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	373x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
146	373x	: epoll_fd_(-1)
147	373x	, event_fd_(-1)
148	373x	, timer_fd_(-1)
149	746x	, event_buffer_(max_events_per_poll_)
150		{
151	373x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
152	373x	if (epoll_fd_ < 0)
153	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
154
155	373x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
156	373x	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	373x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
164	373x	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	373x	epoll_event ev{};
173	373x	ev.events = EPOLLIN \| EPOLLET;
174	373x	ev.data.ptr = nullptr;
175	373x	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	373x	epoll_event timer_ev{};
185	373x	timer_ev.events = EPOLLIN \| EPOLLERR;
186	373x	timer_ev.data.ptr = &timer_fd_;
187	373x	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	373x	timer_svc_ = &get_timer_service(ctx, *this);
197	373x	timer_svc_->set_on_earliest_changed(
198	5383x	timer_service::callback(this, [](void* p) {
199	5010x	auto* self = static_cast<epoll_scheduler*>(p);
200	5010x	self->timerfd_stale_.store(true, std::memory_order_release);
201	5010x	self->interrupt_reactor();
202	5010x	}));
203
204	373x	get_resolver_service(ctx, *this);
205	373x	get_signal_service(ctx, *this);
206	373x	get_stream_file_service(ctx, *this);
207	373x	get_random_access_file_service(ctx, *this);
208
209	373x	completed_ops_.push(&task_op_);
210	373x	}
211
212	746x	inline epoll_scheduler::~epoll_scheduler()
213		{
214	373x	if (timer_fd_ >= 0)
215	373x	::close(timer_fd_);
216	373x	if (event_fd_ >= 0)
217	373x	::close(event_fd_);
218	373x	if (epoll_fd_ >= 0)
219	373x	::close(epoll_fd_);
220	746x	}
221
222		inline void
223	373x	epoll_scheduler::shutdown()
224		{
225	373x	shutdown_drain();
226
227	373x	if (event_fd_ >= 0)
228	373x	interrupt_reactor();
229	373x	}
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	9761x	epoll_scheduler::register_descriptor(int fd, reactor_descriptor_state* desc) const
245		{
246	9761x	epoll_event ev{};
247	9761x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
248	9761x	ev.data.ptr = desc;
249
250	9761x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
251	✗	detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
252
253	9761x	desc->registered_events = ev.events;
254	9761x	desc->fd = fd;
255	9761x	desc->scheduler_ = this;
256	9761x	desc->mutex.set_enabled(!single_threaded_);
257	9761x	desc->ready_events_.store(0, std::memory_order_relaxed);
258
259	9761x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
260	9761x	desc->impl_ref_.reset();
261	9761x	desc->read_ready = false;
262	9761x	desc->write_ready = false;
263	9761x	}
264
265		inline void
266	9761x	epoll_scheduler::deregister_descriptor(int fd) const
267		{
268	9761x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
269	9761x	}
270
271		inline void
272	5706x	epoll_scheduler::interrupt_reactor() const
273		{
274	5706x	bool expected = false;
275	5706x	if (eventfd_armed_.compare_exchange_strong(
276		expected, true, std::memory_order_release,
277		std::memory_order_relaxed))
278		{
279	5469x	std::uint64_t val = 1;
280	5469x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
281		}
282	5706x	}
283
284		inline void
285	9979x	epoll_scheduler::update_timerfd() const
286		{
287	9979x	auto nearest = timer_svc_->nearest_expiry();
288
289	9979x	itimerspec ts{};
290	9979x	int flags = 0;
291
292	9979x	if (nearest == timer_service::time_point::max())
293		{
294		// No timers — disarm by setting to 0 (relative)
295		}
296		else
297		{
298	9921x	auto now = std::chrono::steady_clock::now();
299	9921x	if (nearest <= now)
300		{
301		// Use 1ns instead of 0 — zero disarms the timerfd
302	388x	ts.it_value.tv_nsec = 1;
303		}
304		else
305		{
306	9533x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
307	9533x	nearest - now)
308	9533x	.count();
309	9533x	ts.it_value.tv_sec = nsec / 1000000000;
310	9533x	ts.it_value.tv_nsec = nsec % 1000000000;
311	9533x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
312	✗	ts.it_value.tv_nsec = 1;
313		}
314		}
315
316	9979x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
317	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
318	9979x	}
319
320		inline void
321	57009x	epoll_scheduler::run_task(
322		lock_type& lock, context_type* ctx, long timeout_us)
323		{
324		int timeout_ms;
325	57009x	if (task_interrupted_)
326	43249x	timeout_ms = 0;
327	13760x	else if (timeout_us < 0)
328	13756x	timeout_ms = -1;
329		else
330	4x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
331
332	57009x	if (lock.owns_lock())
333	13760x	lock.unlock();
334
335	57009x	task_cleanup on_exit{this, &lock, ctx};
336
337		// Flush deferred timerfd programming before blocking
338	57009x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
339	4988x	update_timerfd();
340
341	57009x	int nfds = ::epoll_wait(
342		epoll_fd_, event_buffer_.data(),
343	57009x	static_cast<int>(event_buffer_.size()), timeout_ms);
344
345	57009x	if (nfds < 0 && errno != EINTR)
346	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
347
348	57009x	bool check_timers = false;
349	57009x	op_queue local_ops;
350
351	131385x	for (int i = 0; i < nfds; ++i)
352		{
353	74376x	if (event_buffer_[i].data.ptr == nullptr)
354		{
355		std::uint64_t val;
356		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
357	5096x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
358	5096x	eventfd_armed_.store(false, std::memory_order_relaxed);
359	5096x	continue;
360	5096x	}
361
362	69280x	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	4991x	::read(timer_fd_, &expirations, sizeof(expirations));
368	4991x	check_timers = true;
369	4991x	continue;
370	4991x	}
371
372		auto* desc =
373	64289x	static_cast<reactor_descriptor_state*>(event_buffer_[i].data.ptr);
374	64289x	desc->add_ready_events(event_buffer_[i].events);
375
376	64289x	bool expected = false;
377	64289x	if (desc->is_enqueued_.compare_exchange_strong(
378		expected, true, std::memory_order_release,
379		std::memory_order_relaxed))
380		{
381	64289x	local_ops.push(desc);
382		}
383		}
384
385	57009x	if (check_timers)
386		{
387	4991x	timer_svc_->process_expired();
388	4991x	update_timerfd();
389		}
390
391	57009x	lock.lock();
392
393	57009x	if (!local_ops.empty())
394	42707x	completed_ops_.splice(local_ops);
395	57009x	}
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