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

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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/detail/reactor/reactor_scheduler.hpp>
21
22		#include <boost/corosio/native/detail/epoll/epoll_traits.hpp>
23		#include <boost/corosio/detail/timer_service.hpp>
24		#include <boost/corosio/native/detail/make_err.hpp>
25		#include <boost/corosio/native/detail/posix/posix_resolver_service.hpp>
26		#include <boost/corosio/native/detail/posix/posix_signal_service.hpp>
27		#include <boost/corosio/native/detail/posix/posix_stream_file_service.hpp>
28		#include <boost/corosio/native/detail/posix/posix_random_access_file_service.hpp>
29
30		#include <boost/corosio/detail/except.hpp>
31
32		#include <atomic>
33		#include <chrono>
34		#include <cstdint>
35		#include <mutex>
36		#include <vector>
37
38		#include <errno.h>
39		#include <sys/epoll.h>
40		#include <sys/eventfd.h>
41		#include <sys/timerfd.h>
42		#include <unistd.h>
43
44		namespace boost::corosio::detail {
45
46		/** Linux scheduler using epoll for I/O multiplexing.
47
48		This scheduler implements the scheduler interface using Linux epoll
49		for efficient I/O event notification. It uses a single reactor model
50		where one thread runs epoll_wait while other threads
51		wait on a condition variable for handler work. This design provides:
52
53		- Handler parallelism: N posted handlers can execute on N threads
54		- No thundering herd: condition_variable wakes exactly one thread
55		- IOCP parity: Behavior matches Windows I/O completion port semantics
56
57		When threads call run(), they first try to execute queued handlers.
58		If the queue is empty and no reactor is running, one thread becomes
59		the reactor and runs epoll_wait. Other threads wait on a condition
60		variable until handlers are available.
61
62		@par Thread Safety
63		All public member functions are thread-safe.
64		*/
65		class BOOST_COROSIO_DECL epoll_scheduler final : public reactor_scheduler
66		{
67		public:
68		/** Construct the scheduler.
69
70		Creates an epoll instance, eventfd for reactor interruption,
71		and timerfd for kernel-managed timer expiry.
72
73		@param ctx Reference to the owning execution_context.
74		@param concurrency_hint Hint for expected thread count (unused).
75		*/
76		epoll_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
77
78		/// Destroy the scheduler.
79		~epoll_scheduler() override;
80
81		epoll_scheduler(epoll_scheduler const&) = delete;
82		epoll_scheduler& operator=(epoll_scheduler const&) = delete;
83
84		/// Shut down the scheduler, draining pending operations.
85		void shutdown() override;
86
87		/// Apply runtime configuration, resizing the event buffer.
88		void configure_reactor(
89		unsigned max_events,
90		unsigned budget_init,
91		unsigned budget_max,
92		unsigned unassisted) override;
93
94		/** Return the epoll file descriptor.
95
96		Used by socket services to register file descriptors
97		for I/O event notification.
98
99		@return The epoll file descriptor.
100		*/
101		int epoll_fd() const noexcept
102		{
103		return epoll_fd_;
104		}
105
106		/** Register a descriptor for persistent monitoring.
107
108		The fd is registered once and stays registered until explicitly
109		deregistered. Events are dispatched via reactor_descriptor_state which
110		tracks pending read/write/connect operations.
111
112		@param fd The file descriptor to register.
113		@param desc Pointer to descriptor data (stored in epoll_event.data.ptr).
114		*/
115		void register_descriptor(int fd, reactor_descriptor_state* desc) const;
116
117		/** Deregister a persistently registered descriptor.
118
119		@param fd The file descriptor to deregister.
120		*/
121		void deregister_descriptor(int fd) const;
122
123		private:
124		void
125		run_task(lock_type& lock, context_type* ctx,
126		long timeout_us) override;
127		void interrupt_reactor() const override;
128		void update_timerfd() const;
129
130		int epoll_fd_;
131		int event_fd_;
132		int timer_fd_;
133
134		// Edge-triggered eventfd state
135		mutable std::atomic<bool> eventfd_armed_{false};
136
137		// Set when the earliest timer changes; flushed before epoll_wait
138		mutable std::atomic<bool> timerfd_stale_{false};
139
140		// Event buffer sized from max_events_per_poll_ (set at construction,
141		// resized by configure_reactor via io_context_options).
