I’ve been trying to refresh my understanding of sleeping in the kernel with regards to wait queues. So started browsing the source code for bcmgenet.c (kernel version 4.4) which is the driver responsible for driving the 7xxx series of Broadcom SoC for their set top box solution.
As part of the probe callback, this driver initializes a work queue which is part of the driver’s private structure and adds itself to the Q. But I do not see any blocking of any kind anywhere. Then it goes on to initialize a work queue with a function to call when woken up.
Now coming to the ISR0 for the driver, within that is an explicit call to the scheduler as part of the ISR (bcmgenet_isr0) if certain conditions are met. Now AFAIK, this call is used to defer work to a later time, much like a tasklet does.
Post this we check some MDIO status flags and if the conditions are met, we wake up the process which was blocked in process context. But where exactly is the process blocked?
Also, most of the time, wait queues seem to be used in conjunction with work queues. Is that the typical way to use them?
As part of the probe callback, this driver initializes a work queue which is part of the driver’s private structure and adds itself to the Q. But I do not see any blocking of any kind anywhere.
I think you meant the wait queue head, not the work queue. I do not see any evidence of the probe adding itself to the queue; it is merely initializing the queue.
The queue is used by the calls to the wait_event_timeout() macro in the bcmgenet_mii_read() and bcmgenet_mii_write() functions in bcmmii.c. These calls will block until either the condition they are waiting for becomes true or the timeout period elapses. They are woken up by the wake_up(&priv->wq); call in the ISR0 interrupt handler.
Then it goes on to initialize a work queue with a function to call when woken up.
It is initializing a work item, not a work queue. The function will be called from a kernel thread as a result of the work item being added to the system work queue.
Now coming to the ISR0 for the driver, within that is an explicit call to the scheduler as part of the ISR (bcmgenet_isr0) if certain conditions are met. Now AFAIK, this call is used to defer work to a later time, much like a tasklet does.
You are referring to the schedule_work(&priv->bcmgenet_irq_work); call in the ISR0 interrupt handler. This is adding the previously mentioned work item to the system work queue. It is similar to as tasklet, but tasklets are run in a softirq context whereas work items are run in a process context.
Post this we check some MDIO status flags and if the conditions are met, we wake up the process which was blocked in process context. But where exactly is the process blocked?
As mentioned above, the process is blocked in the bcmgenet_mii_read() and bcmgenet_mii_write() functions, although they use a timeout to avoid blocking for long periods. (This timeout is especially important for those versions of GENET that do not support MDIO-related interrupts!)
Also, most of the time, wait queues seem to be used in conjunction with work queues. Is that the typical way to use them?
Not especially. This particular driver uses both a wait queue and a work item, but I wouldn't describe them as being used "in conjunction" since they are being used to handle different interrupt conditions.
Related
According to here, GetMessage() is a blocking call which won't return until there's a message can be retrieved from the message queue.
So, how is this blocking behavior implemented?
Does GetMessage() use some kind of spin lock so that the UI thread just busy waits for new messages showing up in the message queue? If so, I guess at least one of my CPU cores should have high usage when a UI application is running. But I didn't see that happen. So how does it work?
ADD 1
Thanks for the hint in the comments. Spin lock is meant to reduce the cost of thread context switch. It shouldn't be used here. I also thought about maybe some event paradigm is used here. But if it is event-driven, how does this event model work?
My guess is like this:
An event for input checking is raised periodically. Perhaps via some hardware timer interrupt. Then the timer interrupt handler will check for various input device buffers for input events. And then put that into certain application's message queue based on the current desktop context, such as which is the active window.
And I guess maybe some other things are also based on the timer interrupt, such as thread context switching.
ADD 2
Based on replies so far. There's some event object that the UI thread waits on. But since UI thread is waiting on something, it is not active and can do nothing by itself yet. And the event object is just some passive state information. So there has to be someone else to wake up the thread upon the event state change. I tink it should be the thread scheduler. And the thread scheduler may be pulsed by the timer interrupt.
The thread scheduler will check the event state periodically and wake up thread and put messages into its queue as necessary.
Am I right about the whole picture?
