
203
204 std::size_t do_one(
205 std::unique_lock<std::mutex>& lock,
206 long timeout_us,
207 epoll::scheduler_context* ctx);
208 void
209 run_task(std::unique_lock<std::mutex>& lock, epoll::scheduler_context* ctx);
210 void wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const;
211 void interrupt_reactor() const;
212 void update_timerfd() const;
213
214 /** Set the signaled state and wake all waiting threads.
215
216 @par Preconditions
217 Mutex must be held.
218
219 @param lock The held mutex lock.
220 */
221 void signal_all(std::unique_lock<std::mutex>& lock) const;
222
223 /** Set the signaled state and wake one waiter if any exist.
224
225 Only unlocks and signals if at least one thread is waiting.
226 Use this when the caller needs to perform a fallback action
227 (such as interrupting the reactor) when no waiters exist.
228
229 @par Preconditions
230 Mutex must be held.
231
232 @param lock The held mutex lock.
233
234 @return `true` if unlocked and signaled, `false` if lock still held.
235 */
236 bool maybe_unlock_and_signal_one(std::unique_lock<std::mutex>& lock) const;
237
238 /** Set the signaled state, unlock, and wake one waiter if any exist.
239
240 Always unlocks the mutex. Use this when the caller will release
241 the lock regardless of whether a waiter exists.
242
243 @par Preconditions
244 Mutex must be held.
245
246 @param lock The held mutex lock.
247
248 @return `true` if a waiter was signaled, `false` otherwise.
249 */
250 bool unlock_and_signal_one(std::unique_lock<std::mutex>& lock) const;
251
252 /** Clear the signaled state before waiting.
253
254 @par Preconditions
255 Mutex must be held.
256 */
257 void clear_signal() const;
258
259 /** Block until the signaled state is set.
260
261 Returns immediately if already signaled (fast-path). Otherwise
262 increments the waiter count, waits on the condition variable,
263 and decrements the waiter count upon waking.
264
265 @par Preconditions
266 Mutex must be held.
267
268 @param lock The held mutex lock.
269 */
270 void wait_for_signal(std::unique_lock<std::mutex>& lock) const;
271
272 /** Block until signaled or timeout expires.
273
274 @par Preconditions
275 Mutex must be held.
276
277 @param lock The held mutex lock.
278 @param timeout_us Maximum time to wait in microseconds.
279 */
280 void wait_for_signal_for(
281 std::unique_lock<std::mutex>& lock, long timeout_us) const;
282
283 int epoll_fd_;
284 int event_fd_; // for interrupting reactor
285 int timer_fd_; // timerfd for kernel-managed timer expiry
286 mutable std::mutex mutex_;
287 mutable std::condition_variable cond_;
288 mutable op_queue completed_ops_;
289 mutable std::atomic<long> outstanding_work_;
290 bool stopped_;
291
292 // True while a thread is blocked in epoll_wait. Used by
293 // wake_one_thread_and_unlock and work_finished to know when
294 // an eventfd interrupt is needed instead of a condvar signal.
295 mutable std::atomic<bool> task_running_{false};
296
297 // True when the reactor has been told to do a non-blocking poll
298 // (more handlers queued or poll mode). Prevents redundant eventfd
299 // writes and controls the epoll_wait timeout.
300 mutable bool task_interrupted_ = false;
301
302 // Signaling state: bit 0 = signaled, upper bits = waiter count (incremented by 2)
303 mutable std::size_t state_ = 0;
304
305 // Edge-triggered eventfd state
306 mutable std::atomic<bool> eventfd_armed_{false};
307
308 // Set when the earliest timer changes; flushed before epoll_wait
309 // blocks. Avoids timerfd_settime syscalls for timers that are
310 // scheduled then cancelled without being waited on.
311 mutable std::atomic<bool> timerfd_stale_{false};
312
313 // Sentinel operation for interleaving reactor runs with handler execution.
314 // Ensures the reactor runs periodically even when handlers are continuously
315 // posted, preventing starvation of I/O events, timers, and signals.
316 struct task_op final : scheduler_op
317 {
318 ✗ void operator()() override {}
319 ✗ void destroy() override {}
320 };
321 task_op task_op_;
322 };
323
324 //--------------------------------------------------------------------------
325 //
326 // Implementation
327 //
328 //--------------------------------------------------------------------------
329
330 /*
331 epoll Scheduler - Single Reactor Model
332 ======================================
333
334 This scheduler uses a thread coordination strategy to provide handler
335 parallelism and avoid the thundering herd problem.
