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 bool shutdown_;
292
293 // True while a thread is blocked in epoll_wait. Used by
294 // wake_one_thread_and_unlock and work_finished to know when
295 // an eventfd interrupt is needed instead of a condvar signal.
296 mutable std::atomic<bool> task_running_{false};
297
298 // True when the reactor has been told to do a non-blocking poll
299 // (more handlers queued or poll mode). Prevents redundant eventfd
300 // writes and controls the epoll_wait timeout.
301 mutable bool task_interrupted_ = false;
302
303 // Signaling state: bit 0 = signaled, upper bits = waiter count (incremented by 2)
304 mutable std::size_t state_ = 0;
305
306 // Edge-triggered eventfd state
307 mutable std::atomic<bool> eventfd_armed_{false};
308
309 // Set when the earliest timer changes; flushed before epoll_wait
310 // blocks. Avoids timerfd_settime syscalls for timers that are
311 // scheduled then cancelled without being waited on.
312 mutable std::atomic<bool> timerfd_stale_{false};
313
314 // Sentinel operation for interleaving reactor runs with handler execution.
315 // Ensures the reactor runs periodically even when handlers are continuously
316 // posted, preventing starvation of I/O events, timers, and signals.
317 struct task_op final : scheduler_op
318 {
319 ✗ void operator()() override {}
320 ✗ void destroy() override {}
321 };
322 task_op task_op_;
323 };
324
325 //--------------------------------------------------------------------------
326 //
327 // Implementation
328 //
329 //--------------------------------------------------------------------------
330
331 /*
332 epoll Scheduler - Single Reactor Model
333 ======================================
334
335 This scheduler uses a thread coordination strategy to provide handler
336 parallelism and avoid the thundering herd problem.
337 Instead of all threads blocking on epoll_wait(), one thread becomes the
338 "reactor" while others wait on a condition variable for handler work.
339
340 Thread Model
341 ------------
342 - ONE thread runs epoll_wait() at a time (the reactor thread)
343 - OTHER threads wait on cond_ (condition variable) for handlers
344 - When work is posted, exactly one waiting thread wakes via notify_one()
345 - This matches Windows IOCP semantics where N posted items wake N threads
346
347 Event Loop Structure (do_one)
348 -----------------------------
349 1. Lock mutex, try to pop handler from queue
350 2. If got handler: execute it (unlocked), return
351 3. If queue empty and no reactor running: become reactor
352 - Run epoll_wait (unlocked), queue I/O completions, loop back
353 4. If queue empty and reactor running: wait on condvar for work
354
355 The task_running_ flag ensures only one thread owns epoll_wait().
356 After the reactor queues I/O completions, it loops back to try getting
357 a handler, giving priority to handler execution over more I/O polling.
358
359 Signaling State (state_)
360 ------------------------
361 The state_ variable encodes two pieces of information:
362 - Bit 0: signaled flag (1 = signaled, persists until cleared)
363 - Upper bits: waiter count (each waiter adds 2 before blocking)
364
365 This allows efficient coordination:
366 - Signalers only call notify when waiters exist (state_ > 1)
367 - Waiters check if already signaled before blocking (fast-path)
368
369 Wake Coordination (wake_one_thread_and_unlock)
370 ----------------------------------------------
371 When posting work:
372 - If waiters exist (state_ > 1): signal and notify_one()
373 - Else if reactor running: interrupt via eventfd write
374 - Else: no-op (thread will find work when it checks queue)
375
376 This avoids waking threads unnecessarily. With cascading wakes,
377 each handler execution wakes at most one additional thread if
378 more work exists in the queue.
379
380 Work Counting
381 -------------
382 outstanding_work_ tracks pending operations. When it hits zero, run()
383 returns. Each operation increments on start, decrements on completion.
384
385 Timer Integration
386 -----------------
387 Timers are handled by timer_service. The reactor adjusts epoll_wait
388 timeout to wake for the nearest timer expiry. When a new timer is
389 scheduled earlier than current, timer_service calls interrupt_reactor()
390 to re-evaluate the timeout.
