linux-kernel - Re: [take12 0/3] kevent: Generic event handling mechanism.

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Message-ID: <20060822100316.GA31820@2ka.mipt.ru>
Date:	Tue, 22 Aug 2006 14:03:16 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Nicholas Miell <nmiell@...cast.net>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Ulrich Drepper <drepper@...hat.com>,
	Andrew Morton <akpm@...l.org>, netdev <netdev@...r.kernel.org>,
	Zach Brown <zach.brown@...cle.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [take12 0/3] kevent: Generic event handling mechanism.

On Tue, Aug 22, 2006 at 02:29:48AM -0700, Nicholas Miell (nmiell@...cast.net) wrote:
> > > Is any of this documented anywhere? I'd think that any new userspace
> > > interfaces should have man pages explaining their use and some example
> > > code before getting merged into the kernel to shake out any interface
> > > problems.
> > 
> > There are two excellent articles on lwn.net
> 
> Google knows of one and it doesn't actually explain how to use kevents.

http://lwn.net/Articles/192964/
http://lwn.net/Articles/172844/

In the thread there were enough links to homepage where you can find
several examples of how to use kevents (and timers among others) with
old interfaces and new ones.

> > I will ask just one question, do _you_ propose anything here?
> >  
> 
> struct sigevent sigev = {
> 	.sigev_notify = SIGEV_KEVENT,
> 	.sigev_kevent_fd = kev_fd,
> 	.sigev_value.sival_ptr = &MyCookie
> };
> 
> struct itimerspec its = {
> 	.it_value = { ... },
> 	.it_interval = { ... }
> };
> 
> struct timespec timeout = { .. };
> 
> struct ukevent events[max];
> 
> timer_t timer;
> 
> timer_create(CLOCK_MONOTONIC, &sigev, &timer);
> timer_settime(timer, 0, &its, NULL);
> 
> /* ... */
> 
> kevent_get_events(kev_fd, min, max, &timeout, events, 0);
> 
> 
> 
> Which isn't all that different from what Ulrich Drepper suggested and
> Solaris does right now. (timer_create would probably end up calling
> kevent_ctl itself, but it obviously can't do that unless kevents
> actually support real interval timers).

Ugh, rtsignals... Their's problems forced me to not implement
"interrupt"-like mechanism for kevents in addition to dequeueing.

Anyway, it seems you did not read the whole thread, homepage, lwn and
userpsace examples, so you do not understand what kevents are.

They are userspace requests which are returned back when they are ready.
It means that userspace must provide something to kernel and ask it to
notify when that "something" is ready. For example it can provide a
timeout value and ask kernel to fire a timer with it and inform
userspace when timeout has expired.
It does not matter what timer is used there - feel free to use
high-resolution one, usual timer, busyloop or anything else. Main issue 
that userspace request must be completed.

What you are trying to do is to put kevents under POSIX API.
That means that those kevents can not be read using
kevent_get_events(), basicaly because there are no user-known kevents,
i.e. user has not requested timer, so it should not receive it's
notifications (otherwise it will receive everything requested by other
threads and other issues, i.e. how to differentiate timer request made
by timer_create(), which is not supposed to be caught by
kevent_get_events()).

You could implement POSIX timer _fully_ on top of kevents, i.e. both
create and read, for example network AIO is implemented in that way -
there is a system calls aio_send()/aio_recv() and aio_sendfile() which
create kevent internally and then get it's readiness notifications over
provided callback, process data and finally remove kevent,
so POSIX timers could create timer kevent, wait until it is ready, in
completeness callback it would call signal delivering mechanism...

But there are no reading mechanism in POSIX timers (I mean not reading
pending timeout values or remaining time), they use signals for 
completeness delivering... So where do you want to put kevent's
userspace there?

What you are trying to achive is not POSIX timers in any way, you want
completely new machanism which has similar to POSIX API, and I give it to
you (well, with API which can be used not only with timers, but with any 
other type of notifications you like). 
You need clockid_t? Put it in raw.id[0] and make kevent_timer_enqueue()
callback select different type of timers.
What else?

> -- 
> Nicholas Miell <nmiell@...cast.net>

-- 
	Evgeniy Polyakov
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