336 Instead of all threads blocking on epoll_wait(), one thread becomes the
337 "reactor" while others wait on a condition variable for handler work.
338
339 Thread Model
340 ------------
341 - ONE thread runs epoll_wait() at a time (the reactor thread)
342 - OTHER threads wait on cond_ (condition variable) for handlers
343 - When work is posted, exactly one waiting thread wakes via notify_one()
344 - This matches Windows IOCP semantics where N posted items wake N threads
345
346 Event Loop Structure (do_one)
347 -----------------------------
348 1. Lock mutex, try to pop handler from queue
349 2. If got handler: execute it (unlocked), return
350 3. If queue empty and no reactor running: become reactor
351 - Run epoll_wait (unlocked), queue I/O completions, loop back
352 4. If queue empty and reactor running: wait on condvar for work
353
354 The task_running_ flag ensures only one thread owns epoll_wait().
355 After the reactor queues I/O completions, it loops back to try getting
356 a handler, giving priority to handler execution over more I/O polling.
357
358 Signaling State (state_)
359 ------------------------
360 The state_ variable encodes two pieces of information:
361 - Bit 0: signaled flag (1 = signaled, persists until cleared)
362 - Upper bits: waiter count (each waiter adds 2 before blocking)
363
364 This allows efficient coordination:
365 - Signalers only call notify when waiters exist (state_ > 1)
366 - Waiters check if already signaled before blocking (fast-path)
367
368 Wake Coordination (wake_one_thread_and_unlock)
369 ----------------------------------------------
370 When posting work:
371 - If waiters exist (state_ > 1): signal and notify_one()
372 - Else if reactor running: interrupt via eventfd write
373 - Else: no-op (thread will find work when it checks queue)
374
375 This avoids waking threads unnecessarily. With cascading wakes,
376 each handler execution wakes at most one additional thread if
377 more work exists in the queue.
378
379 Work Counting
380 -------------
381 outstanding_work_ tracks pending operations. When it hits zero, run()
382 returns. Each operation increments on start, decrements on completion.
383
384 Timer Integration
385 -----------------
386 Timers are handled by timer_service. The reactor adjusts epoll_wait
387 timeout to wake for the nearest timer expiry. When a new timer is
388 scheduled earlier than current, timer_service calls interrupt_reactor()
389 to re-evaluate the timeout.
390 */
391
392 namespace epoll {
393
394 struct BOOST_COROSIO_SYMBOL_VISIBLE scheduler_context
395 {
396 epoll_scheduler const* key;
397 scheduler_context* next;
398 op_queue private_queue;
399 long private_outstanding_work;
400 int inline_budget;
401 int inline_budget_max;
402 bool unassisted;
403
404 219 scheduler_context(epoll_scheduler const* k, scheduler_context* n)
405 219 : key(k)
406 219 , next(n)
407 219 , private_outstanding_work(0)
408 219 , inline_budget(0)
409 219 , inline_budget_max(2)
410 219 , unassisted(false)
411 {
412 219 }
413 };
414
415 inline thread_local_ptr<scheduler_context> context_stack;
416
417 struct thread_context_guard
418 {
419 scheduler_context frame_;
420
421 219 explicit thread_context_guard(epoll_scheduler const* ctx) noexcept
422 219 : frame_(ctx, context_stack.get())
423 {
424 219 context_stack.set(&frame_);
425 219 }
426
427 219 ~thread_context_guard() noexcept
428 {
429 219 if (!frame_.private_queue.empty())
430 ✗ frame_.key->drain_thread_queue(
431 ✗ frame_.private_queue, frame_.private_outstanding_work);
432 219 context_stack.set(frame_.next);
433 219 }
434 };
435
436 inline scheduler_context*
437 618092 find_context(epoll_scheduler const* self) noexcept
438 {
439 618092 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
440 616368 if (c->key == self)
441 616368 return c;
442 1724 return nullptr;
443 }
444
445 } // namespace epoll
446
447 inline void
448 86343 epoll_scheduler::reset_inline_budget() const noexcept
449 {
450 86343 if (auto* ctx = epoll::find_context(this))
451 {
452 // Cap when no other thread absorbed queued work. A moderate
453 // cap (4) amortizes scheduling for small buffers while avoiding
454 // bursty I/O that fills socket buffers and stalls large transfers.
455 86343 if (ctx->unassisted)
456 {
457 86343 ctx->inline_budget_max = 4;
458 86343 ctx->inline_budget = 4;
459 86343 return;
460 }
461 // Ramp up when previous cycle fully consumed budget.
