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SubjectRe: [take24 0/6] kevent: Generic event handling mechanism.
On Sun, Nov 19, 2006 at 04:02:03PM -0800, Ulrich Drepper (drepper@redhat.com) wrote:
> Evgeniy Polyakov wrote:
> >>Possible solution:
> >>
> >>a) it would be possible to have a "used" flag in each ring buffer entry.
> >> That's too expensive, I guess.
> >>
> >>b) kevent_wait needs another parameter which specifies the which is the
> >> last (i.e., least recently added) entry in the ring buffer.
> >> Everything between this entry and the current head (in ->kidx) is
> >> occupied. If multiple threads arrive in kevent_wait the highest idx
> >> (with wrap around possibly lowest) is used.
> >>
> >> kevent_wait will not try to move more entries into the ring buffer
> >> if ->kidx and the higest index passed in to any kevent_wait call
> >> is equal (i.e., the ring buffer is full).
> >>
> >> There is one issue, though, and that is that a system call is needed
> >> to signal to the kernel that more entries in the ring buffer are
> >> processed and that they can be refilled. This goes against the
> >> kernel filling the ring buffer automatically (see below)
> >
> >If thread calls kevent_wait() it means it has processed previous entries,
> >one can call kevent_wait() with $num parameter as zero, which
> >means that thread does not want any new events, so nothing will be
> >copied.
>
> This doesn't solve the problem. You could only request new events when
> all previously reported events are processed. Plus: how do you report
> events if the you don't allow get_event pass them on?

Userspace should itself maintain order and possibility to get event in
this implementation, kernel just returns events which were requested.

> >Writable ring buffer does not sound too good to me - what if one thread
> >will overwrite the whole ring buffer so kernel's indexes can be screwed?
>
> Agreed, there are problems. This is why I suggested the ring buffer can
> be a structured. Parts of it might be read-only, other parts
> read/write. I don't necessarily think the 'used' flag is the right way.
> And front/tail pointer solution seems to be better.
>
>
> >Ring buffer processed not in FIFO order is wrong idea
>
> Not necessarily, see my comments about CPU affinity in the previous mail.
>
>
> >- ring buffer can
> >be potentially very big and searching there for the entry, which was
> >been marked as 'free' by userspace is not a solution at all - userspace
> >in that case must provide ukevent so fast tree search would be used,
> >(and although it is already possible) it requires userspace to make
> >additional syscalls which is not what we want.
>
> It is not necessary. I've proposed to only have a fron and tail
> pointer. The tail pointer is maintained by the application and passed
> to the kernel explicitly or via shared memory. The kernel maintains the
> front pointer. No tree needed.

There was such implementation (in previous patchset) - sine no one
commented, I changed that.

> >As a solution I can create folowing scheme:
> >there are two syscalls (or one with a switch) which get events and
> >commits them.
> >
> >kevent_wait() becomes a syscall which waits until number of events or
> >one of them becomes ready and just copies them into ring buffer and
> >returns. kevent_wait() will fail with special error code when ring
> >buffer is full.
> >
> >kevent_commit() frees requested number of events _from the beginning_,
> >i.e. from special index, visible from userspace. Userspace can create
> >special counters for events (and even put them into read-only ring
> >buffer overwriting some fields of kevent, especially if we will increase
> >it's size) and only call kevent_commit() when all events have zero usage
> >counter.
>
> Right, that's basically the front/tail pointer implementation. That
> would work. You just have to make sure that the kevent_wait() call
> takes the current front pointer/index as a parameter. This way if the
> buffer gets filled between the thread checking the ring buffer (and
> finding it empty) and the syscall being handled the thread is not suspended.

It is exactly how previous ring buffer (in mapped area though) was
implemented.