391 */
392
393 namespace epoll {
394
395 struct BOOST_COROSIO_SYMBOL_VISIBLE scheduler_context
396 {
397 epoll_scheduler const* key;
398 scheduler_context* next;
399 op_queue private_queue;
400 long private_outstanding_work;
401 int inline_budget;
402 int inline_budget_max;
403 bool unassisted;
404
405 367 scheduler_context(epoll_scheduler const* k, scheduler_context* n)
406 367 : key(k)
407 367 , next(n)
408 367 , private_outstanding_work(0)
409 367 , inline_budget(0)
410 367 , inline_budget_max(2)
411 367 , unassisted(false)
412 {
413 367 }
414 };
415
416 inline thread_local_ptr<scheduler_context> context_stack;
417
418 struct thread_context_guard
419 {
420 scheduler_context frame_;
421
422 367 explicit thread_context_guard(epoll_scheduler const* ctx) noexcept
423 367 : frame_(ctx, context_stack.get())
424 {
425 367 context_stack.set(&frame_);
426 367 }
427
428 367 ~thread_context_guard() noexcept
429 {
430 367 if (!frame_.private_queue.empty())
431 ✗ frame_.key->drain_thread_queue(
432 ✗ frame_.private_queue, frame_.private_outstanding_work);
433 367 context_stack.set(frame_.next);
434 367 }
435 };
436
437 inline scheduler_context*
438 416588 find_context(epoll_scheduler const* self) noexcept
439 {
440 416588 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
441 414545 if (c->key == self)
442 414545 return c;
443 2043 return nullptr;
444 }
445
446 } // namespace epoll
447
448 inline void
449 61396 epoll_scheduler::reset_inline_budget() const noexcept
450 {
451 61396 if (auto* ctx = epoll::find_context(this))
452 {
453 // Cap when no other thread absorbed queued work. A moderate
454 // cap (4) amortizes scheduling for small buffers while avoiding
455 // bursty I/O that fills socket buffers and stalls large transfers.
456 61396 if (ctx->unassisted)
457 {
458 61396 ctx->inline_budget_max = 4;
459 61396 ctx->inline_budget = 4;
460 61396 return;
461 }
462 // Ramp up when previous cycle fully consumed budget.
463 // Reset on partial consumption (EAGAIN hit or peer got scheduled).
464 ✗ if (ctx->inline_budget == 0)
465 ✗ ctx->inline_budget_max = (std::min)(ctx->inline_budget_max * 2, 16);
466 ✗ else if (ctx->inline_budget < ctx->inline_budget_max)
467 ✗ ctx->inline_budget_max = 2;
468 ✗ ctx->inline_budget = ctx->inline_budget_max;
469 }
470 }
471
472 inline bool
473 257654 epoll_scheduler::try_consume_inline_budget() const noexcept
474 {
475 257654 if (auto* ctx = epoll::find_context(this))
476 {
477 257654 if (ctx->inline_budget > 0)
478 {
479 206105 --ctx->inline_budget;
480 206105 return true;
481 }
482 }
483 51549 return false;
484 }
485
486 inline void
487 43367 descriptor_state::operator()()
488 {
489 43367 is_enqueued_.store(false, std::memory_order_relaxed);
490
491 // Take ownership of impl ref set by close_socket() to prevent
492 // the owning impl from being freed while we're executing
493 43367 auto prevent_impl_destruction = std::move(impl_ref_);
494
495 43367 std::uint32_t ev = ready_events_.exchange(0, std::memory_order_acquire);
496 43367 if (ev == 0)
497 {
498 ✗ scheduler_->compensating_work_started();
499 ✗ return;
500 }
501
502 43367 op_queue local_ops;
503
504 43367 int err = 0;
505 43367 if (ev & EPOLLERR)
506 {
507 3 socklen_t len = sizeof(err);
508 3 if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
509 ✗ err = errno;
510 3 if (err == 0)
511 1 err = EIO;
512 }
513
514 {
515 43367 std::lock_guard lock(mutex);
516 43367 if (ev & EPOLLIN)
517 {
518 12607 if (read_op)
519 {
520 4903 auto* rd = read_op;
521 4903 if (err)
522 2 rd->complete(err, 0);
523 else