ADD 3
And there's a remaining question: who modify the state of an event object? Based on here, it seems events are just some data structures that can be modified by any active parties. I think thread scheduler just use the relations among threads and events to decide which thread to run or not.
And by the time a thread is scheduled to run, all it's requirements should already been fulfilled. Such as a message should have been put into its queue before the event it waits on is raised. This is reasonable because otherwise it may be too late. (thanks to RbMm's comments.)
ADD 4
In JDK, the LinkedBlockingDeque type also offers a similar blocking behavior with the take() method.
Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque), waiting if
necessary until an element becomes available.
And the .NET counterpart is the BlockingCollection< T > type. A thread to discuss it.
Here is a thread about how to implement a blocking queue in C#.
The exact implementation is internal and not documented.
Most of the window manager in Windows is implemented in kernel mode. GetMessage is no different. It waits on an event object in kernel mode. When a thread is waiting on an event (or any other kernel-based synchronization object), it does not use any CPU time, because it is not scheduled to run until the wait is satisfied.
When a message is posted to the waiting thread's message queue, or another thread sends a message to a window belonging to the waiting thread, or a timer in the waiting thread fires, or a window in the waiting thread is ready to be painted, the event is signaled, the thread wakes up, and GetMessage() acts accordingly, whether that is to return a posted message to the caller, or to dispatch a sent message directly to the target window procedure and then go back to waiting on the event.
You can see the implementation leaking into the public API if you compare the maximum number of objects you can wait on in WaitForMultipleObjects and MsgWaitForMultipleObjects:
The maximum number of object handles is MAXIMUM_WAIT_OBJECTS.
vs
The maximum number of object handles is MAXIMUM_WAIT_OBJECTS minus
one.
I don't understand part of the latest Windows threadpool API. I need help with that.
From the documentation, the recipe to use it for I/O (in my case, for SOCKET) can be summarized as follows:
Call CreateThreadpoolIo.
Call StartThreadpoolIo. You can find this warning there:
You must call this function before initiating each asynchronous I/O operation on the file handle bound to the I/O completion object. Failure to do so will cause the thread pool to ignore an I/O operation when it completes and will cause memory corruption.
Call the operation on the file handle (e.g., WSARecvFrom). If it fails, call CancelThreadpoolIo. Otherwise, process the result when it is available. WSARecvFrom, when used asynchronously, asks for a WSAOVERLAPPED (that you have to create beforehand) but not for any information that links it to the previous call to StartThreadpoolIo. CancelThreadpoolIo only asks for the PTP_IO, but not for any additional information to derive a specific asynchronous operation.
Repeat steps 2 and 3.
Call CloseThreadpoolIo to finish. You can find this warning there:
It may be necessary to cancel threadpool I/O notifications to prevent memory leaks. For more information, see CancelThreadpoolIo.
I usually need it for UDP, so I strive to have several reception operations queued (asynchronous WSARecvFrom operations started) at any given time. That way I don't have to rush to start another reception operation at the beginning of the callback function nor synchronize access to the reception buffers (I can have a pool of them, each one able to contain a datagram, and reissue the reception operation when I finish processing each message; in the interim, other queued operations will keep the receiver busy). Datagrams are independent and self contained. I'm aware that this approach may not be valid for TCP.
StartThreadpoolIo/CancelThreadpoolIo seem to me the source of the problem: StartThreadpoolIo and WSARecvFrom are not directly bound (they don't share any arguments). So:
How can the framework know which operation to cancel when you call CancelThreadpoolIo? How does it cancel just the operation that failed and not any of the pending ones?
You can say, "don't call StartThreadpoolIo concurrently". I can live without several concurrent WSARecvFrom's, but I can't live without concurrent WSARecvFrom and WSASendTo. So I think being unable to have several asynchronous operations at the same time can't be the way the API was designed.
You can say, "call StartThreadpoolIo only once, that will suffice to register the callback; it is an on/off process". But the documentation says:
You must call this function before initiating each asynchronous I/O operation on the file handle...