I think I need to quickly setup my slightly used (bought on ebay) but
still working mind reader, I will try to tune it to work with your brain
waves so next time I would not spent weeks changing something which
could be reused, while others keep silent :)

> >I disagree that having possibility to have holes in the ring buffer is a
> >good idea at all - it requires much more complex protocol, which will
> >fill and reuse that holes, and the main disavantge - it requires to
> >transfer much more information from userspace to kernelspace to free the
> >ring entry in the hole - in that case it is already possible just to
> >call kevent_ctl(KEVENT_REMOVE) and do not wash the brain with new
> >approach at all.
>
> Well, it would require more data transport of we'd use writable shared
> memory. But I agree, it's far too complicated and might not scale with
> growing ring buffer sizes.
>
>
> >>- implementing the kevent_wait syscall the proposed way means we are
> >> missing out on one possible optimization. The ring buffer is
> >> currently only filled on kevent_wait calls. I expect that in really
> >> high traffic situations requests are coming in at a higher rate than
> >> the can be processed. At least for periods of time. If such
> >> situations it would be nice to not have to call into the kernel at
> >> all. If the kernel would deliver into the ring buffer on its own
> >> this would be possible.
> >
> >Well, it can be done on behalf of workqueue or dedicated thread which
> >will bring up appropriate mm context,
>
> I think it should be done. It's potentially a huge advantage.
>
>
> >although it means that userspace
> >can not handle the load it requested, which is a bad sign...
>
> I don't understand. What is not supposed to work? There is nothing
> which cannot work with automatic posting since the get_event() call does
> nothing but copying the event data over and wake a thread.

If userspace is too slow to get events, dedicated thread or workqueue
will be busy unneded things, although they can allow to remove peaks in
the load.

> >>- the kevent_get_event syscall is not needed at all. All reporting
> >> should be done using a ring buffer. There really is not reason to
> >> keep two interfaces around which serve the same purpose. Making
> >> the argument the kevent_get_event is so much easier to use is not
> >> valid. The exposed interface to access the ring buffer will be easy,
> >> too. In the OLS paper I more or wait hinted at the interfaces. I
> >> think they should be like this (names are irrelevant):
> >
> >Well, kevent_get_events() _is_ much easier to use. And actually having
> >only that interface it is possible to implement ring buffer with any
> >kind or protocol for its controlling - userspace can have a wrapper
> >which will call kevent_get_events() with pointer which shows to the
> >place in the shared ring buffer where to place new events, that wrapper
> >can handle essentially any kind of flags/parameters which are suitable
> >for that ring buffer implementation.
>
> That's far too slow. The whole point behind the ring buffer is speed.
> And emulation would defeat the purpose.

It was an example, I do not say ring-buffer maintained in kernelspace is
bad idea. Actually it is possible to create several threads which will
only read events into the buffer, which will be processed by some pool of
'working' threads. There are a lot of possibilities to work with only
one syscall and create scalable system.

> >But since we started to implement ring buffer as a additional feature of
> >kevent, let's find the way all people will be happy with before removing
> >something which was proven to work correctly.
>
> The get_event interface is basically the userlevel interface the runtime
> (glibc probably) would provide. Programmers don't see the complexity.
>
> I'm concerned about the get_event interface holding the kernel
> implementation back. For instance, automatic filling the ring buffer.
> This would not be possible if the program is free to mix
> kevent_get_event and kevent_wait calls freely. If you do away with the
> get_event syscall the automatic ring buffer filling is possible and a
> logical extension.

Yes, that is why only one should be used.
If there are several threads, then ring buffer implementation should be
used otherwise just kevent_get_events().
In theory yes, access library like glibc can provide kevent_get_events()
which will read event from ring buffer, but there is no such call right
now, so kernel's kevent_get_events() looks reasonable.

> >The last three are exactly kevent_get_events() with different set of
> >parameters - it is possible to get events without sleeping, it is
> >possible to wait until at least something is ready and it is possible to
> >sleep for timeout.
>
> Exactly. But these interfaces should be implemented at userlevel, not
> at the syscall level. It's not necessary. The kernel interface should
> be kept as small as possible and the get_event syscall is pure duplication.