462 // Reset on partial consumption (EAGAIN hit or peer got scheduled).
463 ✗ if (ctx->inline_budget == 0)
464 ✗ ctx->inline_budget_max = (std::min)(ctx->inline_budget_max * 2, 16);
465 ✗ else if (ctx->inline_budget < ctx->inline_budget_max)
466 ✗ ctx->inline_budget_max = 2;
467 ✗ ctx->inline_budget = ctx->inline_budget_max;
468 }
469 }
470
471 inline bool
472 387992 epoll_scheduler::try_consume_inline_budget() const noexcept
473 {
474 387992 if (auto* ctx = epoll::find_context(this))
475 {
476 387992 if (ctx->inline_budget > 0)
477 {
478 310466 --ctx->inline_budget;
479 310466 return true;
480 }
481 }
482 77526 return false;
483 }
484
485 inline void
486 63689 descriptor_state::operator()()
487 {
488 63689 is_enqueued_.store(false, std::memory_order_relaxed);
489
490 // Take ownership of impl ref set by close_socket() to prevent
491 // the owning impl from being freed while we're executing
492 63689 auto prevent_impl_destruction = std::move(impl_ref_);
493
494 63689 std::uint32_t ev = ready_events_.exchange(0, std::memory_order_acquire);
495 63689 if (ev == 0)
496 {
497 ✗ scheduler_->compensating_work_started();
498 ✗ return;
499 }
500
501 63689 op_queue local_ops;
502
503 63689 int err = 0;
504 63689 if (ev & EPOLLERR)
505 {
506 1 socklen_t len = sizeof(err);
507 1 if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
508 ✗ err = errno;
509 1 if (err == 0)
510 ✗ err = EIO;
511 }
512
513 {
514 63689 std::lock_guard lock(mutex);
515 63689 if (ev & EPOLLIN)
516 {
517 20477 if (read_op)
518 {
519 4304 auto* rd = read_op;
520 4304 if (err)
521 ✗ rd->complete(err, 0);
522 else
523 4304 rd->perform_io();
524
525 4304 if (rd->errn == EAGAIN || rd->errn == EWOULDBLOCK)
526 {
527 ✗ rd->errn = 0;
528 }
529 else
530 {
531 4304 read_op = nullptr;
532 4304 local_ops.push(rd);
533 }
534 }
535 else
536 {
537 16173 read_ready = true;
538 }
539 }
540 63689 if (ev & EPOLLOUT)
541 {
542 59389 bool had_write_op = (connect_op || write_op);
543 59389 if (connect_op)
544 {
545 4304 auto* cn = connect_op;
546 4304 if (err)
547 1 cn->complete(err, 0);
548 else
549 4303 cn->perform_io();
550 4304 connect_op = nullptr;
551 4304 local_ops.push(cn);
552 }
553 59389 if (write_op)
554 {
555 ✗ auto* wr = write_op;
556 ✗ if (err)
557 ✗ wr->complete(err, 0);
558 else
559 ✗ wr->perform_io();
560
561 ✗ if (wr->errn == EAGAIN || wr->errn == EWOULDBLOCK)
562 {
563 ✗ wr->errn = 0;
564 }
565 else
566 {
567 ✗ write_op = nullptr;
568 ✗ local_ops.push(wr);
569 }
570 }
571 59389 if (!had_write_op)
572 55085 write_ready = true;
573 }
574 63689 if (err)
575 {
576 1 if (read_op)
577 {
578 ✗ read_op->complete(err, 0);
579 ✗ local_ops.push(std::exchange(read_op, nullptr));
580 }
581 1 if (write_op)
582 {
583 ✗ write_op->complete(err, 0);
584 ✗ local_ops.push(std::exchange(write_op, nullptr));
585 }
586 1 if (connect_op)
587 {
588 ✗ connect_op->complete(err, 0);
589 ✗ local_ops.push(std::exchange(connect_op, nullptr));
590 }
591 }
592 63689 }
593
594 // Execute first handler inline — the scheduler's work_cleanup
595 // accounts for this as the "consumed" work item
596 63689 scheduler_op* first = local_ops.pop();
597 63689 if (first)
598 {
599 8608 scheduler_->post_deferred_completions(local_ops);
600 8608 (*first)();
601 }
602 else
603 {
604 55081 scheduler_->compensating_work_started();
605 }
606 63689 }
607
608 229 inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
609 229 : epoll_fd_(-1)
610 229 , event_fd_(-1)
611 229 , timer_fd_(-1)
612 229 , outstanding_work_(0)
613 229 , stopped_(false)
614 229 , task_running_{false}
615 229 , task_interrupted_(false)
616 458 , state_(0)
617 {
618 229 epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
619 229 if (epoll_fd_ < 0)
620 ✗ detail::throw_system_error(make_err(errno), "epoll_create1");
621
622 229 event_fd_ = ::eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
