Kernel Space: New revisions to the kevent API

The long process of revising kervent can be seen as evidence that the system is working as it should

Some patches make it into the kernel in something very close to their original form. Others have to go through a few changes first. The all-time record for development iterations may be held by devfs; Richard Gooch had just released the 157th revision when this ill-fated subsystem was merged for 2.3.46. On that scale, Evgeniy Polyakov is just getting started with kevent take 26; even so, the process must be starting to seem like a long one.

In this case, however, the long process can be seen as evidence that the system is working as it should. The kevent subsystem is a major addition to the Linux system call API. Once it goes in, it will have to be supported forever (to a finite-precision arithmetic approximation, at least). Adding a kevent interface with warts, or which does not provide the best performance possible, would be a serious mistake. Nobody wants to be faced with designing and implementing a new event interface in a few years while supporting the old one indefinitely. So it makes sense to go slow and make sure that things have been thought out well.

The number of people posting comments on the kevent patches has been relatively small; for whatever reason, many normally vocal developers do not seem to have much to say on this new API. Fortunately, Ulrich Drepper (the glibc maintainer) has taken a strong interest in this interface and has pushed hard for the changes he thought were necessary. One gets the sense the Ulrich and Evgeniy have gotten a little tired of each other over the last month or so. But, to their credit, they have stuck to the task. As of this writing, Ulrich has not commented on the version of the API implemented in the "take 26" patch set. It does, however, clearly reflect some of the things he has been asking for.

While Evgeniy has been concerned with getting events out of the kernel, Ulrich has been worried about performance and robustness. So he wanted ways for multi-threaded programs to cancel threads at any time without losing track of which events have been processed. Whenever possible, he would like to be able to process events without involving the kernel at all. And he has pushed strongly for timeout values to be represented in an absolute format. Evgeniy has (a bit grudgingly, at times) addressed most of these wishes.

It is still possible to get a kevent file descriptor by opening /dev/kevent, though that is no longer the only way. The kevent_ctl() system call is still used for the management of events:

int kevent_ctl(int fd, unsigned int cmd, unsigned int num,
&nbsp&nbsp&nbsp struct ukevent *arg);

With kevent_ctl(), an application can add requests for events, remove them, or modify them in place. There is a new KEVENT_CTL_READY operation which can be used to mark specific events as being "ready" and cause the kernel to wake up one or more processes waiting for events.

The synchronous interface has been changed slightly:

int kevent_get_events(int ctl_fd, unsigned int min_nr,
&nbsp&nbsp&nbspunsigned int max_nr, struct timespec timeout,
struct ukevent *buf, unsigned flags);

The difference is that the timeout value now is a struct timespec. That value is still interpreted as a relative timeout, however, unless flags contains KEVENT_FLAGS_ABSTIME. In the latter case, timeout is an absolute time, and the code will print a warning to the effect that Evgeniy was wrong in believing that nobody would ever want to use absolute times.

It is expected, however, that performance-aware applications will use the user-space ring buffer rather than the synchronous interface. That ring buffer is still set up with kevent_init():

int kevent_init(struct kevent_ring *ring, unsigned int ring_size,
&nbsp&nbsp&nbsp unsigned int flags);

The file descriptor argument has been removed from this system call; instead, kevent_init() opens a new file descriptor and passes it back as its return value. Thus, there is no separate need to open /dev/kevent.

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