142		std::vector<epoll_event> event_buffer_;
143		};
144
145	644x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
146	644x	: epoll_fd_(-1)
147	644x	, event_fd_(-1)
148	644x	, timer_fd_(-1)
149	1288x	, event_buffer_(max_events_per_poll_)
150		{
151	644x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
152	644x	if (epoll_fd_ < 0)
153	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
154
155	644x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
156	644x	if (event_fd_ < 0)
157		{
158	✗	int errn = errno;
159	✗	::close(epoll_fd_);
160	✗	detail::throw_system_error(make_err(errn), "eventfd");
161		}
162
163	644x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
164	644x	if (timer_fd_ < 0)
165		{
166	✗	int errn = errno;
167	✗	::close(event_fd_);
168	✗	::close(epoll_fd_);
169	✗	detail::throw_system_error(make_err(errn), "timerfd_create");
170		}
171
172	644x	epoll_event ev{};
173	644x	ev.events = EPOLLIN \| EPOLLET;
174	644x	ev.data.ptr = nullptr;
175	644x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, event_fd_, &ev) < 0)
176		{
177	✗	int errn = errno;
178	✗	::close(timer_fd_);
179	✗	::close(event_fd_);
180	✗	::close(epoll_fd_);
181	✗	detail::throw_system_error(make_err(errn), "epoll_ctl");
182		}
183
184	644x	epoll_event timer_ev{};
185	644x	timer_ev.events = EPOLLIN \| EPOLLERR;
186	644x	timer_ev.data.ptr = &timer_fd_;
187	644x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, timer_fd_, &timer_ev) < 0)
188		{
189	✗	int errn = errno;
190	✗	::close(timer_fd_);
191	✗	::close(event_fd_);
192	✗	::close(epoll_fd_);
193	✗	detail::throw_system_error(make_err(errn), "epoll_ctl (timerfd)");
194		}
195
196	644x	timer_svc_ = &get_timer_service(ctx, *this);
197	644x	timer_svc_->set_on_earliest_changed(
198	5448x	timer_service::callback(this, [](void* p) {
199	4804x	auto* self = static_cast<epoll_scheduler*>(p);
200	4804x	self->timerfd_stale_.store(true, std::memory_order_release);
201	4804x	self->interrupt_reactor();
202	4804x	}));
203
204	644x	get_resolver_service(ctx, *this);
205	644x	get_signal_service(ctx, *this);
206	644x	get_stream_file_service(ctx, *this);
207	644x	get_random_access_file_service(ctx, *this);
208
209	644x	completed_ops_.push(&task_op_);
210	644x	}
211
212	1288x	inline epoll_scheduler::~epoll_scheduler()
213		{
214	644x	if (timer_fd_ >= 0)
215	644x	::close(timer_fd_);
216	644x	if (event_fd_ >= 0)
217	644x	::close(event_fd_);
218	644x	if (epoll_fd_ >= 0)
219	644x	::close(epoll_fd_);
220	1288x	}
221
222		inline void
223	644x	epoll_scheduler::shutdown()
224		{
225	644x	shutdown_drain();
226
227	644x	if (event_fd_ >= 0)
228	644x	interrupt_reactor();
229	644x	}
230
231		inline void
232	10x	epoll_scheduler::configure_reactor(
233		unsigned max_events,
234		unsigned budget_init,
235		unsigned budget_max,
236		unsigned unassisted)
237		{
238	10x	reactor_scheduler::configure_reactor(
239		max_events, budget_init, budget_max, unassisted);
240	9x	event_buffer_.resize(max_events_per_poll_);
241	9x	}
242
243		inline void
244	9509x	epoll_scheduler::register_descriptor(int fd, reactor_descriptor_state* desc) const
245		{
246	9509x	epoll_event ev{};
247	9509x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
248	9509x	ev.data.ptr = desc;
249
250	9509x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
251	✗	detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
252
253	9509x	desc->registered_events = ev.events;
254	9509x	desc->fd = fd;
255	9509x	desc->scheduler_ = this;
256	9509x	desc->mutex.set_enabled(!single_threaded_);
257	9509x	desc->ready_events_.store(0, std::memory_order_relaxed);
258
259	9509x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
260	9509x	desc->impl_ref_.reset();
261	9509x	desc->read_ready = false;
262	9509x	desc->write_ready = false;
263	9509x	}
264
265		inline void
266	9509x	epoll_scheduler::deregister_descriptor(int fd) const