524 4901 rd->perform_io();
525
526 4903 if (rd->errn == EAGAIN || rd->errn == EWOULDBLOCK)
527 {
528 ✗ rd->errn = 0;
529 }
530 else
531 {
532 4903 read_op = nullptr;
533 4903 local_ops.push(rd);
534 }
535 }
536 else
537 {
538 7704 read_ready = true;
539 }
540 }
541 43367 if (ev & EPOLLOUT)
542 {
543 38645 bool had_write_op = (connect_op || write_op);
544 38645 if (connect_op)
545 {
546 4725 auto* cn = connect_op;
547 4725 if (err)
548 ✗ cn->complete(err, 0);
549 else
550 4725 cn->perform_io();
551 4725 connect_op = nullptr;
552 4725 local_ops.push(cn);
553 }
554 38645 if (write_op)
555 {
556 ✗ auto* wr = write_op;
557 ✗ if (err)
558 ✗ wr->complete(err, 0);
559 else
560 ✗ wr->perform_io();
561
562 ✗ if (wr->errn == EAGAIN || wr->errn == EWOULDBLOCK)
563 {
564 ✗ wr->errn = 0;
565 }
566 else
567 {
568 ✗ write_op = nullptr;
569 ✗ local_ops.push(wr);
570 }
571 }
572 38645 if (!had_write_op)
573 33920 write_ready = true;
574 }
575 43367 if (err)
576 {
577 3 if (read_op)
578 {
579 ✗ read_op->complete(err, 0);
580 ✗ local_ops.push(std::exchange(read_op, nullptr));
581 }
582 3 if (write_op)
583 {
584 ✗ write_op->complete(err, 0);
585 ✗ local_ops.push(std::exchange(write_op, nullptr));
586 }
587 3 if (connect_op)
588 {
589 ✗ connect_op->complete(err, 0);
590 ✗ local_ops.push(std::exchange(connect_op, nullptr));
591 }
592 }
593 43367 }
594
595 // Execute first handler inline — the scheduler's work_cleanup
596 // accounts for this as the "consumed" work item
597 43367 scheduler_op* first = local_ops.pop();
598 43367 if (first)
599 {
600 9628 scheduler_->post_deferred_completions(local_ops);
601 9628 (*first)();
602 }
603 else
604 {
605 33739 scheduler_->compensating_work_started();
606 }
607 43367 }
608
609 270 inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
610 270 : epoll_fd_(-1)
611 270 , event_fd_(-1)
612 270 , timer_fd_(-1)
613 270 , outstanding_work_(0)
614 270 , stopped_(false)
615 270 , shutdown_(false)
616 270 , task_running_{false}
617 270 , task_interrupted_(false)
618 540 , state_(0)
619 {
620 270 epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
621 270 if (epoll_fd_ < 0)
622 ✗ detail::throw_system_error(make_err(errno), "epoll_create1");
623
624 270 event_fd_ = ::eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
625 270 if (event_fd_ < 0)
626 {
627 ✗ int errn = errno;
628 ✗ ::close(epoll_fd_);
629 ✗ detail::throw_system_error(make_err(errn), "eventfd");
630 }
631
632 270 timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK | TFD_CLOEXEC);
633 270 if (timer_fd_ < 0)
634 {
635 ✗ int errn = errno;
636 ✗ ::close(event_fd_);
637 ✗ ::close(epoll_fd_);
638 ✗ detail::throw_system_error(make_err(errn), "timerfd_create");
639 }
640
641 270 epoll_event ev{};
642 270 ev.events = EPOLLIN | EPOLLET;
643 270 ev.data.ptr = nullptr;
644 270 if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, event_fd_, &ev) < 0)
645 {
646 ✗ int errn = errno;
647 ✗ ::close(timer_fd_);
648 ✗ ::close(event_fd_);
649 ✗ ::close(epoll_fd_);
650 ✗ detail::throw_system_error(make_err(errn), "epoll_ctl");
651 }
652
653 270 epoll_event timer_ev{};
654 270 timer_ev.events = EPOLLIN | EPOLLERR;
655 270 timer_ev.data.ptr = &timer_fd_;
656 270 if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, timer_fd_, &timer_ev) < 0)
657 {
658 ✗ int errn = errno;
659 ✗ ::close(timer_fd_);
660 ✗ ::close(event_fd_);
661 ✗ ::close(epoll_fd_);
662 ✗ detail::throw_system_error(make_err(errn), "epoll_ctl (timerfd)");
663 }
664
665 270 timer_svc_ = &get_timer_service(ctx, *this);
666 270 timer_svc_->set_on_earliest_changed(
