[take25 1/6] kevent: Description.

[Evgeniy Polyakov <johnpol@2ka.mipt.ru>]

Description.


diff --git a/Documentation/kevent.txt b/Documentation/kevent.txt
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+Description.
+
+int kevent_init(struct kevent_ring *ring, unsigned int ring_size);
+
+num - size of the ring buffer in events 
+ring - pointer to allocated ring buffer
+
+Return value: kevent control file descriptor or negative error value.
+
+ struct kevent_ring
+ {
+   unsigned int ring_kidx, ring_uidx, ring_over;
+   struct ukevent event[0];
+ }
+
+ring_kidx - index in the ring buffer where kernel will put new events 
+		when kevent_wait() or kevent_get_events() is called 
+ring_uidx - index of the first entry userspace can start reading from
+ring_over - number of overflows of ring_uidx happend from the start.
+	Overflow counter is used to prevent situation when two threads 
+	are going to free the same events, but one of them was scheduled 
+	away for too long, so ring indexes were wrapped, so when that 
+	thread will be awakened, it will free not those events, which 
+	it suppose to free.
+
+Example userspace code (ring_buffer.c) can be found on project's homepage.
+
+Each kevent syscall can be so called cancellation point in glibc, i.e. when 
+thread has been cancelled in kevent syscall, thread can be safely removed 
+and no events will be lost, since each syscall (kevent_wait() or 
+kevent_get_events()) will copy event into special ring buffer, accessible 
+from other threads or even processes (if shared memory is used).
+
+When kevent is removed (not dequeued when it is ready, but just removed), 
+even if it was ready, it is not copied into ring buffer, since if it is 
+removed, no one cares about it (otherwise user would wait until it becomes 
+ready and got it through usual way using kevent_get_events() or kevent_wait()) 
+and thus no need to copy it to the ring buffer.
+
+-------------------------------------------------------------------------------
+
+
+int kevent_ctl(int fd, unsigned int cmd, unsigned int num, struct ukevent *arg);
+
+fd - is the file descriptor referring to the kevent queue to manipulate. 
+It is created by opening "/dev/kevent" char device, which is created with 
+dynamic minor number and major number assigned for misc devices. 
+
+cmd - is the requested operation. It can be one of the following:
+    KEVENT_CTL_ADD - add event notification 
+    KEVENT_CTL_REMOVE - remove event notification 
+    KEVENT_CTL_MODIFY - modify existing notification 
+
+num - number of struct ukevent in the array pointed to by arg 
+arg - array of struct ukevent
+
+Return value: 
+ number of events processed or negative error value.
+
+When called, kevent_ctl will carry out the operation specified in the 
+cmd parameter.
+-------------------------------------------------------------------------------
+
+ int kevent_get_events(int ctl_fd, unsigned int min_nr, unsigned int max_nr, 
+ 			__u64 timeout, struct ukevent *buf, unsigned flags);
+
+ctl_fd - file descriptor referring to the kevent queue 
+min_nr - minimum number of completed events that kevent_get_events will block 
+	 waiting for 
+max_nr - number of struct ukevent in buf 
+timeout - number of nanoseconds to wait before returning less than min_nr 
+	  events. If this is -1, then wait forever. 
+buf - pointer to an array of struct ukevent. 
+flags - unused 
+
+Return value:
+ number of events copied or negative error value.
+
+kevent_get_events will wait timeout milliseconds for at least min_nr completed 
+events, copying completed struct ukevents to buf and deleting any 
+KEVENT_REQ_ONESHOT event requests. In nonblocking mode it returns as many 
+events as possible, but not more than max_nr. In blocking mode it waits until 
+timeout or if at least min_nr events are ready.
+
+This function copies event into ring buffer if it was initialized, if ring buffer
+is full, KEVENT_RET_COPY_FAILED flag is set in ret_flags field.
+-------------------------------------------------------------------------------
+
+ int kevent_wait(int ctl_fd, unsigned int num, __u64 timeout);
+
+ctl_fd - file descriptor referring to the kevent queue 
+num - number of processed kevents 
+timeout - this timeout specifies number of nanoseconds to wait until there is 
+		free space in kevent queue 
+
+Return value:
+ number of events copied into ring buffer or negative error value.
+
+This syscall waits until either timeout expires or at least one event becomes 
+ready. It also copies events into special ring buffer. If ring buffer is full,
+it waits until there are ready events and then return.
+If kevent is one-shot kevent it is removed in this syscall.
+If kevent is edge-triggered (KEVENT_REQ_ET flag is set in 'req_flags') it is 
+requeued in this syscall for performance reasons.
+-------------------------------------------------------------------------------
+
+ int kevent_commit(int ctl_fd, unsigned int start, 
+ 	unsigned int num, unsigned int over);
+
+ctl_fd - file descriptor referring to the kevent queue 
+start - index of the first index in the ring buffer to start to commit from