623 229 if (event_fd_ < 0)
624 {
625 ✗ int errn = errno;
626 ✗ ::close(epoll_fd_);
627 ✗ detail::throw_system_error(make_err(errn), "eventfd");
628 }
629
630 229 timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK | TFD_CLOEXEC);
631 229 if (timer_fd_ < 0)
632 {
633 ✗ int errn = errno;
634 ✗ ::close(event_fd_);
635 ✗ ::close(epoll_fd_);
636 ✗ detail::throw_system_error(make_err(errn), "timerfd_create");
637 }
638
639 229 epoll_event ev{};
640 229 ev.events = EPOLLIN | EPOLLET;
641 229 ev.data.ptr = nullptr;
642 229 if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, event_fd_, &ev) < 0)
643 {
644 ✗ int errn = errno;
645 ✗ ::close(timer_fd_);
646 ✗ ::close(event_fd_);
647 ✗ ::close(epoll_fd_);
648 ✗ detail::throw_system_error(make_err(errn), "epoll_ctl");
649 }
650
651 229 epoll_event timer_ev{};
652 229 timer_ev.events = EPOLLIN | EPOLLERR;
653 229 timer_ev.data.ptr = &timer_fd_;
654 229 if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, timer_fd_, &timer_ev) < 0)
655 {
656 ✗ int errn = errno;
657 ✗ ::close(timer_fd_);
658 ✗ ::close(event_fd_);
659 ✗ ::close(epoll_fd_);
660 ✗ detail::throw_system_error(make_err(errn), "epoll_ctl (timerfd)");
661 }
662
663 229 timer_svc_ = &get_timer_service(ctx, *this);
664 229 timer_svc_->set_on_earliest_changed(
665 4737 timer_service::callback(this, [](void* p) {
666 4508 auto* self = static_cast<epoll_scheduler*>(p);
667 4508 self->timerfd_stale_.store(true, std::memory_order_release);
668 4508 if (self->task_running_.load(std::memory_order_acquire))
669 ✗ self->interrupt_reactor();
670 4508 }));
671
672 // Initialize resolver service
673 229 get_resolver_service(ctx, *this);
674
675 // Initialize signal service
676 229 get_signal_service(ctx, *this);
677
678 // Push task sentinel to interleave reactor runs with handler execution
679 229 completed_ops_.push(&task_op_);
680 229 }
681
682 458 inline epoll_scheduler::~epoll_scheduler()
683 {
684 229 if (timer_fd_ >= 0)
685 229 ::close(timer_fd_);
686 229 if (event_fd_ >= 0)
687 229 ::close(event_fd_);
688 229 if (epoll_fd_ >= 0)
689 229 ::close(epoll_fd_);
690 458 }
691
692 inline void
693 229 epoll_scheduler::shutdown()
694 {
695 {
696 229 std::unique_lock lock(mutex_);
697
698 498 while (auto* h = completed_ops_.pop())
699 {
700 269 if (h == &task_op_)
701 229 continue;
702 40 lock.unlock();
703 40 h->destroy();
704 40 lock.lock();
705 269 }
706
707 229 signal_all(lock);
708 229 }
709
710 229 if (event_fd_ >= 0)
711 229 interrupt_reactor();
712 229 }
713
714 inline void
715 6381 epoll_scheduler::post(std::coroutine_handle<> h) const
716 {
717 struct post_handler final : scheduler_op
718 {
719 std::coroutine_handle<> h_;
720
721 6381 explicit post_handler(std::coroutine_handle<> h) : h_(h) {}
722
723 12762 ~post_handler() override = default;
724
725 6375 void operator()() override
726 {
727 6375 auto h = h_;
728 6375 delete this;
729 6375 h.resume();
730 6375 }
731
732 6 void destroy() override
733 {
734 6 auto h = h_;
735 6 delete this;
736 6 h.destroy();
737 6 }
738 };
739
740 6381 auto ph = std::make_unique<post_handler>(h);
741
742 // Fast path: same thread posts to private queue
743 // Only count locally; work_cleanup batches to global counter
744 6381 if (auto* ctx = epoll::find_context(this))
745 {
746 4687 ++ctx->private_outstanding_work;
747 4687 ctx->private_queue.push(ph.release());
748 4687 return;
749 }
750
751 // Slow path: cross-thread post requires mutex
752 1694 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
753
754 1694 std::unique_lock lock(mutex_);
755 1694 completed_ops_.push(ph.release());
756 1694 wake_one_thread_and_unlock(lock);
757 6381 }
758
759 inline void
760 82295 epoll_scheduler::post(scheduler_op* h) const
761 {
762 // Fast path: same thread posts to private queue
763 // Only count locally; work_cleanup batches to global counter