667 5161 timer_service::callback(this, [](void* p) {
668 4891 auto* self = static_cast<epoll_scheduler*>(p);
669 4891 self->timerfd_stale_.store(true, std::memory_order_release);
670 4891 if (self->task_running_.load(std::memory_order_acquire))
671 ✗ self->interrupt_reactor();
672 4891 }));
673
674 // Initialize resolver service
675 270 get_resolver_service(ctx, *this);
676
677 // Initialize signal service
678 270 get_signal_service(ctx, *this);
679
680 // Push task sentinel to interleave reactor runs with handler execution
681 270 completed_ops_.push(&task_op_);
682 270 }
683
684 540 inline epoll_scheduler::~epoll_scheduler()
685 {
686 270 if (timer_fd_ >= 0)
687 270 ::close(timer_fd_);
688 270 if (event_fd_ >= 0)
689 270 ::close(event_fd_);
690 270 if (epoll_fd_ >= 0)
691 270 ::close(epoll_fd_);
692 540 }
693
694 inline void
695 270 epoll_scheduler::shutdown()
696 {
697 {
698 270 std::unique_lock lock(mutex_);
699 270 shutdown_ = true;
700
701 588 while (auto* h = completed_ops_.pop())
702 {
703 318 if (h == &task_op_)
704 270 continue;
705 48 lock.unlock();
706 48 h->destroy();
707 48 lock.lock();
708 318 }
709
710 270 signal_all(lock);
711 270 }
712
713 270 outstanding_work_.store(0, std::memory_order_release);
714
715 270 if (event_fd_ >= 0)
716 270 interrupt_reactor();
717 270 }
718
719 inline void
720 7089 epoll_scheduler::post(std::coroutine_handle<> h) const
721 {
722 struct post_handler final : scheduler_op
723 {
724 std::coroutine_handle<> h_;
725
726 7089 explicit post_handler(std::coroutine_handle<> h) : h_(h) {}
727
728 14178 ~post_handler() override = default;
729
730 7089 void operator()() override
731 {
732 7089 auto h = h_;
733 7089 delete this;
734 7089 h.resume();
735 7089 }
736
737 ✗ void destroy() override
738 {
739 ✗ delete this;
740 ✗ }
741 };
742
743 7089 auto ph = std::make_unique<post_handler>(h);
744
745 // Fast path: same thread posts to private queue
746 // Only count locally; work_cleanup batches to global counter
747 7089 if (auto* ctx = epoll::find_context(this))
748 {
749 5072 ++ctx->private_outstanding_work;
750 5072 ctx->private_queue.push(ph.release());
751 5072 return;
752 }
753
754 // Slow path: cross-thread post requires mutex
755 2017 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
756
757 2017 std::unique_lock lock(mutex_);
758 2017 completed_ops_.push(ph.release());
759 2017 wake_one_thread_and_unlock(lock);
760 7089 }
761
762 inline void
763 56710 epoll_scheduler::post(scheduler_op* h) const
764 {
765 // Fast path: same thread posts to private queue
766 // Only count locally; work_cleanup batches to global counter
767 56710 if (auto* ctx = epoll::find_context(this))
768 {
769 56684 ++ctx->private_outstanding_work;
770 56684 ctx->private_queue.push(h);
771 56684 return;
772 }
773
774 // Slow path: cross-thread post requires mutex
775 26 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
776
777 26 std::unique_lock lock(mutex_);
778 26 completed_ops_.push(h);
779 26 wake_one_thread_and_unlock(lock);
780 26 }
781
782 inline bool
783 1064 epoll_scheduler::running_in_this_thread() const noexcept
784 {
785 1064 for (auto* c = epoll::context_stack.get(); c != nullptr; c = c->next)
786 460 if (c->key == this)
787 460 return true;
788 604 return false;
789 }
790
791 inline void
792 383 epoll_scheduler::stop()
793 {
794 383 std::unique_lock lock(mutex_);
795 383 if (!stopped_)
796 {
797 347 stopped_ = true;
798 347 signal_all(lock);
799 347 interrupt_reactor();
800 }
801 383 }
802
803 inline bool
804 18 epoll_scheduler::stopped() const noexcept