You can say, "it cancels the operation started by the same thread that just called StartThreadpoolIo". But then the advice of calling CancelThreadpoolIo in the context of calling CloseThreadpoolIo doesn't make sense (I will call CloseThreadpoolIo from the thread that triggers stopping, which will be completely independent from the threads issuing the asynchronous operations; and a single call to CancelThreadpoolIo may not be enough to cancel several operations). Being unable to trigger cancellation from a different thread is a serious limitation, anyway. I'm aware of the existence of CreateThreadpoolCleanupGroup, but my question is more fundamental. I want to understand how this API can be fundamentally right and useful.
You can say "call CreateThreadpoolIo several times, so that you have independent PTP_IO's to work with". It doesn't work. When I call CreateThreadpoolIo a second time, nullptr is returned.
Am I wrong, or is this API awkward? Normally, other asynchronous APIs work with one of these patterns:
Create an operation and receive a handle => call methods passing the handle.
Create a reusable handle => call methods (including starting operations) passing the handle.
The latest Windows threadpool API, in which the handle seems to be implicit, or there are several handles for the same operation (TP_IO, WSAOVERLAPPED, StartThreadpoolIo) and they aren't all explicitly linked together, uses neither of them.
Thank you very much for your help.
How can the framework know which operation to cancel when you call CancelThreadpoolIo? How does it cancel just the operation that failed
and not any of the pending ones?
CancelThreadpoolIo() doesn't cancel IO. It is reciprocal to StartThreadpoolIo(). StartThreadpoolIo() prepares threadpool to accept a completion. If threadpool doesn't expect a completion, it won't wait for it, thus you may miss it. If threadpool expects a completion but completion doesn't happen, threadpool may waste resources.
CancelThreadpoolIo() undoes whatever StartThreadpoolIo() did.
I have a Win32 MFC app that creates a thread which listens on the RS232 port. When new data is received that listener thread allocates memory using new and posts a message to a window using PostMessage. This carries on just fine and the window handles the incoming data and deletes the memory as necessary using delete. I'm noticing some small memory leaks right as my program closes. My suspicion is that one or two final messages are being posted and are still sitting in the message queue at the moment the user shuts the program and the thread closes before that memory gets properly deleted. Is there a way I can insure certain things happen before the program closes? Can I make sure the message queue is empty or at least has processed some of these important messages? I have tried looking at WaitForInputIdle or PeekMessage in destructors and things like that. Any ideas on a good way to solve this?
I 100% agree that all allocated memory should be explicitly free'd. (Just as you should fixed all compiler warnings). This eliminates the diagnostic noise, allowing you to quickly spot real issues.
Building on Harry Johnston's suggestion, I would push all new data into some kind of a queue and simply post a command "check the queue", removing and freeing data in the message handler. That way you can easily free everything left in the queue before exiting.
For a small utility, that leak might be acceptable - but it might cover other causes that are less benign.
PostMessage does not guarantee delivery. So other options are
using a blocking SendMessage
add the data to a deque, use Post Message to notify the receiver new data is available
(Remote code review: if PostMessage returns false, do you delete the memory right away?)
The folks arguing to not worry about it have a valid point. The process is about to end, and the OS will release all the memory, so there's not much point in spending time cleaning up first.
However, this does create noise that might obscure ongoing memory leaks that could become real problems before you application exits. It also means your program would be harder to turn into a library that could be incorporated into another app later.
I'm a fan of writing clean shutdown code, and then, in opt builds, adding an early out to skip the unnecessary work. Thus your debug builds will tell you about real leaks, and your users will get a responsive exit.
To do this cleanly:
You'll need a way for the main thread to tell the listener thread to quit (or at least to stop listening). Otherwise you'll always have a small window of opportunity where the main thread is about the quit just as the listener does another allocation. The main thread will need to know that the listener thread has received and complied with this message. Only then, can the main thread go through the queue to free up all the memory associated with the last messages and know that nothing more will arrive.
Don't use TerminateThread, or you'll end up with additional problems! If the listener thread is waiting on a handle the represents the serial port, then you can make it instead wait on two handle: the serial port handle and the handle of an event. The main thread can raise the event when it wants the listener to quit. The listener thread can raise a different event to signal that it has stopped listening.
When the main thread gets the WM_QUIT, it should raise the event to tell the listener to quit, then wait on the event that says the listener thread is done, then use PeekMessage to pull any messages that the listener posted before it stopped and free the memory associated with them.