764 82295 if (auto* ctx = epoll::find_context(this))
765 {
766 82265 ++ctx->private_outstanding_work;
767 82265 ctx->private_queue.push(h);
768 82265 return;
769 }
770
771 // Slow path: cross-thread post requires mutex
772 30 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
773
774 30 std::unique_lock lock(mutex_);
775 30 completed_ops_.push(h);
776 30 wake_one_thread_and_unlock(lock);
777 30 }
778
779 inline bool
780 732 epoll_scheduler::running_in_this_thread() const noexcept
781 {
782 732 for (auto* c = epoll::context_stack.get(); c != nullptr; c = c->next)
783 457 if (c->key == this)
784 457 return true;
785 275 return false;
786 }
787
788 inline void
789 200 epoll_scheduler::stop()
790 {
791 200 std::unique_lock lock(mutex_);
792 200 if (!stopped_)
793 {
794 181 stopped_ = true;
795 181 signal_all(lock);
796 181 interrupt_reactor();
797 }
798 200 }
799
800 inline bool
801 18 epoll_scheduler::stopped() const noexcept
802 {
803 18 std::unique_lock lock(mutex_);
804 36 return stopped_;
805 18 }
806
807 inline void
808 53 epoll_scheduler::restart()
809 {
810 53 std::unique_lock lock(mutex_);
811 53 stopped_ = false;
812 53 }
813
814 inline std::size_t
815 199 epoll_scheduler::run()
816 {
817 398 if (outstanding_work_.load(std::memory_order_acquire) == 0)
818 {
819 14 stop();
820 14 return 0;
821 }
822
823 185 epoll::thread_context_guard ctx(this);
824 185 std::unique_lock lock(mutex_);
825
826 185 std::size_t n = 0;
827 for (;;)
828 {
829 152505 if (!do_one(lock, -1, &ctx.frame_))
830 185 break;
831 152320 if (n != (std::numeric_limits<std::size_t>::max)())
832 152320 ++n;
833 152320 if (!lock.owns_lock())
834 69889 lock.lock();
835 }
836 185 return n;
837 185 }
838
839 inline std::size_t
840 2 epoll_scheduler::run_one()
841 {
842 4 if (outstanding_work_.load(std::memory_order_acquire) == 0)
843 {
844 ✗ stop();
845 ✗ return 0;
846 }
847
848 2 epoll::thread_context_guard ctx(this);
849 2 std::unique_lock lock(mutex_);
850 2 return do_one(lock, -1, &ctx.frame_);
851 2 }
852
853 inline std::size_t
854 34 epoll_scheduler::wait_one(long usec)
855 {
856 68 if (outstanding_work_.load(std::memory_order_acquire) == 0)
857 {
858 7 stop();
859 7 return 0;
860 }
861
862 27 epoll::thread_context_guard ctx(this);
863 27 std::unique_lock lock(mutex_);
864 27 return do_one(lock, usec, &ctx.frame_);
865 27 }
866
867 inline std::size_t
868 4 epoll_scheduler::poll()
869 {
870 8 if (outstanding_work_.load(std::memory_order_acquire) == 0)
871 {
872 1 stop();
873 1 return 0;
874 }
875
876 3 epoll::thread_context_guard ctx(this);
877 3 std::unique_lock lock(mutex_);
878
879 3 std::size_t n = 0;
880 for (;;)
881 {
882 7 if (!do_one(lock, 0, &ctx.frame_))
883 3 break;
884 4 if (n != (std::numeric_limits<std::size_t>::max)())
885 4 ++n;
886 4 if (!lock.owns_lock())
887 4 lock.lock();
888 }
889 3 return n;
890 3 }
891
892 inline std::size_t
893 4 epoll_scheduler::poll_one()
894 {
895 8 if (outstanding_work_.load(std::memory_order_acquire) == 0)
896 {
897 2 stop();
898 2 return 0;
899 }
900
901 2 epoll::thread_context_guard ctx(this);
902 2 std::unique_lock lock(mutex_);
903 2 return do_one(lock, 0, &ctx.frame_);
904 2 }
905
906 inline void
907 8698 epoll_scheduler::register_descriptor(int fd, descriptor_state* desc) const
908 {
909 8698 epoll_event ev{};
910 8698 ev.events = EPOLLIN | EPOLLOUT | EPOLLET | EPOLLERR | EPOLLHUP;
911 8698 ev.data.ptr = desc;
912
913 8698 if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
914 ✗ detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
915
916 8698 desc->registered_events = ev.events;
917 8698 desc->fd = fd;
918 8698 desc->scheduler_ = this;
919
920 8698 std::lock_guard lock(desc->mutex);
921 8698 desc->read_ready = false;
922 8698 desc->write_ready = false;
923 8698 }
924
925 inline void
926 8698 epoll_scheduler::deregister_descriptor(int fd) const
927 {
