1. 18 May, 2007 1 commit
  2. 11 May, 2007 1 commit
    • Davide Libenzi's avatar
      signal/timer/event: eventfd core · e1ad7468
      Davide Libenzi authored
      This is a very simple and light file descriptor, that can be used as event
      wait/dispatch by userspace (both wait and dispatch) and by the kernel
      (dispatch only).  It can be used instead of pipe(2) in all cases where those
      would simply be used to signal events.  Their kernel overhead is much lower
      than pipes, and they do not consume two fds.  When used in the kernel, it can
      offer an fd-bridge to enable, for example, functionalities like KAIO or
      syslets/threadlets to signal to an fd the completion of certain operations.
      But more in general, an eventfd can be used by the kernel to signal readiness,
      in a POSIX poll/select way, of interfaces that would otherwise be incompatible
      with it.  The API is:
      
      int eventfd(unsigned int count);
      
      The eventfd API accepts an initial "count" parameter, and returns an eventfd
      fd.  It supports poll(2) (POLLIN, POLLOUT, POLLERR), read(2) and write(2).
      
      The POLLIN flag is raised when the internal counter is greater than zero.
      
      The POLLOUT flag is raised when at least a value of "1" can be written to the
      internal counter.
      
      The POLLERR flag is raised when an overflow in the counter value is detected.
      
      The write(2) operation can never overflow the counter, since it blocks (unless
      O_NONBLOCK is set, in which case -EAGAIN is returned).
      
      But the eventfd_signal() function can do it, since it's supposed to not sleep
      during its operation.
      
      The read(2) function reads the __u64 counter value, and reset the internal
      value to zero.  If the value read is equal to (__u64) -1, an overflow happened
      on the internal counter (due to 2^64 eventfd_signal() posts that has never
      been retired - unlickely, but possible).
      
      The write(2) call writes an __u64 count value, and adds it to the current
      counter.  The eventfd fd supports O_NONBLOCK also.
      
      On the kernel side, we have:
      
      struct file *eventfd_fget(int fd);
      int eventfd_signal(struct file *file, unsigned int n);
      
      The eventfd_fget() should be called to get a struct file* from an eventfd fd
      (this is an fget() + check of f_op being an eventfd fops pointer).
      
      The kernel can then call eventfd_signal() every time it wants to post an event
      to userspace.  The eventfd_signal() function can be called from any context.
      An eventfd() simple test and bench is available here:
      
      http://www.xmailserver.org/eventfd-bench.c
      
      This is the eventfd-based version of pipetest-4 (pipe(2) based):
      
      http://www.xmailserver.org/pipetest-4.c
      
      Not that performance matters much in the eventfd case, but eventfd-bench
      shows almost as double as performance than pipetest-4.
      
      [akpm@linux-foundation.org: fix i386 build]
      [akpm@linux-foundation.org: add sys_eventfd to sys_ni.c]
      Signed-off-by: default avatarDavide Libenzi <davidel@xmailserver.org>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      e1ad7468