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	359x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
149	359x	: epoll_fd_(-1)
150	359x	, event_fd_(-1)
151	359x	, timer_fd_(-1)
152	718x	, event_buffer_(max_events_per_poll_)
153		{
154	359x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
155	359x	if (epoll_fd_ < 0)
156	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
157
158	359x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
159	359x	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	359x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
167	359x	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	359x	epoll_event ev{};
176	359x	ev.events = EPOLLIN \| EPOLLET;
177	359x	ev.data.ptr = nullptr;
178	359x	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	359x	epoll_event timer_ev{};
188	359x	timer_ev.events = EPOLLIN \| EPOLLERR;
189	359x	timer_ev.data.ptr = &timer_fd_;
190	359x	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	359x	timer_svc_ = &get_timer_service(ctx, *this);
200	359x	timer_svc_->set_on_earliest_changed(
201	5119x	timer_service::callback(this, [](void* p) {
202	4760x	auto* self = static_cast<epoll_scheduler*>(p);
203	4760x	self->timerfd_stale_.store(true, std::memory_order_release);
204	4760x	self->interrupt_reactor();
205	4760x	}));
206
207	359x	get_resolver_service(ctx, *this);
208	359x	get_signal_service(ctx, *this);
209	359x	get_stream_file_service(ctx, *this);
210	359x	get_random_access_file_service(ctx, *this);
211
212	359x	completed_ops_.push(&task_op_);
213	359x	}
214
215	718x	inline epoll_scheduler::~epoll_scheduler()
216		{
217	359x	if (timer_fd_ >= 0)
218	359x	::close(timer_fd_);
219	359x	if (event_fd_ >= 0)
220	359x	::close(event_fd_);
221	359x	if (epoll_fd_ >= 0)
222	359x	::close(epoll_fd_);
223	718x	}
224
225		inline void
226	359x	epoll_scheduler::shutdown()
227		{
228	359x	shutdown_drain();
229
230	359x	if (event_fd_ >= 0)
231	359x	interrupt_reactor();
232	359x	}
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	9235x	epoll_scheduler::register_descriptor(int fd, descriptor_state* desc) const
248		{
249	9235x	epoll_event ev{};
250	9235x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
251	9235x	ev.data.ptr = desc;
252
253	9235x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
254	✗	detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
255
256	9235x	desc->registered_events = ev.events;
257	9235x	desc->fd = fd;
258	9235x	desc->scheduler_ = this;
259	9235x	desc->mutex.set_enabled(!single_threaded_);
260	9235x	desc->ready_events_.store(0, std::memory_order_relaxed);
261
262	9235x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
263	9235x	desc->impl_ref_.reset();
264	9235x	desc->read_ready = false;
265	9235x	desc->write_ready = false;
266	9235x	}
267
268		inline void
269	9235x	epoll_scheduler::deregister_descriptor(int fd) const
270		{
271	9235x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
272	9235x	}
273
274		inline void
275	5423x	epoll_scheduler::interrupt_reactor() const
276		{
277	5423x	bool expected = false;
278	5423x	if (eventfd_armed_.compare_exchange_strong(
279		expected, true, std::memory_order_release,
280		std::memory_order_relaxed))
281		{
282	5198x	std::uint64_t val = 1;
283	5198x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
284		}
285	5423x	}
286
287		inline void
288	9480x	epoll_scheduler::update_timerfd() const
289		{
290	9480x	auto nearest = timer_svc_->nearest_expiry();
291
292	9480x	itimerspec ts{};
293	9480x	int flags = 0;
294
295	9480x	if (nearest == timer_service::time_point::max())
296		{
297		// No timers — disarm by setting to 0 (relative)
298		}
299		else
300		{
301	9427x	auto now = std::chrono::steady_clock::now();
302	9427x	if (nearest <= now)
303		{
304		// Use 1ns instead of 0 — zero disarms the timerfd
305	238x	ts.it_value.tv_nsec = 1;
306		}
307		else
308		{
309	9189x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
310	9189x	nearest - now)
311	9189x	.count();
312	9189x	ts.it_value.tv_sec = nsec / 1000000000;
313	9189x	ts.it_value.tv_nsec = nsec % 1000000000;
314	9189x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
315	✗	ts.it_value.tv_nsec = 1;
316		}
317		}
318
319	9480x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
320	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
321	9480x	}
322
323		inline void
324	36559x	epoll_scheduler::run_task(
325		lock_type& lock, context_type* ctx, long timeout_us)
326		{
327		int timeout_ms;
328	36559x	if (task_interrupted_)
329	23013x	timeout_ms = 0;
330	13546x	else if (timeout_us < 0)
331	13538x	timeout_ms = -1;
332		else
333	8x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
334
335	36559x	if (lock.owns_lock())
336	13546x	lock.unlock();
337
338	36559x	task_cleanup on_exit{this, &lock, ctx};
339
340		// Flush deferred timerfd programming before blocking
341	36559x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
342	4738x	update_timerfd();
343
344	36559x	int nfds = ::epoll_wait(
345		epoll_fd_, event_buffer_.data(),
346	36559x	static_cast<int>(event_buffer_.size()), timeout_ms);
347
348	36559x	if (nfds < 0 && errno != EINTR)
349	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
350
351	36559x	bool check_timers = false;
352	36559x	op_queue local_ops;
353
354	81730x	for (int i = 0; i < nfds; ++i)
355		{
356	45171x	if (event_buffer_[i].data.ptr == nullptr)
357		{
358		std::uint64_t val;
359		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
360	4839x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
361	4839x	eventfd_armed_.store(false, std::memory_order_relaxed);
362	4839x	continue;
363	4839x	}
364
365	40332x	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	4742x	::read(timer_fd_, &expirations, sizeof(expirations));
371	4742x	check_timers = true;
372	4742x	continue;
373	4742x	}
374
375		auto* desc =
376	35590x	static_cast<descriptor_state*>(event_buffer_[i].data.ptr);
377	35590x	desc->add_ready_events(event_buffer_[i].events);
378
379	35590x	bool expected = false;
380	35590x	if (desc->is_enqueued_.compare_exchange_strong(
381		expected, true, std::memory_order_release,
382		std::memory_order_relaxed))
383		{
384	35590x	local_ops.push(desc);
385		}
386		}
387
388	36559x	if (check_timers)
389		{
390	4742x	timer_svc_->process_expired();
391	4742x	update_timerfd();
392		}
393
394	36559x	lock.lock();
395
396	36559x	if (!local_ops.empty())
397	22475x	completed_ops_.splice(local_ops);
398	36559x	}
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