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Subject[take24 0/6] kevent: Generic event handling mechanism.

Generic event handling mechanism.

Kevent is a generic subsytem which allows to handle event notifications.
It supports both level and edge triggered events. It is similar to
poll/epoll in some cases, but it is more scalable, it is faster and
allows to work with essentially eny kind of events.

Events are provided into kernel through control syscall and can be read
back through ring buffer or using usual syscalls.
Kevent update (i.e. readiness switching) happens directly from internals
of the appropriate state machine of the underlying subsytem (like
network, filesystem, timer or any other).


Documentation page:

Consider for inclusion.

Changes from 'take23' patchset:
* kevent PIPE notifications
* KEVENT_REQ_LAST_CHECK flag, which allows to perform last check in dequeuing time
* fixed poll/select notifications (were broken due to tree manipulations)
* made Documentation/kevent.txt look nice in 80-col terminal
* fix for copy_to_user() failure report for the first kevent (Andrew Morton)
* minor fucntion renames
Here is pipe result with kevent_pipe kernel kevent part with 2000 pipes
(Eric Dumazet's application):
epoll (edge-triggered): 248408 events/sec
kevent (edge-triggered): 269282 events/sec
Busy reading loop: 269519 events/sec

Changes from 'take22' patchset:
* new ring buffer implementation in process' memory
* wakeup-one-thread flag
* edge-triggered behaviour
With this release additional independent benchmark shows kevent speed compared to epoll:
Eric Dumazet created special benchmark which creates set of AF_INET sockets and two threads
start to simultaneously read and write data from/into them.
Here is results:
epoll (no EPOLLET): 57428 events/sec
kevent (no ET): 59794 events/sec
epoll (with EPOLLET): 71000 events/sec
kevent (with ET): 78265 events/sec
Maximum (busy loop reading events): 88482 events/sec

Changes from 'take21' patchset:
* minor cleanups (different return values, removed unneded variables, whitespaces and so on)
* fixed bug in kevent removal in case when kevent being removed
is the same as overflow_kevent (spotted by Eric Dumazet)

Changes from 'take20' patchset:
* new ring buffer implementation
* removed artificial limit on possible number of kevents
With this release and fixed userspace web server it was possible to
achive 3960+ req/s with client connection rate of 4000 con/s
over 100 Mbit lan, data IO over network was about 10582.7 KB/s, which
is too close to wire speed if we get into account headers and the like.

Changes from 'take19' patchset:
* use __init instead of __devinit
* removed 'default N' from config for user statistic
* removed kevent_user_fini() since kevent can not be unloaded
* use KERN_INFO for statistic output

Changes from 'take18' patchset:
* use __init instead of __devinit
* removed 'default N' from config for user statistic
* removed kevent_user_fini() since kevent can not be unloaded
* use KERN_INFO for statistic output

Changes from 'take17' patchset:
* Use RB tree instead of hash table.
At least for a web sever, frequency of addition/deletion of new kevent
is comparable with number of search access, i.e. most of the time events
are added, accesed only couple of times and then removed, so it justifies
RB tree usage over AVL tree, since the latter does have much slower deletion
time (max O(log(N)) compared to 3 ops),
although faster search time (1.44*O(log(N)) vs. 2*O(log(N))).
So for kevents I use RB tree for now and later, when my AVL tree implementation
is ready, it will be possible to compare them.
* Changed readiness check for socket notifications.

With both above changes it is possible to achieve more than 3380 req/second compared to 2200,
sometimes 2500 req/second for epoll() for trivial web-server and httperf client on the same
It is possible that above kevent limit is due to maximum allowed kevents in a time limit, which is
4096 events.

