Re: [PATCH 2 of 4] Introduce i386 fibril scheduling

[Davide Libenzi]

On Fri, 2 Feb 2007, Linus Torvalds wrote:

> > You get some other funny things from co-routines which are very powerful,
> > very dangerous, or plain insane
> 
> You forgot "very hard to think about". 
> 
> We DO NOT want coroutines in general. It's clever, but it's
>  (a) impossible to do without language support that C doesn't have, or 
>      some really really horrid macro constructs that really only work for 
>      very specific and simple cases.
>  (b) very non-intuitive unless you've worked with coroutines a lot (and 
>      almost nobody has)

Actually, coroutines are not too bad to program once you have a 
total-coverage async scheduler to run them. The attached (very sketchy) 
example uses libpcl ( http://www.xmailserver.org/libpcl.html ) and epoll 
as scheduler (but here you can really use anything). You can implement 
coroutines in many way, from C preprocessor macros up to anything, but in 
the libpcl case they are simply switched stacks. Like fibrils are supposed 
to be. The problem is that in order to make a real-life example of 
coroutine-based application work, you need everything that can put you at 
sleep (syscalls or any external library call you have no control on) 
implemented in an async way. And what I ended up doing is exactly what Zab 
did inside the kernel. In my case a dynamic pool of (userspace) threads 
servicing any non-native potentially pre-emptive call, and signaling the 
result to a pollable fd (pipe in my case) that is integrated in the epoll 
(poll/select whatever) scheduler.
I personally find Zab idea a really good one, since it allows for generic 
kernel async implementation, w/out the burden of dirtying kernel code 
paths with AIO knowledge. Being it fibrils or real kthreads, it is IMO 
definitely worth a very close look.




- Davide


struct eph_conn {
	int sfd;
	unsigned int events, revents;
	coroutine_t co;
};



int eph_new_conn(int sfd, void *func) {
	struct eph_conn *conn;
	struct epoll_event ev;

	conn = (struct eph_conn *) malloc(sizeof(struct eph_conn));

	conn->sfd = sfd;
	conn->co = co_create(func, conn, NULL, STACKSIZE);

	ev.events = 0;
	ev.data.ptr = conn;
	epoll_ctl(kdpfd, EPOLL_CTL_ADD, sfd, &ev);

	co_call(conn->co);

	return 0;
}

void eph_exit_conn(struct eph_conn *conn) {
	struct epoll_event ev;

	epoll_ctl(kdpfd, EPOLL_CTL_DEL, conn->sfd, &ev);
	co_exit();
}

int eph_connect(struct eph_conn *conn, const struct sockaddr *serv_addr, socklen_t addrlen) {

	if (connect(conn->sfd, serv_addr, addrlen) == -1) {
		if (errno != EWOULDBLOCK && errno != EINPROGRESS)
			return -1;
		co_resume();
		if (conn->revents & (EPOLLERR | EPOLLHUP))
			return -1;
	}
	return 0;
}

int eph_read(struct eph_conn *conn, void *buf, int nbyte) {
	int n;

	while ((n = read(conn->sfd, buf, nbyte)) < 0) {
		if (errno == EINTR)
			continue;
		if (errno != EAGAIN && errno != EWOULDBLOCK)
			return -1;
		co_resume();
	}
	return n;
}

int eph_write(struct eph_conn *conn, void const *buf, int nbyte) {
	int n;

	while ((n = write(conn->sfd, buf, nbyte)) < 0) {
		if (errno == EINTR)
			continue;
		if (errno != EAGAIN && errno != EWOULDBLOCK)
			return -1;
		co_resume();
	}
	return n;
}

int eph_accept(struct eph_conn *conn, struct sockaddr *addr, int *addrlen) {
	int sfd;

	while ((sfd = accept(conn->sfd, addr, (socklen_t *) addrlen)) < 0) {
		if (errno == EINTR)
			continue;
		if (errno != EAGAIN && errno != EWOULDBLOCK)
			return -1;
		co_resume();
	}
	return sfd;
}

int eph_scheduler(int loop, long timeout) {
	int i, nfds;
	struct eph_conn *conn;
	struct epoll_event *cevents;

	do {
		nfds = epoll_wait(kdpfd, events, maxfds, timeout);

		for (i = 0, cevents = events; i < nfds; i++, cevents++) {
			conn = cevents->data.ptr;
			conn->revents = cevents->events;
			if (conn->revents & conn->events)
				co_call(conn->co);
		}
	} while (loop);

	return 0;
}