I would say that ring-buffer mainpulating syscalls are duplicatino, but
it is just matter of a view :)

> >They all already imeplemented. Just all above, and it was done several
> >months ago already. No need to reinvent what is already there.
> >Even if we will decide to remove kevent_get_events() in favour of ring
> >buffer-only implementation, winting-for-event syscall will be
> >essentially kevent_get_events() without pointer to the place where to
> >put events.
>
> Right, but this limitation of the interface is important. It means the
> interface of the kernel is smaller: fewer possibilities for problems and
> fewer constraints if in future something should be changed (and smaller
> kernel).

Ok, lets see for ring buffer implementation right now, and then we will
decide if we want to remove or to stay with kevent_get_events() syscall.

> >I agree that having special syscall to initialize kevent is a good idea,
> >and initial kevent implementation had it, but it was removed due to API
> >cleanup work by Cristoph Hellwing.
>
> Well, he is wrong. If, for instance, init or any of the programs which
> start first wants to use the syscall it couldn't because /dev isn't
> mounted. The program might use libraries and therefore not have any
> influence on whether the kevent stuff is used or not.
>
> Yes, the /dev interface is useful for some/many other kernel interfaces.
> But this is a core interface. For the same reason epoll_create is a
> syscall.

Ok, I will create initialization syscall.

> >Do you have _any_ kind of benchmarks with epoll() which would show that
> >it is feasible? ukevent is one cache line (well, 2 cache lines on old
> >CPUs), which can be setup way too far away from the time when it is
> >ready, and CPU which origianlly set that up can be busy, so we will lose
> >performance waiting until CPU becomes free instead of calling other
> >thread on different CPU.
>
> If the period between the generation of the event (e.g., incoming
> network traffic or sent data) and the delivery of the event by waking a
> thread is too long, it makes not too much sense. But if the L2 cache
> hasn't hasn't been flushed it might be a big advantage.
>
> I think it's reasonable to only have the last queued entry for a CPU
> handled special. And note, this is only ever a hint. If an event entry
> was created by the kernel in one CPU but none of the threads which wait
> to be waken is on that CPU, nothing has to be done.
>
> No, I don't have a benchmark. But it is likely quite easily possible to
> create a synthetic benachmark. Maybe with pipes.
>
>
> >It is possible to specify CPU id in kevent (not in ukevent, i.e. not
> >in shared by userspace structure, but in it's kernel representation),
> >and then check if currently active CPU is the same or not, but what if
> >it is not the same CPU?
>
> Nothing special. It's up to the userlevel wrapper code. The CPU number
> would only be a hint.
>
>
> >Entry order is important, since application can
> >take advantage of synchronization, so idea to skip some entries is bad.
>
> That's something the application should be make a call about. It's not
> always (or even mostly) the case that the ordering of the notification
> is important. Furthermore, this would also require the kernel to
> enforce an ordering. This is expensive on SMP machines. A locally
> generated event (i.e., source and the thread reporting the event) can be
> delivered faster than an event created on another CPU.

How come? If signal was delivered earlier than data arrived, userspace
should get signal before data - that is the rule. Ordering is maintained
not for event insertion, but for marking them ready - it is atomic, so
who first starts to mark even ready, that event will be read first from
the ready queue.

> >It is management task - kernel should not even know about someone has
> >died and can not process events it requested.
>
> But the kernel has to be involed.
>
>
> >Userspace can open a control pipe (and setup a kevent handler for it)
> >and glibc will write there a byte thus awakening some other thread.
> >It can be done in userspace and should be done in userspace.
>
> That's invasive. The problem is that no userlevel interface should have
> to implicitly keep file descriptors open. This would mean the
> application would be influenced since suddenly a file descriptor is not
> available anymore. Yes, applications shouldn't care but they
> unfortunately sometimes do.

Then I propose userspace notifications - each new thread can register
'wake me up when userspace event 1 is ready' and 'event 1' will be
marked as ready by glibc when it removes the thread.

> >Will we discuss it for death?
> >
> >Kevent does not need to have absolute timeout.
>
> Of course it does. Just because you don't see a need for it for your
> applications right now it doesn't mean it's not a valid use.

