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_op.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		struct epoll_op;
47		struct descriptor_state;
48
49		/** Linux scheduler using epoll for I/O multiplexing.
50
51		This scheduler implements the scheduler interface using Linux epoll
52		for efficient I/O event notification. It uses a single reactor model
53		where one thread runs epoll_wait while other threads
54		wait on a condition variable for handler work. This design provides:
55
56		- Handler parallelism: N posted handlers can execute on N threads
57		- No thundering herd: condition_variable wakes exactly one thread
58		- IOCP parity: Behavior matches Windows I/O completion port semantics
59
60		When threads call run(), they first try to execute queued handlers.
61		If the queue is empty and no reactor is running, one thread becomes
62		the reactor and runs epoll_wait. Other threads wait on a condition
63		variable until handlers are available.
64
65		@par Thread Safety
66		All public member functions are thread-safe.
67		*/
68		class BOOST_COROSIO_DECL epoll_scheduler final : public reactor_scheduler
69		{
70		public:
71		/** Construct the scheduler.
72
73		Creates an epoll instance, eventfd for reactor interruption,
74		and timerfd for kernel-managed timer expiry.
75
76		@param ctx Reference to the owning execution_context.
77		@param concurrency_hint Hint for expected thread count (unused).
78		*/
79		epoll_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
80
81		/// Destroy the scheduler.
82		~epoll_scheduler() override;
83
84		epoll_scheduler(epoll_scheduler const&) = delete;
85		epoll_scheduler& operator=(epoll_scheduler const&) = delete;
86
87		/// Shut down the scheduler, draining pending operations.
88		void shutdown() override;
89
90		/// Apply runtime configuration, resizing the event buffer.
91		void configure_reactor(
92		unsigned max_events,
93		unsigned budget_init,
94		unsigned budget_max,
95		unsigned unassisted) override;
96
97		/** Return the epoll file descriptor.
98
99		Used by socket services to register file descriptors
100		for I/O event notification.
101
102		@return The epoll file descriptor.
103		*/
104		int epoll_fd() const noexcept
105		{
106		return epoll_fd_;
107		}
108
109		/** Register a descriptor for persistent monitoring.
110
111		The fd is registered once and stays registered until explicitly
112		deregistered. Events are dispatched via descriptor_state which
113		tracks pending read/write/connect operations.
114
115		@param fd The file descriptor to register.
116		@param desc Pointer to descriptor data (stored in epoll_event.data.ptr).
117		*/
118		void register_descriptor(int fd, descriptor_state* desc) const;
119
120		/** Deregister a persistently registered descriptor.
121
122		@param fd The file descriptor to deregister.
123		*/
124		void deregister_descriptor(int fd) const;
125
126		private:
127		void
128		run_task(lock_type& lock, context_type* ctx,
129		long timeout_us) override;
130		void interrupt_reactor() const override;
131		void update_timerfd() const;
132
133		int epoll_fd_;
134		int event_fd_;
135		int timer_fd_;
136
137		// Edge-triggered eventfd state
138		mutable std::atomic<bool> eventfd_armed_{false};
139
140		// Set when the earliest timer changes; flushed before epoll_wait
141		mutable std::atomic<bool> timerfd_stale_{false};
142
143		// Event buffer sized from max_events_per_poll_ (set at construction,
144		// resized by configure_reactor via io_context_options).
