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	650x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
146	650x	: epoll_fd_(-1)
147	650x	, event_fd_(-1)
148	650x	, timer_fd_(-1)
149	1300x	, event_buffer_(max_events_per_poll_)
150		{
151	650x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
152	650x	if (epoll_fd_ < 0)
153	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
154
155	650x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
156	650x	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	650x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
164	650x	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	650x	epoll_event ev{};
173	650x	ev.events = EPOLLIN \| EPOLLET;
174	650x	ev.data.ptr = nullptr;
175	650x	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	650x	epoll_event timer_ev{};
185	650x	timer_ev.events = EPOLLIN \| EPOLLERR;
186	650x	timer_ev.data.ptr = &timer_fd_;
187	650x	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	650x	timer_svc_ = &get_timer_service(ctx, *this);
197	650x	timer_svc_->set_on_earliest_changed(
198	5636x	timer_service::callback(this, [](void* p) {
199	4986x	auto* self = static_cast<epoll_scheduler*>(p);
200	4986x	self->timerfd_stale_.store(true, std::memory_order_release);
201	4986x	self->interrupt_reactor();
202	4986x	}));
203
204	650x	get_resolver_service(ctx, *this);
205	650x	get_signal_service(ctx, *this);
206	650x	get_stream_file_service(ctx, *this);
207	650x	get_random_access_file_service(ctx, *this);
208
209	650x	completed_ops_.push(&task_op_);
210	650x	}
211
212	1300x	inline epoll_scheduler::~epoll_scheduler()
213		{
214	650x	if (timer_fd_ >= 0)
215	650x	::close(timer_fd_);
216	650x	if (event_fd_ >= 0)
217	650x	::close(event_fd_);
218	650x	if (epoll_fd_ >= 0)
219	650x	::close(epoll_fd_);
220	1300x	}
221
222		inline void
223	650x	epoll_scheduler::shutdown()
224		{
225	650x	shutdown_drain();
226
227	650x	if (event_fd_ >= 0)
228	650x	interrupt_reactor();
229	650x	}
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	9875x	epoll_scheduler::register_descriptor(int fd, reactor_descriptor_state* desc) const
245		{
246	9875x	epoll_event ev{};
247	9875x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
248	9875x	ev.data.ptr = desc;
249
250	9875x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
251	✗	detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
252
253	9875x	desc->registered_events = ev.events;
254	9875x	desc->fd = fd;
255	9875x	desc->scheduler_ = this;
256	9875x	desc->mutex.set_enabled(!single_threaded_);
257	9875x	desc->ready_events_.store(0, std::memory_order_relaxed);
258
259	9875x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
260	9875x	desc->impl_ref_.reset();
261	9875x	desc->read_ready = false;
262	9875x	desc->write_ready = false;
263	9875x	}
264
265		inline void
266	9875x	epoll_scheduler::deregister_descriptor(int fd) const
267		{
268	9875x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
269	9875x	}
270
271		inline void
272	6128x	epoll_scheduler::interrupt_reactor() const
273		{
274	6128x	bool expected = false;
275	6128x	if (eventfd_armed_.compare_exchange_strong(
276		expected, true, std::memory_order_release,
277		std::memory_order_relaxed))
278		{
279	5743x	std::uint64_t val = 1;
280	5743x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
281		}
282	6128x	}
283
284		inline void
285	9930x	epoll_scheduler::update_timerfd() const
286		{
287	9930x	auto nearest = timer_svc_->nearest_expiry();
288
289	9930x	itimerspec ts{};
290	9930x	int flags = 0;
291
292	9930x	if (nearest == timer_service::time_point::max())
293		{
294		// No timers — disarm by setting to 0 (relative)
295		}
296		else
297		{
298	9819x	auto now = std::chrono::steady_clock::now();
299	9819x	if (nearest <= now)
300		{
301		// Use 1ns instead of 0 — zero disarms the timerfd
302	511x	ts.it_value.tv_nsec = 1;
303		}
304		else
305		{
306	9308x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
307	9308x	nearest - now)
308	9308x	.count();
309	9308x	ts.it_value.tv_sec = nsec / 1000000000;
310	9308x	ts.it_value.tv_nsec = nsec % 1000000000;
311	9308x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
312	✗	ts.it_value.tv_nsec = 1;
313		}
314		}
315
316	9930x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
317	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
318	9930x	}
319
320		inline void
321	41795x	epoll_scheduler::run_task(
322		lock_type& lock, context_type* ctx, long timeout_us)
323		{
324		int timeout_ms;
325	41795x	if (task_interrupted_)
326	28184x	timeout_ms = 0;
327	13611x	else if (timeout_us < 0)
328	13607x	timeout_ms = -1;
329		else
330	4x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
331
332	41795x	if (lock.owns_lock())
333	13611x	lock.unlock();
334
335	41795x	task_cleanup on_exit{this, &lock, ctx};
336
337		// Flush deferred timerfd programming before blocking
338	41795x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
339	4963x	update_timerfd();
340
341	41795x	int nfds = ::epoll_wait(
342		epoll_fd_, event_buffer_.data(),
343	41795x	static_cast<int>(event_buffer_.size()), timeout_ms);
344
345	41795x	if (nfds < 0 && errno != EINTR)
346	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
347
348	41795x	bool check_timers = false;
349	41795x	op_queue local_ops;
350
351	95652x	for (int i = 0; i < nfds; ++i)
352		{
353	53857x	if (event_buffer_[i].data.ptr == nullptr)
354		{
355		std::uint64_t val;
356		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
357	5093x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
358	5093x	eventfd_armed_.store(false, std::memory_order_relaxed);
359	5093x	continue;
360	5093x	}
361
362	48764x	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	4967x	::read(timer_fd_, &expirations, sizeof(expirations));
368	4967x	check_timers = true;
369	4967x	continue;
370	4967x	}
371
372		auto* desc =
373	43797x	static_cast<reactor_descriptor_state*>(event_buffer_[i].data.ptr);
374	43797x	desc->add_ready_events(event_buffer_[i].events);
375
376	43797x	bool expected = false;
377	43797x	if (desc->is_enqueued_.compare_exchange_strong(
378		expected, true, std::memory_order_release,
379		std::memory_order_relaxed))
380		{
381	43797x	local_ops.push(desc);
382		}
383		}
384
385	41795x	if (check_timers)
386		{
387	4967x	timer_svc_->process_expired();
388	4967x	update_timerfd();
389		}
390
391	41795x	lock.lock();
392
393	41795x	if (!local_ops.empty())
394	27739x	completed_ops_.splice(local_ops);
395	41795x	}
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