805 {
806 18 std::unique_lock lock(mutex_);
807 36 return stopped_;
808 18 }
809
810 inline void
811 178 epoll_scheduler::restart()
812 {
813 178 std::unique_lock lock(mutex_);
814 178 stopped_ = false;
815 178 }
816
817 inline std::size_t
818 366 epoll_scheduler::run()
819 {
820 732 if (outstanding_work_.load(std::memory_order_acquire) == 0)
821 {
822 31 stop();
823 31 return 0;
824 }
825
826 335 epoll::thread_context_guard ctx(this);
827 335 std::unique_lock lock(mutex_);
828
829 335 std::size_t n = 0;
830 for (;;)
831 {
832 107468 if (!do_one(lock, -1, &ctx.frame_))
833 335 break;
834 107133 if (n != (std::numeric_limits<std::size_t>::max)())
835 107133 ++n;
836 107133 if (!lock.owns_lock())
837 50266 lock.lock();
838 }
839 335 return n;
840 335 }
841
842 inline std::size_t
843 2 epoll_scheduler::run_one()
844 {
845 4 if (outstanding_work_.load(std::memory_order_acquire) == 0)
846 {
847 ✗ stop();
848 ✗ return 0;
849 }
850
851 2 epoll::thread_context_guard ctx(this);
852 2 std::unique_lock lock(mutex_);
853 2 return do_one(lock, -1, &ctx.frame_);
854 2 }
855
856 inline std::size_t
857 34 epoll_scheduler::wait_one(long usec)
858 {
859 68 if (outstanding_work_.load(std::memory_order_acquire) == 0)
860 {
861 7 stop();
862 7 return 0;
863 }
864
865 27 epoll::thread_context_guard ctx(this);
866 27 std::unique_lock lock(mutex_);
867 27 return do_one(lock, usec, &ctx.frame_);
868 27 }
869
870 inline std::size_t
871 2 epoll_scheduler::poll()
872 {
873 4 if (outstanding_work_.load(std::memory_order_acquire) == 0)
874 {
875 1 stop();
876 1 return 0;
877 }
878
879 1 epoll::thread_context_guard ctx(this);
880 1 std::unique_lock lock(mutex_);
881
882 1 std::size_t n = 0;
883 for (;;)
884 {
885 3 if (!do_one(lock, 0, &ctx.frame_))
886 1 break;
887 2 if (n != (std::numeric_limits<std::size_t>::max)())
888 2 ++n;
889 2 if (!lock.owns_lock())
890 2 lock.lock();
891 }
892 1 return n;
893 1 }
894
895 inline std::size_t
896 4 epoll_scheduler::poll_one()
897 {
898 8 if (outstanding_work_.load(std::memory_order_acquire) == 0)
899 {
900 2 stop();
901 2 return 0;
902 }
903
904 2 epoll::thread_context_guard ctx(this);
905 2 std::unique_lock lock(mutex_);
906 2 return do_one(lock, 0, &ctx.frame_);
907 2 }
908
909 inline void
910 9588 epoll_scheduler::register_descriptor(int fd, descriptor_state* desc) const
911 {
912 9588 epoll_event ev{};
913 9588 ev.events = EPOLLIN | EPOLLOUT | EPOLLET | EPOLLERR | EPOLLHUP;
914 9588 ev.data.ptr = desc;
915
916 9588 if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
917 ✗ detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
918
919 9588 desc->registered_events = ev.events;
920 9588 desc->fd = fd;
921 9588 desc->scheduler_ = this;
922
923 9588 std::lock_guard lock(desc->mutex);
924 9588 desc->read_ready = false;
925 9588 desc->write_ready = false;
926 9588 }
927
928 inline void
929 9588 epoll_scheduler::deregister_descriptor(int fd) const
930 {
931 9588 ::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
932 9588 }
933
934 inline void
935 15827 epoll_scheduler::work_started() noexcept
936 {
937 15827 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
938 15827 }
939
940 inline void
941 22748 epoll_scheduler::work_finished() noexcept
942 {
943 45496 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
944 341 stop();
945 22748 }
946
947 inline void
948 33739 epoll_scheduler::compensating_work_started() const noexcept
949 {
950 33739 auto* ctx = epoll::find_context(this);
951 33739 if (ctx)
952 33739 ++ctx->private_outstanding_work;
953 33739 }
954