+num - number of kevents to commit
+over - overflow count for given $start value
+
+Return value:
+ number of committed kevents or negative error value.
+
+This function commits, i.e. marks as empty, slots in the ring buffer, so
+they can be reused when userspace completes that entries processing.
+
+Overflow counter is used to prevent situation when two threads are going 
+to free the same events, but one of them was scheduled away for too long, 
+so ring indexes were wrapped, so when that thread will be awakened, it 
+will free not those events, which it suppose to free.
+
+It is possible that returned number of committed events will be smaller than
+requested number - it is possible when several threads try to commit the
+same events.
+-------------------------------------------------------------------------------
+
+The bulk of the interface is entirely done through the ukevent struct. 
+It is used to add event requests, modify existing event requests, 
+specify which event requests to remove, and return completed events.
+
+struct ukevent contains the following members:
+
+struct kevent_id id
+    Id of this request, e.g. socket number, file descriptor and so on 
+__u32 type
+    Event type, e.g. KEVENT_SOCK, KEVENT_INODE, KEVENT_TIMER and so on 
+__u32 event
+    Event itself, e.g. SOCK_ACCEPT, INODE_CREATED, TIMER_FIRED 
+__u32 req_flags
+    Per-event request flags,
+
+    KEVENT_REQ_ONESHOT
+        event will be removed when it is ready 
+
+    KEVENT_REQ_WAKEUP_ALL
+        Kevent wakes up only first thread interested in given event, 
+	or all threads if this flag is set.
+
+    KEVENT_REQ_ET
+        Edge Triggered behaviour. It is an optimisation which allows to move 
+	ready and dequeued (i.e. copied to userspace) event to move into set 
+	of interest for given storage (socket, inode and so on) again. It is 
+	very usefull for cases when the same event should be used many times 
+	(like reading from pipe). It is similar to epoll()'s EPOLLET flag. 
+
+    KEVENT_REQ_LAST_CHECK
+        if set allows to perform the last check on kevent (call appropriate 
+	callback) when kevent is marked as ready and has been removed from 
+	ready queue. If it will be confirmed that kevent is ready 
+	(k->callbacks.callback(k) returns true) then kevent will be copied 
+	to userspace, otherwise it will be requeued back to storage. 
+	Second (checking) call is performed with this bit cleared, so callback 
+	can detect when it was called from kevent_storage_ready() - bit is set, 
+	or kevent_dequeue_ready() - bit is cleared. If kevent will be requeued, 
+	bit will be set again.
+
+   KEVENT_REQ_ALWAYS_QUEUE
+        If this flag is set kevent will be queued into ready queue if it is 
+	ready at enqueue time, otherwise it will be copied back to userspace
+	and will not be queued into the storage.
+
+__u32 ret_flags
+    Per-event return flags
+
+    KEVENT_RET_BROKEN
+        Kevent is broken 
+
+    KEVENT_RET_DONE
+        Kevent processing was finished successfully 
+
+    KEVENT_RET_COPY_FAILED
+        Kevent was not copied into ring buffer due to some error conditions. 
+
+__u32 ret_data
+    Event return data. Event originator fills it with anything it likes 
+    (for example timer notifications put number of milliseconds when timer 
+    has fired 
+union { __u32 user[2]; void *ptr; }
+    User's data. It is not used, just copied to/from user. The whole structure 
+    is aligned to 8 bytes already, so the last union is aligned properly. 
+
+-------------------------------------------------------------------------------
+
+Usage
+
+For KEVENT_CTL_ADD, all fields relevant to the event type must be filled 
+(id, type, event, req_flags). 
+After kevent_ctl(..., KEVENT_CTL_ADD, ...) returns each struct's ret_flags 
+should be checked to see if the event is already broken or done.
+
+For KEVENT_CTL_MODIFY, the id, req_flags, and user and event fields must be 
+set and an existing kevent request must have matching id and user fields. If 
+match is found, req_flags and event are replaced with the newly supplied 
+values and requeueing is started, so modified kevent can be checked and 
+probably marked as ready immediately. If a match can't be found, the 
+passed in ukevent's ret_flags has KEVENT_RET_BROKEN set. KEVENT_RET_DONE is 
+always set.
+
+For KEVENT_CTL_REMOVE, the id and user fields must be set and an existing 
+kevent request must have matching id and user fields. If a match is found, 
+the kevent request is removed. If a match can't be found, the passed in 
+ukevent's ret_flags has KEVENT_RET_BROKEN set. KEVENT_RET_DONE is always set.
+
+For kevent_get_events, the entire structure is returned.
+
+-------------------------------------------------------------------------------
+
+Usage cases
+
+kevent_timer
+struct ukevent should contain following fields:
+    type - KEVENT_TIMER 
+    event - KEVENT_TIMER_FIRED 
+    req_flags - KEVENT_REQ_ONESHOT if you want to fire that timer only once 
+    id.raw[0] - number of seconds after commit when this timer shout expire 
+    id.raw[0] - additional to number of seconds number of nanoseconds