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

From: Evgeniy Polyakov
Date: Tue Aug 22 2006 - 04:35:34 EST

On Tue, Aug 22, 2006 at 01:17:52AM -0700, Nicholas Miell (nmiell@xxxxxxxxxxx) wrote:
> On Tue, 2006-08-22 at 11:24 +0400, Evgeniy Polyakov wrote:
> > On Tue, Aug 22, 2006 at 12:00:51AM -0700, Nicholas Miell (nmiell@xxxxxxxxxxx) wrote:
> > > On Mon, 2006-08-21 at 14:19 +0400, Evgeniy Polyakov wrote:
> > > > Generic event handling mechanism.
> > >
> > > Since this is the sixth[1] event notification system that's getting
> > > added to the kernel, could somebody please convince me that the
> > > userspace API is right this time? (Evidently, the others weren't and are
> > > now just backward compatibility bloat.)
> > >
> > > Just looking at the proposed kevent API, it appears that the timer event
> > > queuing mechanism can't be used for the queuing of POSIX.1b interval
> > > timer events (i.e. via a SIGEV_KEVENT notification value in a struct
> > > sigevent) because (being a very thin veneer over the internal kernel
> > > timer system) you can't specify a clockid, the time value doesn't have
> > > the flexibility of a struct itimerspec (no re-arm timeout or absolute
> > > times), and there's no way to alter, disable or query a pending timer or
> > > query a timer overrun count.
> > >
> > > Overall, kevent timers appear to be inconvenient to use and limited
> > > compared to POSIX interval timers (excepting the fact you can read their
> > > expiry events out of a queue, of course).
> >
> > Kevent timers are just trivial kevent user.
> > But even as is it is not that bad solution.
> > I, as user, do not want to know which timer is used - I only need to
> > get some signal when interval completed, especially I do not want to
> > have some troubles when timer with given clockid has disappeared.
> > Kevent timer can be trivially rearmed (acutally it is always rearmed
> > until one-shot flag is set).
> > Of course it can be disabled by removing requested kevent.
> > I can add possibility to alter timeout without removing kevent if there
> > is strong requirement for that.
> >
> 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

> > Timer notifications were designed not from committee point of view, when
> > theoretical discussions end up in multi-megabyte documentation 99.9% of
> > which can not be used without major brain surgery.
> Do you have any technical objections to the POSIX.1b interval timer
> design to back up your insults?

POSIX timers can have any design, but do not force others to use the

> > I just implemented what I use, if you want more - say what you need.
> I don't know what I need, I just know what POSIX already has, and your

And I do know what I need, that is why I do it.

> extensions don't appear to be compatible with that model and
> deliberately designing something that has no hope of ever getting into
> the POSIX standard or serving as the basis for whatever comes out of the
> standard committee seems rather stupid. (Especially considering that
> Linux's only viable competitor has already shipped a unified event
> queuing API that does fit into the existing POSIX design.)

I think I even know what it is :)

> Ulrich Drepper is probably better able to speak on this than I am,
> considering that he's involved with POSIX and is probably going to be
> involved in the Linux libc work, whatever it may be.

Feel free to use POSIX timers, but do not force others to it too.

> >
> > > [1] Previously: select, poll, AIO, epoll, and inotify. Did I miss any?
> >
> > Let me guess - kevent, which can do all above and a lot of other things?
> > And you forget netlink-based notificators - netlink, rtnetlink,
> > gennetlink, connector and tons of accounting application based on them,
> > kobject, kobject_uevent.
> > There also filessytem based ones - sysfs, procfs, debugfs, relayfs.
> OK, so with literally a dozen different interfaces to queue events to
> userspace, all of which are apparently inadequate and in need of
> replacement by kevent, don't you want to slow down a bit and make sure
> that the kevent API is correct before it becomes permanent and then just
> has to be replaced *again* ?

I only though that license issues remains unresolved in that
linux-kernel@ flood, but not, I was wrong :)

I will ask just one question, do _you_ propose anything here?

> --
> Nicholas Miell <nmiell@xxxxxxxxxxx>

Evgeniy Polyakov
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