Please explain why glibc AIO uses relatinve timeouts then :)

> >Because timeout specified there is always related to the start of
> >syscall, since it is a timeout which specifies maximum time frame
> >syscall can live.
>
> That's your current implementation. There is absolutely no reason
> whatsoever why this couldn't be changed.

It has nothing with implementation - it is logic. Something starts and
it has its maximum lifetime, but not something starts and should be
stopped Jan 1, 2008. In the latter case one can setup a timer, but it
does not allow to specify maximum lifetime. If glibc posix sleeping
functions converts relatinve AIO timeouts into absolute it does not mean
all should do it. It is just not needed.

> >I created kevent_signal notifications - it allows user to setup any set
> >of interested signals before call to kevent_get_events() and friends.
> >
> >No need to solve a problem with operation way when there is tactical and
> >strategical ones
>
> Of course there is a need and I explained it before. Getting signal
> notifications is in no way the same as changing the signal mask
> temporarily. You cannot correctly emulate the case where you want to
> block a signal while in the call as reenable it afterwards. Receiving
> the signal as an event and then artificially raising it is not the same.
> Especially timing-wise, the signal kevent might not be seen long after
> the syscall returns because other entries are worked on first.
>
> The opposite case is equally impossible to emulate: unblocking a signal
> just for the duration of the syscall. These are all possible and used
> cases.

Add and remove appropriate kevent - it is as simple as call for one
function.

> >>- the KEVENT_REQ_WAKEUP_ONE functionality is good and needed. But I
> >> would reverse the default. I cannot see many places where you want
> >> all threads to be woken. Introduce KEVENT_REQ_WAKEUP_ALL instead.
> >
> >I.e. to wake up only first thread always and in addon those threads
> >which have specified flag set? Ok, will put into todo foer the next
> >release.
>
> It's a flag for an event. So the threads won't have the flag set. If
> an event is delivered with the flag set, wake all threads. Otherwise
> just one.

Ok.

> >>- there is really no reason to invent yet another timer implementation.
> >> We have the POSIX timers which are feature rich and nicely
> >> implemented. All that is needed is to implement SIGEV_KEVENT as a
> >> notification mechanism. The timer is registered as part of the
> >> timer_create() syscalls.
> >
> >Feel free to add any interface you like - it is as simple as call for
> >kevent_user_add_ukevent() in userspace.
>
> No, that's not what I mean. There is no need for the special
> timer-related part of your patch. Instead the existing POSIX timer
> syscalls should be modified to handle SIGEV_KEVENT notification. Again,
> keep the interface as small as possible. Plus, the POSIX timer
> interface is very flexible. You don't want to duplicate all that
> functionality.

Interface is already there with kevent_ctl(KEVENT_ADD), I just created
additional entry, which describes timers enqueue/dequeue callbacks - I
have not invented new interfaces, just reused existing generic kevent
facilities. It is possible to add timer events from any other place.

> >And I almost silently stay behind with the fact that it is possbile to
> >implement _all_ above ring buffer things in userspace with
> >kevent_get_events() and this functionality is there for almost a year :)
>
> Again, this defeats the purpose completely. The ring buffer is the
> faster interface, especially when coupled with asynchronous filling of
> ring buffer (i.e., without a syscal).

It is still possible to have very scalable system with it, for example
with one thread dedicated for syscall reading (with big number of events
transferred in one shot syscall overhead becomes negligible) and pool of
working threads. It is not about 'let's remove kernelspace ring buffer
management', but about possibilities and flexibility of the existing
model.

> >Let's solve problem in order of theirs appearance - what do you think
> >about above interface for ring buffer?
>
> Looks better, yes.

Ok, I will implement this new (old) ring buffer in present it in the
next release. I will also schedule there userspace notifications,
'wake-up-one-thread' flag changes and other small updates.

> --
> ➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View,
> CA ❖

--
Evgeniy Polyakov
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