955 inline void
956 ✗ epoll_scheduler::drain_thread_queue(op_queue& queue, long count) const
957 {
958 // Note: outstanding_work_ was already incremented when posting
959 ✗ std::unique_lock lock(mutex_);
960 ✗ completed_ops_.splice(queue);
961 ✗ if (count > 0)
962 ✗ maybe_unlock_and_signal_one(lock);
963 ✗ }
964
965 inline void
966 9628 epoll_scheduler::post_deferred_completions(op_queue& ops) const
967 {
968 9628 if (ops.empty())
969 9628 return;
970
971 // Fast path: if on scheduler thread, use private queue
972 ✗ if (auto* ctx = epoll::find_context(this))
973 {
974 ✗ ctx->private_queue.splice(ops);
975 ✗ return;
976 }
977
978 // Slow path: add to global queue and wake a thread
979 ✗ std::unique_lock lock(mutex_);
980 ✗ completed_ops_.splice(ops);
981 ✗ wake_one_thread_and_unlock(lock);
982 ✗ }
983
984 inline void
985 643 epoll_scheduler::interrupt_reactor() const
986 {
987 // Only write if not already armed to avoid redundant writes
988 643 bool expected = false;
989 643 if (eventfd_armed_.compare_exchange_strong(
990 expected, true, std::memory_order_release,
991 std::memory_order_relaxed))
992 {
993 453 std::uint64_t val = 1;
994 453 [[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
995 }
996 643 }
997
998 inline void
999 617 epoll_scheduler::signal_all(std::unique_lock<std::mutex>&) const
1000 {
1001 617 state_ |= 1;
1002 617 cond_.notify_all();
1003 617 }
1004
1005 inline bool
1006 2043 epoll_scheduler::maybe_unlock_and_signal_one(
1007 std::unique_lock<std::mutex>& lock) const
1008 {
1009 2043 state_ |= 1;
1010 2043 if (state_ > 1)
1011 {
1012 ✗ lock.unlock();
1013 ✗ cond_.notify_one();
1014 ✗ return true;
1015 }
1016 2043 return false;
1017 }
1018
1019 inline bool
1020 132514 epoll_scheduler::unlock_and_signal_one(std::unique_lock<std::mutex>& lock) const
1021 {
1022 132514 state_ |= 1;
1023 132514 bool have_waiters = state_ > 1;
1024 132514 lock.unlock();
1025 132514 if (have_waiters)
1026 ✗ cond_.notify_one();
1027 132514 return have_waiters;
1028 }
1029
1030 inline void
1031 ✗ epoll_scheduler::clear_signal() const
1032 {
1033 ✗ state_ &= ~std::size_t(1);
1034 ✗ }
1035
1036 inline void
1037 ✗ epoll_scheduler::wait_for_signal(std::unique_lock<std::mutex>& lock) const
1038 {
1039 ✗ while ((state_ & 1) == 0)
1040 {
1041 ✗ state_ += 2;
1042 ✗ cond_.wait(lock);
1043 ✗ state_ -= 2;
1044 }
1045 ✗ }
1046
1047 inline void
1048 ✗ epoll_scheduler::wait_for_signal_for(
1049 std::unique_lock<std::mutex>& lock, long timeout_us) const
1050 {
1051 ✗ if ((state_ & 1) == 0)
1052 {
1053 ✗ state_ += 2;
1054 ✗ cond_.wait_for(lock, std::chrono::microseconds(timeout_us));
1055 ✗ state_ -= 2;
1056 }
1057 ✗ }
1058
1059 inline void
1060 2043 epoll_scheduler::wake_one_thread_and_unlock(
1061 std::unique_lock<std::mutex>& lock) const
1062 {
1063 2043 if (maybe_unlock_and_signal_one(lock))
1064 ✗ return;
1065
1066 2043 if (task_running_.load(std::memory_order_relaxed) && !task_interrupted_)
1067 {
1068 26 task_interrupted_ = true;
1069 26 lock.unlock();
1070 26 interrupt_reactor();
1071 }
1072 else
1073 {
1074 2017 lock.unlock();
1075 }
1076 }
1077
1078 107166 inline epoll_scheduler::work_cleanup::~work_cleanup()
1079 {
1080 107166 if (ctx)
1081 {
1082 107166 long produced = ctx->private_outstanding_work;
1083 107166 if (produced > 1)
1084 7 scheduler->outstanding_work_.fetch_add(
1085 produced - 1, std::memory_order_relaxed);
1086 107159 else if (produced < 1)
1087 16549 scheduler->work_finished();
1088 107166 ctx->private_outstanding_work = 0;
1089
1090 107166 if (!ctx->private_queue.empty())