928 8698 ::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
929 8698 }
930
931 inline void
932 14084 epoll_scheduler::work_started() noexcept
933 {
934 14084 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
935 14084 }
936
937 inline void
938 20308 epoll_scheduler::work_finished() noexcept
939 {
940 40616 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
941 174 stop();
942 20308 }
943
944 inline void
945 55081 epoll_scheduler::compensating_work_started() const noexcept
946 {
947 55081 auto* ctx = epoll::find_context(this);
948 55081 if (ctx)
949 55081 ++ctx->private_outstanding_work;
950 55081 }
951
952 inline void
953 ✗ epoll_scheduler::drain_thread_queue(op_queue& queue, long count) const
954 {
955 // Note: outstanding_work_ was already incremented when posting
956 ✗ std::unique_lock lock(mutex_);
957 ✗ completed_ops_.splice(queue);
958 ✗ if (count > 0)
959 ✗ maybe_unlock_and_signal_one(lock);
960 ✗ }
961
962 inline void
963 8608 epoll_scheduler::post_deferred_completions(op_queue& ops) const
964 {
965 8608 if (ops.empty())
966 8608 return;
967
968 // Fast path: if on scheduler thread, use private queue
969 ✗ if (auto* ctx = epoll::find_context(this))
970 {
971 ✗ ctx->private_queue.splice(ops);
972 ✗ return;
973 }
974
975 // Slow path: add to global queue and wake a thread
976 ✗ std::unique_lock lock(mutex_);
977 ✗ completed_ops_.splice(ops);
978 ✗ wake_one_thread_and_unlock(lock);
979 ✗ }
980
981 inline void
982 436 epoll_scheduler::interrupt_reactor() const
983 {
984 // Only write if not already armed to avoid redundant writes
985 436 bool expected = false;
986 436 if (eventfd_armed_.compare_exchange_strong(
987 expected, true, std::memory_order_release,
988 std::memory_order_relaxed))
989 {
990 304 std::uint64_t val = 1;
991 304 [[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
992 }
993 436 }
994
995 inline void
996 410 epoll_scheduler::signal_all(std::unique_lock<std::mutex>&) const
997 {
998 410 state_ |= 1;
999 410 cond_.notify_all();
1000 410 }
1001
1002 inline bool
1003 1724 epoll_scheduler::maybe_unlock_and_signal_one(
1004 std::unique_lock<std::mutex>& lock) const
1005 {
1006 1724 state_ |= 1;
1007 1724 if (state_ > 1)
1008 {
1009 ✗ lock.unlock();
1010 ✗ cond_.notify_one();
1011 ✗ return true;
1012 }
1013 1724 return false;
1014 }
1015
1016 inline bool
1017 195183 epoll_scheduler::unlock_and_signal_one(std::unique_lock<std::mutex>& lock) const
1018 {
1019 195183 state_ |= 1;
1020 195183 bool have_waiters = state_ > 1;
1021 195183 lock.unlock();
1022 195183 if (have_waiters)
1023 ✗ cond_.notify_one();
1024 195183 return have_waiters;
1025 }
1026
1027 inline void
1028 2 epoll_scheduler::clear_signal() const
1029 {
1030 2 state_ &= ~std::size_t(1);
1031 2 }
1032
1033 inline void
1034 2 epoll_scheduler::wait_for_signal(std::unique_lock<std::mutex>& lock) const
1035 {
1036 4 while ((state_ & 1) == 0)
1037 {
1038 2 state_ += 2;
1039 2 cond_.wait(lock);
1040 2 state_ -= 2;
1041 }
1042 2 }
1043
1044 inline void
1045 ✗ epoll_scheduler::wait_for_signal_for(
1046 std::unique_lock<std::mutex>& lock, long timeout_us) const
1047 {
1048 ✗ if ((state_ & 1) == 0)
1049 {
1050 ✗ state_ += 2;
1051 ✗ cond_.wait_for(lock, std::chrono::microseconds(timeout_us));
1052 ✗ state_ -= 2;
1053 }
1054 ✗ }
1055
1056 inline void
1057 1724 epoll_scheduler::wake_one_thread_and_unlock(
1058 std::unique_lock<std::mutex>& lock) const
1059 {
1060 1724 if (maybe_unlock_and_signal_one(lock))
1061 ✗ return;
1062
1063 1724 if (task_running_.load(std::memory_order_relaxed) && !task_interrupted_)
1064 {
1065 26 task_interrupted_ = true;
1066 26 lock.unlock();
1067 26 interrupt_reactor();
1068 }
1069 else
1070 {
1071 1698 lock.unlock();
1072 }
1073 }
1074
1075 152355 inline epoll_scheduler::work_cleanup::~work_cleanup()
1076 {
1077 152355 if (ctx)
1078 {
1079 152355 long produced = ctx->private_outstanding_work;