267		{
268	9509x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
269	9509x	}
270
271		inline void
272	5951x	epoll_scheduler::interrupt_reactor() const
273		{
274	5951x	bool expected = false;
275	5951x	if (eventfd_armed_.compare_exchange_strong(
276		expected, true, std::memory_order_release,
277		std::memory_order_relaxed))
278		{
279	5567x	std::uint64_t val = 1;
280	5567x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
281		}
282	5951x	}
283
284		inline void
285	9564x	epoll_scheduler::update_timerfd() const
286		{
287	9564x	auto nearest = timer_svc_->nearest_expiry();
288
289	9564x	itimerspec ts{};
290	9564x	int flags = 0;
291
292	9564x	if (nearest == timer_service::time_point::max())
293		{
294		// No timers — disarm by setting to 0 (relative)
295		}
296		else
297		{
298	9453x	auto now = std::chrono::steady_clock::now();
299	9453x	if (nearest <= now)
300		{
301		// Use 1ns instead of 0 — zero disarms the timerfd
302	329x	ts.it_value.tv_nsec = 1;
303		}
304		else
305		{
306	9124x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
307	9124x	nearest - now)
308	9124x	.count();
309	9124x	ts.it_value.tv_sec = nsec / 1000000000;
310	9124x	ts.it_value.tv_nsec = nsec % 1000000000;
311	9124x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
312	✗	ts.it_value.tv_nsec = 1;
313		}
314		}
315
316	9564x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
317	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
318	9564x	}
319
320		inline void
321	39561x	epoll_scheduler::run_task(
322		lock_type& lock, context_type* ctx, long timeout_us)
323		{
324		int timeout_ms;
325	39561x	if (task_interrupted_)
326	25725x	timeout_ms = 0;
327	13836x	else if (timeout_us < 0)
328	13832x	timeout_ms = -1;
329		else
330	4x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
331
332	39561x	if (lock.owns_lock())
333	13836x	lock.unlock();
334
335	39561x	task_cleanup on_exit{this, &lock, ctx};
336
337		// Flush deferred timerfd programming before blocking
338	39561x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
339	4781x	update_timerfd();
340
341	39561x	int nfds = ::epoll_wait(
342		epoll_fd_, event_buffer_.data(),
343	39561x	static_cast<int>(event_buffer_.size()), timeout_ms);
344
345	39561x	if (nfds < 0 && errno != EINTR)
346	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
347
348	39561x	bool check_timers = false;
349	39561x	op_queue local_ops;
350
351	89936x	for (int i = 0; i < nfds; ++i)
352		{
353	50375x	if (event_buffer_[i].data.ptr == nullptr)
354		{
355		std::uint64_t val;
356		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
357	4923x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
358	4923x	eventfd_armed_.store(false, std::memory_order_relaxed);
359	4923x	continue;
360	4923x	}
361
362	45452x	if (event_buffer_[i].data.ptr == &timer_fd_)
363		{
364		std::uint64_t expirations;
365		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
366		[[maybe_unused]] auto r =
367	4783x	::read(timer_fd_, &expirations, sizeof(expirations));
368	4783x	check_timers = true;
369	4783x	continue;
370	4783x	}
371
372		auto* desc =
373	40669x	static_cast<reactor_descriptor_state*>(event_buffer_[i].data.ptr);
374	40669x	desc->add_ready_events(event_buffer_[i].events);
375
376	40669x	bool expected = false;
377	40669x	if (desc->is_enqueued_.compare_exchange_strong(
378		expected, true, std::memory_order_release,
379		std::memory_order_relaxed))
380		{
381	40669x	local_ops.push(desc);
382		}
383		}
384
385	39561x	if (check_timers)
386		{
387	4783x	timer_svc_->process_expired();
388	4783x	update_timerfd();
389		}
390
391	39561x	lock.lock();
392
393	39561x	if (!local_ops.empty())
394	25289x	completed_ops_.splice(local_ops);
395	39561x	}
396
397		} // namespace boost::corosio::detail
398
399		#endif // BOOST_COROSIO_HAS_EPOLL
400
401		#endif // BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
402