145		std::vector<epoll_event> event_buffer_;
146		};
147
148	333x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
149	333x	: epoll_fd_(-1)
150	333x	, event_fd_(-1)
151	333x	, timer_fd_(-1)
152	666x	, event_buffer_(max_events_per_poll_)
153		{
154	333x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
155	333x	if (epoll_fd_ < 0)
156	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
157
158	333x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
159	333x	if (event_fd_ < 0)
160		{
161	✗	int errn = errno;
162	✗	::close(epoll_fd_);
163	✗	detail::throw_system_error(make_err(errn), "eventfd");
164		}
165
166	333x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
167	333x	if (timer_fd_ < 0)
168		{
169	✗	int errn = errno;
170	✗	::close(event_fd_);
171	✗	::close(epoll_fd_);
172	✗	detail::throw_system_error(make_err(errn), "timerfd_create");
173		}
174
175	333x	epoll_event ev{};
176	333x	ev.events = EPOLLIN \| EPOLLET;
177	333x	ev.data.ptr = nullptr;
178	333x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, event_fd_, &ev) < 0)
179		{
180	✗	int errn = errno;
181	✗	::close(timer_fd_);
182	✗	::close(event_fd_);
183	✗	::close(epoll_fd_);
184	✗	detail::throw_system_error(make_err(errn), "epoll_ctl");
185		}
186
187	333x	epoll_event timer_ev{};
188	333x	timer_ev.events = EPOLLIN \| EPOLLERR;
189	333x	timer_ev.data.ptr = &timer_fd_;
190	333x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, timer_fd_, &timer_ev) < 0)
191		{
192	✗	int errn = errno;
193	✗	::close(timer_fd_);
194	✗	::close(event_fd_);
195	✗	::close(epoll_fd_);
196	✗	detail::throw_system_error(make_err(errn), "epoll_ctl (timerfd)");
197		}
198
199	333x	timer_svc_ = &get_timer_service(ctx, *this);
200	333x	timer_svc_->set_on_earliest_changed(
201	2965x	timer_service::callback(this, [](void* p) {
202	2632x	auto* self = static_cast<epoll_scheduler*>(p);
203	2632x	self->timerfd_stale_.store(true, std::memory_order_release);
204	2632x	self->interrupt_reactor();
205	2632x	}));
206
207	333x	get_resolver_service(ctx, *this);
208	333x	get_signal_service(ctx, *this);
209	333x	get_stream_file_service(ctx, *this);
210	333x	get_random_access_file_service(ctx, *this);
211
212	333x	completed_ops_.push(&task_op_);
213	333x	}
214
215	666x	inline epoll_scheduler::~epoll_scheduler()
216		{
217	333x	if (timer_fd_ >= 0)
218	333x	::close(timer_fd_);
219	333x	if (event_fd_ >= 0)
220	333x	::close(event_fd_);
221	333x	if (epoll_fd_ >= 0)
222	333x	::close(epoll_fd_);
223	666x	}
224
225		inline void
226	333x	epoll_scheduler::shutdown()
227		{
228	333x	shutdown_drain();
229
230	333x	if (event_fd_ >= 0)
231	333x	interrupt_reactor();
232	333x	}
233
234		inline void
235	✗	epoll_scheduler::configure_reactor(
236		unsigned max_events,
237		unsigned budget_init,
238		unsigned budget_max,
239		unsigned unassisted)
240		{
241	✗	reactor_scheduler::configure_reactor(
242		max_events, budget_init, budget_max, unassisted);
243	✗	event_buffer_.resize(max_events_per_poll_);
244	✗	}
245
246		inline void
247	4937x	epoll_scheduler::register_descriptor(int fd, descriptor_state* desc) const
248		{
249	4937x	epoll_event ev{};
250	4937x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
251	4937x	ev.data.ptr = desc;
252
253	4937x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
254	✗	detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
255
256	4937x	desc->registered_events = ev.events;
257	4937x	desc->fd = fd;
258	4937x	desc->scheduler_ = this;
259	4937x	desc->mutex.set_enabled(!single_threaded_);
260	4937x	desc->ready_events_.store(0, std::memory_order_relaxed);
261
262	4937x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
263	4937x	desc->impl_ref_.reset();
264	4937x	desc->read_ready = false;
265	4937x	desc->write_ready = false;
266	4937x	}
267
268		inline void
269	4937x	epoll_scheduler::deregister_descriptor(int fd) const
270		{