1091 {
1092 56878 lock->lock();
1093 56878 scheduler->completed_ops_.splice(ctx->private_queue);
1094 }
1095 }
1096 else
1097 {
1098 ✗ scheduler->work_finished();
1099 }
1100 107166 }
1101
1102 70282 inline epoll_scheduler::task_cleanup::~task_cleanup()
1103 {
1104 35141 if (!ctx)
1105 ✗ return;
1106
1107 35141 if (ctx->private_outstanding_work > 0)
1108 {
1109 4866 scheduler->outstanding_work_.fetch_add(
1110 4866 ctx->private_outstanding_work, std::memory_order_relaxed);
1111 4866 ctx->private_outstanding_work = 0;
1112 }
1113
1114 35141 if (!ctx->private_queue.empty())
1115 {
1116 4866 if (!lock->owns_lock())
1117 ✗ lock->lock();
1118 4866 scheduler->completed_ops_.splice(ctx->private_queue);
1119 }
1120 35141 }
1121
1122 inline void
1123 9745 epoll_scheduler::update_timerfd() const
1124 {
1125 9745 auto nearest = timer_svc_->nearest_expiry();
1126
1127 9745 itimerspec ts{};
1128 9745 int flags = 0;
1129
1130 9745 if (nearest == timer_service::time_point::max())
1131 {
1132 // No timers - disarm by setting to 0 (relative)
1133 }
1134 else
1135 {
1136 9700 auto now = std::chrono::steady_clock::now();
1137 9700 if (nearest <= now)
1138 {
1139 // Use 1ns instead of 0 - zero disarms the timerfd
1140 91 ts.it_value.tv_nsec = 1;
1141 }
1142 else
1143 {
1144 9609 auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
1145 9609 nearest - now)
1146 9609 .count();
1147 9609 ts.it_value.tv_sec = nsec / 1000000000;
1148 9609 ts.it_value.tv_nsec = nsec % 1000000000;
1149 // Ensure non-zero to avoid disarming if duration rounds to 0
1150 9609 if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
1151 ✗ ts.it_value.tv_nsec = 1;
1152 }
1153 }
1154
1155 9745 if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
1156 ✗ detail::throw_system_error(make_err(errno), "timerfd_settime");
1157 9745 }
1158
1159 inline void
1160 35141 epoll_scheduler::run_task(
1161 std::unique_lock<std::mutex>& lock, epoll::scheduler_context* ctx)
1162 {
1163 35141 int timeout_ms = task_interrupted_ ? 0 : -1;
1164
1165 35141 if (lock.owns_lock())
1166 9793 lock.unlock();
1167
1168 35141 task_cleanup on_exit{this, &lock, ctx};
1169
1170 // Flush deferred timerfd programming before blocking
1171 35141 if (timerfd_stale_.exchange(false, std::memory_order_acquire))
1172 4879 update_timerfd();
1173
1174 // Event loop runs without mutex held
1175 epoll_event events[128];
1176 35141 int nfds = ::epoll_wait(epoll_fd_, events, 128, timeout_ms);
1177
1178 35141 if (nfds < 0 && errno != EINTR)
1179 ✗ detail::throw_system_error(make_err(errno), "epoll_wait");
1180
1181 35141 bool check_timers = false;
1182 35141 op_queue local_ops;
1183
1184 // Process events without holding the mutex
1185 83605 for (int i = 0; i < nfds; ++i)
1186 {
1187 48464 if (events[i].data.ptr == nullptr)
1188 {
1189 std::uint64_t val;
1190 // Mutex released above; analyzer can't track unlock via ref
1191 // NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
1192 183 [[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
1193 183 eventfd_armed_.store(false, std::memory_order_relaxed);
1194 183 continue;
1195 183 }
1196
1197 48281 if (events[i].data.ptr == &timer_fd_)
1198 {
1199 std::uint64_t expirations;
1200 // NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
1201 [[maybe_unused]] auto r =
1202 4866 ::read(timer_fd_, &expirations, sizeof(expirations));
1203 4866 check_timers = true;
1204 4866 continue;
1205 4866 }
1206
1207 // Deferred I/O: just set ready events and enqueue descriptor
1208 // No per-descriptor mutex locking in reactor hot path!
