[RFC PATCH v3 04/21] apps: common: event_loop: Use single event source for deferred calls

[Jacopo Mondi]

Hi Barnabás

On Thu, Jan 30, 2025 at 11:50:23AM +0000, Barnabás Pőcze wrote:
> Instead of calling `event_base_once()` every time a deferred call
> is added to the loop, create an event source at construction, and
> simply trigger that when a new deferred call is scheduled.
>
> Signed-off-by: Barnabás Pőcze <pobrn at protonmail.com>

Sorry, I've not replied to the previous version.

Now that I understand it better I really like this

Reviewed-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>

> ---
>  src/apps/common/event_loop.cpp | 48 +++++++++++++++++-----------------
>  src/apps/common/event_loop.h   |  5 +---
>  2 files changed, 25 insertions(+), 28 deletions(-)
>
> diff --git a/src/apps/common/event_loop.cpp b/src/apps/common/event_loop.cpp
> index bc8cf17ab..b6230f4ba 100644
> --- a/src/apps/common/event_loop.cpp
> +++ b/src/apps/common/event_loop.cpp
> @@ -21,12 +21,35 @@ EventLoop::EventLoop()
>  	evthread_use_pthreads();
>  	base_ = event_base_new();
>  	instance_ = this;
> +
> +	callsTrigger_ = event_new(base_, -1, EV_PERSIST, [](evutil_socket_t, short, void *closure) {
> +		auto *self = static_cast<EventLoop *>(closure);
> +
> +		for (;;) {
> +			std::function<void()> call;
> +
> +			{
> +				std::lock_guard locker(self->lock_);
> +				if (self->calls_.empty())
> +					break;
> +
> +				call = std::move(self->calls_.front());
> +				self->calls_.pop_front();
> +			}
> +
> +			call();
> +		}
> +	}, this);
> +	assert(callsTrigger_);
> +	event_add(callsTrigger_, nullptr);
>  }
>
>  EventLoop::~EventLoop()
>  {
>  	instance_ = nullptr;
>
> +	event_free(callsTrigger_);
> +
>  	events_.clear();
>  	event_base_free(base_);
>  	libevent_global_shutdown();
> @@ -57,7 +80,7 @@ void EventLoop::callLater(std::function<void()> &&func)
>  		calls_.push_back(std::move(func));
>  	}
>
> -	event_base_once(base_, -1, EV_TIMEOUT, dispatchCallback, this, nullptr);
> +	event_active(callsTrigger_, 0, 0);
>  }
>
>  void EventLoop::addFdEvent(int fd, EventType type,
> @@ -108,29 +131,6 @@ void EventLoop::addTimerEvent(const std::chrono::microseconds period,
>  	events_.push_back(std::move(event));
>  }
>
> -void EventLoop::dispatchCallback([[maybe_unused]] evutil_socket_t fd,
> -				 [[maybe_unused]] short flags, void *param)
> -{
> -	EventLoop *loop = static_cast<EventLoop *>(param);
> -	loop->dispatchCall();
> -}
> -
> -void EventLoop::dispatchCall()
> -{
> -	std::function<void()> call;
> -
> -	{
> -		std::unique_lock<std::mutex> locker(lock_);
> -		if (calls_.empty())
> -			return;
> -
> -		call = calls_.front();
> -		calls_.pop_front();
> -	}
> -
> -	call();
> -}
> -
>  EventLoop::Event::Event(std::function<void()> &&callback)
>  	: callback_(std::move(callback)), event_(nullptr)
>  {
> diff --git a/src/apps/common/event_loop.h b/src/apps/common/event_loop.h
> index 760075885..aac5ed3cc 100644
> --- a/src/apps/common/event_loop.h
> +++ b/src/apps/common/event_loop.h
> @@ -65,11 +65,8 @@ private:
>  	int exitCode_;
>
>  	std::deque<std::function<void()>> calls_;
> +	::event *callsTrigger_ = nullptr;
>
>  	std::list<std::unique_ptr<Event>> events_;
>  	std::mutex lock_;
> -
> -	static void dispatchCallback(evutil_socket_t fd, short flags,
> -				     void *param);
> -	void dispatchCall();
>  };
> --
> 2.48.1
>
>