271	4937x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
272	4937x	}
273
274		inline void
275	3249x	epoll_scheduler::interrupt_reactor() const
276		{
277	3249x	bool expected = false;
278	3249x	if (eventfd_armed_.compare_exchange_strong(
279		expected, true, std::memory_order_release,
280		std::memory_order_relaxed))
281		{
282	3037x	std::uint64_t val = 1;
283	3037x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
284		}
285	3249x	}
286
287		inline void
288	5222x	epoll_scheduler::update_timerfd() const
289		{
290	5222x	auto nearest = timer_svc_->nearest_expiry();
291
292	5222x	itimerspec ts{};
293	5222x	int flags = 0;
294
295	5222x	if (nearest == timer_service::time_point::max())
296		{
297		// No timers — disarm by setting to 0 (relative)
298		}
299		else
300		{
301	5168x	auto now = std::chrono::steady_clock::now();
302	5168x	if (nearest <= now)
303		{
304		// Use 1ns instead of 0 — zero disarms the timerfd
305	93x	ts.it_value.tv_nsec = 1;
306		}
307		else
308		{
309	5075x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
310	5075x	nearest - now)
311	5075x	.count();
312	5075x	ts.it_value.tv_sec = nsec / 1000000000;
313	5075x	ts.it_value.tv_nsec = nsec % 1000000000;
314	5075x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
315	✗	ts.it_value.tv_nsec = 1;
316		}
317		}
318
319	5222x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
320	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
321	5222x	}
322
323		inline void
324	29573x	epoll_scheduler::run_task(
325		lock_type& lock, context_type* ctx, long timeout_us)
326		{
327		int timeout_ms;
328	29573x	if (task_interrupted_)
329	22300x	timeout_ms = 0;
330	7273x	else if (timeout_us < 0)
331	7265x	timeout_ms = -1;
332		else
333	8x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
334
335	29573x	if (lock.owns_lock())
336	7273x	lock.unlock();
337
338	29573x	task_cleanup on_exit{this, &lock, ctx};
339
340		// Flush deferred timerfd programming before blocking
341	29573x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
342	2610x	update_timerfd();
343
344	29573x	int nfds = ::epoll_wait(
345		epoll_fd_, event_buffer_.data(),
346	29573x	static_cast<int>(event_buffer_.size()), timeout_ms);
347
348	29573x	if (nfds < 0 && errno != EINTR)
349	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
350
351	29573x	bool check_timers = false;
352	29573x	op_queue local_ops;
353
354	69866x	for (int i = 0; i < nfds; ++i)
355		{
356	40293x	if (event_buffer_[i].data.ptr == nullptr)
357		{
358		std::uint64_t val;
359		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
360	2704x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
361	2704x	eventfd_armed_.store(false, std::memory_order_relaxed);
362	2704x	continue;
363	2704x	}
364
365	37589x	if (event_buffer_[i].data.ptr == &timer_fd_)
366		{
367		std::uint64_t expirations;
368		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
369		[[maybe_unused]] auto r =
370	2612x	::read(timer_fd_, &expirations, sizeof(expirations));
371	2612x	check_timers = true;
372	2612x	continue;
373	2612x	}
374
375		auto* desc =
376	34977x	static_cast<descriptor_state*>(event_buffer_[i].data.ptr);
377	34977x	desc->add_ready_events(event_buffer_[i].events);
378
379	34977x	bool expected = false;
380	34977x	if (desc->is_enqueued_.compare_exchange_strong(
381		expected, true, std::memory_order_release,
382		std::memory_order_relaxed))
383		{
384	34977x	local_ops.push(desc);
385		}
386		}
387
388	29573x	if (check_timers)
389		{
390	2612x	timer_svc_->process_expired();
391	2612x	update_timerfd();
392		}
393
394	29573x	lock.lock();
395
396	29573x	if (!local_ops.empty())
397	21759x	completed_ops_.splice(local_ops);
398	29573x	}
399
400		} // namespace boost::corosio::detail
401
402		#endif // BOOST_COROSIO_HAS_EPOLL
403
404		#endif // BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
405