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	434x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
146	434x	: epoll_fd_(-1)
147	434x	, event_fd_(-1)
148	434x	, timer_fd_(-1)
149	868x	, event_buffer_(max_events_per_poll_)
150		{
151	434x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
152	434x	if (epoll_fd_ < 0)
153	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
154
155	434x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
156	434x	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	434x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
164	434x	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	434x	epoll_event ev{};
173	434x	ev.events = EPOLLIN \| EPOLLET;
174	434x	ev.data.ptr = nullptr;
175	434x	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	434x	epoll_event timer_ev{};
185	434x	timer_ev.events = EPOLLIN \| EPOLLERR;
186	434x	timer_ev.data.ptr = &timer_fd_;
187	434x	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	434x	timer_svc_ = &get_timer_service(ctx, *this);
197	434x	timer_svc_->set_on_earliest_changed(
198	7590x	timer_service::callback(this, [](void* p) {
199	7156x	auto* self = static_cast<epoll_scheduler*>(p);
200	7156x	self->timerfd_stale_.store(true, std::memory_order_release);
201	7156x	self->interrupt_reactor();
202	7156x	}));
203
204	434x	get_resolver_service(ctx, *this);
205	434x	get_signal_service(ctx, *this);
206	434x	get_stream_file_service(ctx, *this);
207	434x	get_random_access_file_service(ctx, *this);
208
209	434x	completed_ops_.push(&task_op_);
210	434x	}
211
212	868x	inline epoll_scheduler::~epoll_scheduler()
213		{
214	434x	if (timer_fd_ >= 0)
215	434x	::close(timer_fd_);
216	434x	if (event_fd_ >= 0)
217	434x	::close(event_fd_);
218	434x	if (epoll_fd_ >= 0)
219	434x	::close(epoll_fd_);
220	868x	}
221
222		inline void
223	434x	epoll_scheduler::shutdown()
224		{
225	434x	shutdown_drain();
226
227	434x	if (event_fd_ >= 0)
228	434x	interrupt_reactor();
229	434x	}
230
231		inline void
232	✗	epoll_scheduler::configure_reactor(
233		unsigned max_events,
234		unsigned budget_init,
235		unsigned budget_max,
236		unsigned unassisted)
237		{
238	✗	reactor_scheduler::configure_reactor(
239		max_events, budget_init, budget_max, unassisted);
240	✗	event_buffer_.resize(max_events_per_poll_);
241	✗	}
242
243		inline void
244	15893x	epoll_scheduler::register_descriptor(int fd, reactor_descriptor_state* desc) const
245		{
246	15893x	epoll_event ev{};
247	15893x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
248	15893x	ev.data.ptr = desc;
249
250	15893x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
251	✗	detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
252
253	15893x	desc->registered_events = ev.events;
254	15893x	desc->fd = fd;
255	15893x	desc->scheduler_ = this;
256	15893x	desc->mutex.set_enabled(!single_threaded_);
257	15893x	desc->ready_events_.store(0, std::memory_order_relaxed);
258
259	15893x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
260	15893x	desc->impl_ref_.reset();
261	15893x	desc->read_ready = false;
262	15893x	desc->write_ready = false;
263	15893x	}
264
265		inline void
266	15893x	epoll_scheduler::deregister_descriptor(int fd) const
267		{
268	15893x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
269	15893x	}
270
271		inline void
272	10508x	epoll_scheduler::interrupt_reactor() const
273		{
274	10508x	bool expected = false;
275	10508x	if (eventfd_armed_.compare_exchange_strong(
276		expected, true, std::memory_order_release,
277		std::memory_order_relaxed))
278		{
279	10207x	std::uint64_t val = 1;
280	10207x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
281		}
282	10508x	}
283
284		inline void
285	13531x	epoll_scheduler::update_timerfd() const
286		{
287	13531x	auto nearest = timer_svc_->nearest_expiry();
288
289	13531x	itimerspec ts{};
290	13531x	int flags = 0;
291
292	13531x	if (nearest == timer_service::time_point::max())
293		{
294		// No timers — disarm by setting to 0 (relative)
295		}
296		else
297		{
298	13474x	auto now = std::chrono::steady_clock::now();
299	13474x	if (nearest <= now)
300		{
301		// Use 1ns instead of 0 — zero disarms the timerfd
302	22x	ts.it_value.tv_nsec = 1;
303		}
304		else
305		{
306	13452x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
307	13452x	nearest - now)
308	13452x	.count();
309	13452x	ts.it_value.tv_sec = nsec / 1000000000;
310	13452x	ts.it_value.tv_nsec = nsec % 1000000000;
311	13452x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
312	✗	ts.it_value.tv_nsec = 1;
313		}
314		}
315
316	13531x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
317	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
318	13531x	}
319
320		inline void
321	63148x	epoll_scheduler::run_task(
322		lock_type& lock, context_type* ctx, long timeout_us)
323		{
324		int timeout_ms;
325	63148x	if (task_interrupted_)
326	42358x	timeout_ms = 0;
327	20790x	else if (timeout_us < 0)
328	20782x	timeout_ms = -1;
329		else
330	8x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
331
332	63148x	if (lock.owns_lock())
333	20790x	lock.unlock();
334
335	63148x	task_cleanup on_exit{this, &lock, ctx};
336
337		// Flush deferred timerfd programming before blocking
338	63148x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
339	7134x	update_timerfd();
340
341	63148x	int nfds = ::epoll_wait(
342		epoll_fd_, event_buffer_.data(),
343	63148x	static_cast<int>(event_buffer_.size()), timeout_ms);
344
345	63148x	if (nfds < 0 && errno != EINTR)
346	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
347
348	63148x	bool check_timers = false;
349	63148x	op_queue local_ops;
350
351	134846x	for (int i = 0; i < nfds; ++i)
352		{
353	71698x	if (event_buffer_[i].data.ptr == nullptr)
354		{
355		std::uint64_t val;
356		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
357	9773x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
358	9773x	eventfd_armed_.store(false, std::memory_order_relaxed);
359	9773x	continue;
360	9773x	}
361
362	61925x	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	6397x	::read(timer_fd_, &expirations, sizeof(expirations));
368	6397x	check_timers = true;
369	6397x	continue;
370	6397x	}
371
372		auto* desc =
373	55528x	static_cast<reactor_descriptor_state*>(event_buffer_[i].data.ptr);
374	55528x	desc->add_ready_events(event_buffer_[i].events);
375
376	55528x	bool expected = false;
377	55528x	if (desc->is_enqueued_.compare_exchange_strong(
378		expected, true, std::memory_order_release,
379		std::memory_order_relaxed))
380		{
381	55528x	local_ops.push(desc);
382		}
383		}
384
385	63148x	if (check_timers)
386		{
387	6397x	timer_svc_->process_expired();
388	6397x	update_timerfd();
389		}
390
391	63148x	lock.lock();
392
393	63148x	if (!local_ops.empty())
394	37701x	completed_ops_.splice(local_ops);
395	63148x	}
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