1209 43415 auto* desc = static_cast<descriptor_state*>(events[i].data.ptr);
1210 43415 desc->add_ready_events(events[i].events);
1211
1212 // Only enqueue if not already enqueued
1213 43415 bool expected = false;
1214 43415 if (desc->is_enqueued_.compare_exchange_strong(
1215 expected, true, std::memory_order_release,
1216 std::memory_order_relaxed))
1217 {
1218 43415 local_ops.push(desc);
1219 }
1220 }
1221
1222 // Process timers only when timerfd fires
1223 35141 if (check_timers)
1224 {
1225 4866 timer_svc_->process_expired();
1226 4866 update_timerfd();
1227 }
1228
1229 35141 lock.lock();
1230
1231 35141 if (!local_ops.empty())
1232 25062 completed_ops_.splice(local_ops);
1233 35141 }
1234
1235 inline std::size_t
1236 107502 epoll_scheduler::do_one(
1237 std::unique_lock<std::mutex>& lock,
1238 long timeout_us,
1239 epoll::scheduler_context* ctx)
1240 {
1241 for (;;)
1242 {
1243 142643 if (stopped_)
1244 334 return 0;
1245
1246 142309 scheduler_op* op = completed_ops_.pop();
1247
1248 // Handle reactor sentinel - time to poll for I/O
1249 142309 if (op == &task_op_)
1250 {
1251 35143 bool more_handlers = !completed_ops_.empty();
1252
1253 // Nothing to run the reactor for: no pending work to wait on,
1254 // or caller requested a non-blocking poll
1255 44938 if (!more_handlers &&
1256 19590 (outstanding_work_.load(std::memory_order_acquire) == 0 ||
1257 timeout_us == 0))
1258 {
1259 2 completed_ops_.push(&task_op_);
1260 2 return 0;
1261 }
1262
1263 35141 task_interrupted_ = more_handlers || timeout_us == 0;
1264 35141 task_running_.store(true, std::memory_order_release);
1265
1266 35141 if (more_handlers)
1267 25348 unlock_and_signal_one(lock);
1268
1269 35141 run_task(lock, ctx);
1270
1271 35141 task_running_.store(false, std::memory_order_relaxed);
1272 35141 completed_ops_.push(&task_op_);
1273 35141 continue;
1274 35141 }
1275
1276 // Handle operation
1277 107166 if (op != nullptr)
1278 {
1279 107166 bool more = !completed_ops_.empty();
1280
1281 107166 if (more)
1282 107166 ctx->unassisted = !unlock_and_signal_one(lock);
1283 else
1284 {
1285 ✗ ctx->unassisted = false;
1286 ✗ lock.unlock();
1287 }
1288
1289 107166 work_cleanup on_exit{this, &lock, ctx};
1290
1291 107166 (*op)();
1292 107166 return 1;
1293 107166 }
1294
1295 // No pending work to wait on, or caller requested non-blocking poll
1296 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0 ||
1297 timeout_us == 0)
1298 ✗ return 0;
1299
1300 ✗ clear_signal();
1301 ✗ if (timeout_us < 0)
1302 ✗ wait_for_signal(lock);
1303 else
1304 ✗ wait_for_signal_for(lock, timeout_us);
1305 35141 }
1306 }
1307
1308 } // namespace boost::corosio::detail
1309
1310 #endif // BOOST_COROSIO_HAS_EPOLL
1311
1312 #endif // BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
1313