Changes from 'take16' patchset:
* misc cleanups (__read_mostly, const ...)
* created special macro which is used for mmap size (number of pages) calculation
* export kevent_socket_notify(), since it is used in network protocols which can be
built as modules (IPv6 for example)

Changes from 'take15' patchset:
* converted kevent_timer to high-resolution timers, this forces timer API update at
* use struct ukevent* instead of void * in syscalls (documentation has been updated)
* added warning in kevent_add_ukevent() if ring has broken index (for testing)

Changes from 'take14' patchset:
* added kevent_wait()
This syscall waits until either timeout expires or at least one event
becomes ready. It also commits that @num events from @start are processed
by userspace and thus can be be removed or rearmed (depending on it's flags).
It can be used for commit events read by userspace through mmap interface.
Example userspace code (evtest.c) can be found on project's homepage.
* added socket notifications (send/recv/accept)

Changes from 'take13' patchset:
* do not get lock aroung user data check in __kevent_search()
* fail early if there were no registered callbacks for given type of kevent
* trailing whitespace cleanup

Changes from 'take12' patchset:
* remove non-chardev interface for initialization
* use pointer to kevent_mring instead of unsigned longs
* use aligned 64bit type in raw user data (can be used by high-res timer if needed)
* simplified enqueue/dequeue callbacks and kevent initialization
* use nanoseconds for timeout
* put number of milliseconds into timer's return data
* move some definitions into user-visible header
* removed filenames from comments

Changes from 'take11' patchset:
* include missing headers into patchset
* some trivial code cleanups (use goto instead of if/else games and so on)
* some whitespace cleanups
* check for ready_callback() callback before main loop which should save us some ticks

Changes from 'take10' patchset:
* removed non-existent prototypes
* added helper function for kevent_registered_callbacks
* fixed 80 lines comments issues
* added shared between userspace and kernelspace header instead of embedd them in one
* core restructuring to remove forward declarations
* s o m e w h i t e s p a c e c o d y n g s t y l e c l e a n u p
* use vm_insert_page() instead of remap_pfn_range()

Changes from 'take9' patchset:
* fixed ->nopage method

Changes from 'take8' patchset:
* fixed mmap release bug
* use module_init() instead of late_initcall()
* use better structures for timer notifications

Changes from 'take7' patchset:
* new mmap interface (not tested, waiting for other changes to be acked)
- use nopage() method to dynamically substitue pages
- allocate new page for events only when new added kevent requres it
- do not use ugly index dereferencing, use structure instead
- reduced amount of data in the ring (id and flags),
maximum 12 pages on x86 per kevent fd

Changes from 'take6' patchset:
* a lot of comments!
* do not use list poisoning for detection of the fact, that entry is in the list
* return number of ready kevents even if copy*user() fails
* strict check for number of kevents in syscall
* use ARRAY_SIZE for array size calculation
* changed superblock magic number
* use SLAB_PANIC instead of direct panic() call
* changed -E* return values
* a lot of small cleanups and indent fixes

Changes from 'take5' patchset:
* removed compilation warnings about unused wariables when lockdep is not turned on
* do not use internal socket structures, use appropriate (exported) wrappers instead
* removed default 1 second timeout
* removed AIO stuff from patchset

Changes from 'take4' patchset:
* use miscdevice instead of chardevice
* comments fixes

Changes from 'take3' patchset:
* removed serializing mutex from kevent_user_wait()
* moved storage list processing to RCU
* removed lockdep screaming - all storage locks are initialized in the same function, so it was
to differentiate between various cases
* remove kevent from storage if is marked as broken after callback
* fixed a typo in mmaped buffer implementation which would end up in wrong index calcualtion

Changes from 'take2' patchset:
* split kevent_finish_user() to locked and unlocked variants
* do not use KEVENT_STAT ifdefs, use inline functions instead
* use array of callbacks of each type instead of each kevent callback initialization
* changed name of ukevent guarding lock
* use only one kevent lock in kevent_user for all hash buckets instead of per-bucket locks
* do not use kevent_user_ctl structure instead provide needed arguments as syscall parameters
* various indent cleanups
* added optimisation, which is aimed to help when a lot of kevents are being copied from
* mapped buffer (initial) implementation (no userspace yet)

Changes from 'take1' patchset:
- rebased against 2.6.18-git tree
- removed ioctl controlling
- added new syscall kevent_get_events(int fd, unsigned int min_nr, unsigned int max_nr,
unsigned int timeout, void __user *buf, unsigned flags)
- use old syscall kevent_ctl for creation/removing, modification and initial kevent
- use mutuxes instead of semaphores
- added file descriptor check and return error if provided descriptor does not match
kevent file operations
- various indent fixes
- removed aio_sendfile() declarations.

Thank you.

Signed-off-by: Evgeniy Polyakov <>

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