1080 152355 if (produced > 1)
1081 7 scheduler->outstanding_work_.fetch_add(
1082 produced - 1, std::memory_order_relaxed);
1083 152348 else if (produced < 1)
1084 14832 scheduler->work_finished();
1085 152355 ctx->private_outstanding_work = 0;
1086
1087 152355 if (!ctx->private_queue.empty())
1088 {
1089 82442 lock->lock();
1090 82442 scheduler->completed_ops_.splice(ctx->private_queue);
1091 }
1092 }
1093 else
1094 {
1095 ✗ scheduler->work_finished();
1096 }
1097 152355 }
1098
1099 103320 inline epoll_scheduler::task_cleanup::~task_cleanup()
1100 {
1101 51660 if (!ctx)
1102 ✗ return;
1103
1104 51660 if (ctx->private_outstanding_work > 0)
1105 {
1106 4498 scheduler->outstanding_work_.fetch_add(
1107 4498 ctx->private_outstanding_work, std::memory_order_relaxed);
1108 4498 ctx->private_outstanding_work = 0;
1109 }
1110
1111 51660 if (!ctx->private_queue.empty())
1112 {
1113 4498 if (!lock->owns_lock())
1114 ✗ lock->lock();
1115 4498 scheduler->completed_ops_.splice(ctx->private_queue);
1116 }
1117 51660 }
1118
1119 inline void
1120 8991 epoll_scheduler::update_timerfd() const
1121 {
1122 8991 auto nearest = timer_svc_->nearest_expiry();
1123
1124 8991 itimerspec ts{};
1125 8991 int flags = 0;
1126
1127 8991 if (nearest == timer_service::time_point::max())
1128 {
1129 // No timers - disarm by setting to 0 (relative)
1130 }
1131 else
1132 {
1133 8945 auto now = std::chrono::steady_clock::now();
1134 8945 if (nearest <= now)
1135 {
1136 // Use 1ns instead of 0 - zero disarms the timerfd
1137 231 ts.it_value.tv_nsec = 1;
1138 }
1139 else
1140 {
1141 8714 auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
1142 8714 nearest - now)
1143 8714 .count();
1144 8714 ts.it_value.tv_sec = nsec / 1000000000;
1145 8714 ts.it_value.tv_nsec = nsec % 1000000000;
1146 // Ensure non-zero to avoid disarming if duration rounds to 0
1147 8714 if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
1148 ✗ ts.it_value.tv_nsec = 1;
1149 }
1150 }
1151
1152 8991 if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
1153 ✗ detail::throw_system_error(make_err(errno), "timerfd_settime");
1154 8991 }
1155
1156 inline void
1157 51660 epoll_scheduler::run_task(
1158 std::unique_lock<std::mutex>& lock, epoll::scheduler_context* ctx)
1159 {
1160 51660 int timeout_ms = task_interrupted_ ? 0 : -1;
1161
1162 51660 if (lock.owns_lock())
1163 8832 lock.unlock();
1164
1165 51660 task_cleanup on_exit{this, &lock, ctx};
1166
1167 // Flush deferred timerfd programming before blocking
1168 51660 if (timerfd_stale_.exchange(false, std::memory_order_acquire))
1169 4493 update_timerfd();
1170
1171 // Event loop runs without mutex held
1172 epoll_event events[128];
1173 51660 int nfds = ::epoll_wait(epoll_fd_, events, 128, timeout_ms);
1174
1175 51660 if (nfds < 0 && errno != EINTR)
1176 ✗ detail::throw_system_error(make_err(errno), "epoll_wait");
1177
1178 51660 bool check_timers = false;
1179 51660 op_queue local_ops;
1180
1181 // Process events without holding the mutex
1182 119952 for (int i = 0; i < nfds; ++i)
1183 {
1184 68292 if (events[i].data.ptr == nullptr)
1185 {
1186 std::uint64_t val;
1187 // Mutex released above; analyzer can't track unlock via ref
1188 // NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
1189 75 [[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
1190 75 eventfd_armed_.store(false, std::memory_order_relaxed);
1191 75 continue;
1192 75 }
1193
1194 68217 if (events[i].data.ptr == &timer_fd_)
1195 {
1196 std::uint64_t expirations;
1197 // NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
1198 [[maybe_unused]] auto r =
1199 4498 ::read(timer_fd_, &expirations, sizeof(expirations));
1200 4498 check_timers = true;
1201 4498 continue;
1202 4498 }
1203
1204 // Deferred I/O: just set ready events and enqueue descriptor
1205 // No per-descriptor mutex locking in reactor hot path!
1206 63719 auto* desc = static_cast<descriptor_state*>(events[i].data.ptr);
1207 63719 desc->add_ready_events(events[i].events);
1208
1209 // Only enqueue if not already enqueued
1210 63719 bool expected = false;
1211 63719 if (desc->is_enqueued_.compare_exchange_strong(
1212 expected, true, std::memory_order_release,
1213 std::memory_order_relaxed))
1214 {
1215 63719 local_ops.push(desc);
1216 }
1217 }
1218
1219 // Process timers only when timerfd fires
1220 51660 if (check_timers)
1221 {
1222 4498 timer_svc_->process_expired();
1223 4498 update_timerfd();
1224 }
1225
1226 51660 lock.lock();
1227
1228 51660 if (!local_ops.empty())
1229 42371 completed_ops_.splice(local_ops);
1230 51660 }
1231
1232 inline std::size_t
1233 152543 epoll_scheduler::do_one(
1234 std::unique_lock<std::mutex>& lock,
1235 long timeout_us,
1236 epoll::scheduler_context* ctx)
1237 {
1238 for (;;)
1239 {
1240 204205 if (stopped_)
1241 182 return 0;
1242
1243 204023 scheduler_op* op = completed_ops_.pop();
1244
1245 // Handle reactor sentinel - time to poll for I/O
1246 204023 if (op == &task_op_)
1247 {
1248 51666 bool more_handlers = !completed_ops_.empty();
1249
1250 // Nothing to run the reactor for: no pending work to wait on,
1251 // or caller requested a non-blocking poll
1252 60504 if (!more_handlers &&
1253 17676 (outstanding_work_.load(std::memory_order_acquire) == 0 ||
1254 timeout_us == 0))
1255 {
1256 6 completed_ops_.push(&task_op_);
1257 6 return 0;
1258 }
1259
1260 51660 task_interrupted_ = more_handlers || timeout_us == 0;
1261 51660 task_running_.store(true, std::memory_order_release);
1262
1263 51660 if (more_handlers)
1264 42828 unlock_and_signal_one(lock);
1265
1266 51660 run_task(lock, ctx);
1267
1268 51660 task_running_.store(false, std::memory_order_relaxed);
1269 51660 completed_ops_.push(&task_op_);
1270 51660 continue;
1271 51660 }
1272
1273 // Handle operation
1274 152357 if (op != nullptr)
1275 {
1276 152355 bool more = !completed_ops_.empty();
1277
1278 152355 if (more)
1279 152355 ctx->unassisted = !unlock_and_signal_one(lock);
1280 else
1281 {
1282 ✗ ctx->unassisted = false;
1283 ✗ lock.unlock();
1284 }
1285
1286 152355 work_cleanup on_exit{this, &lock, ctx};
1287
1288 152355 (*op)();
1289 152355 return 1;
1290 152355 }
1291
1292 // No pending work to wait on, or caller requested non-blocking poll
1293 4 if (outstanding_work_.load(std::memory_order_acquire) == 0 ||
1294 timeout_us == 0)
1295 ✗ return 0;
1296
1297 2 clear_signal();
1298 2 if (timeout_us < 0)
1299 2 wait_for_signal(lock);
1300 else
1301 ✗ wait_for_signal_for(lock, timeout_us);
1302 51662 }
1303 }
1304
1305 } // namespace boost::corosio::detail
1306
1307 #endif // BOOST_COROSIO_HAS_EPOLL
1308
1309 #endif // BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
1310