I've created two threads A & B using CreateThread windows API. I'm trying to send the data from thread A to B.
I know I can use Event object and wait for the Event object in another using "WaitForSingleObject" method. What this event does all is just signal the thread. That's it! But how I can send a data. Also I don't want thread B to wait till thread A signals. It has it own job to do. I can't make it wait.
I can't find a Windows function that will allow me to send data to / from the worker thread and main thread referencing the worker thread either by thread ID or by the returned HANDLE. I do not want to introduce the MFC dependency in my project and would like to hear any suggestions as to how others would or have done in this situation. Thanks in advance for any help!
First of all, you should keep in mind that Windows provides a number of mechanisms to deal with threading for you: I/O Completion Ports, old thread pools and new thread pools. Depending on what you're doing any of them might be useful for your purposes.
As to "sending" data from one thread to another, you have a couple of choices. Windows message queues are thread-safe, and a a thread (even if it doesn't have a window) can have a message queue, which you can post messages to using PostThreadMessage.
I've also posted code for a thread-safe queue in another answer.
As far as having the thread continue executing, but take note when a change has happened, the typical method is to have it call WaitForSingleObject with a timeout value of 0, then check the return value -- if it's WAIT_OBJECT_0, the Event (or whatever) has been set, so it needs to take note of the change. If it's WAIT_TIMEOUT, there's been no change, and it can continue executing. Either way, WaitForSingleObject returns immediately.
Since the two threads are in the same process (at least that's what it sounds like), then it is not necessary to "send" data. They can share it (e.g., a simple global variable). You do need to synchronize access to it via either an event, semaphore, mutex, etc.
Depending on what you are doing, it can be very simple.
Thread1Func() {
Set some global data
Signal semaphore to indicate it is available
}
Thread2Func() {
WaitForSingleObject to check/wait if data is available
use the data
}
If you are concerned with minimizing Windows dependencies, and assuming you are coding in C++, then I recommend using Boost.Threads, which is a pretty nice, Posix-like C++ threading interface. This will give you easy portability between Windows and Linux.
If you go this route, then use a mutex to protect any data shared across threads, and a condition variable (combined with the mutex) to signal one thread from the other.
Don´t use a mutexes when only working in one single process, beacuse it has more overhead (since it is a system-wide defined object)... Place a critical section around Your data and try to enter it (as Jerry Coffin did in his code around for the thread safe queue).
The documentation for the Win32 API PulseEvent() function (kernel32.dll) states that this function is “… unreliable and should not be used by new applications. Instead, use condition variables”. However, condition variables cannot be used across process boundaries like (named) events can.
I have a scenario that is cross-process, cross-runtime (native and managed code) in which a single producer occasionally has something interesting to make known to zero or more consumers. Right now, a well-known named event is used (and set to signaled state) by the producer using this PulseEvent function when it needs to make something known. Zero or more consumers wait on that event (WaitForSingleObject()) and perform an action in response. There is no need for two-way communication in my scenario, and the producer does not need to know if the event has any listeners, nor does it need to know if the event was successfully acted upon. On the other hand, I do not want any consumers to ever miss any events. In other words, the system needs to be perfectly reliable – but the producer does not need to know if that is the case or not. The scenario can be thought of as a “clock ticker” – i.e., the producer provides a semi-regular signal for zero or more consumers to count. And all consumers must have the correct count over any given period of time. No polling by consumers is allowed (performance reasons). The ticker is just a few milliseconds (20 or so, but not perfectly regular).
Raymen Chen (The Old New Thing) has a blog post pointing out the “fundamentally flawed” nature of the PulseEvent() function, but I do not see an alternative for my scenario from Chen or the posted comments.
Can anyone please suggest one?
Please keep in mind that the IPC signal must cross process boundries on the machine, not simply threads. And the solution needs to have high performance in that consumers must be able to act within 10ms of each event.
I think you're going to need something a little more complex to hit your reliability target.
My understanding of your problem is that you have one producer and an unknown number of consumers all of which are different processes. Each consumer can NEVER miss any events.
I'd like more clarification as to what missing an event means.
i) if a consumer started to run and got to just before it waited on your notification method and an event occurred should it process it even though it wasn't quite ready at the point that the notification was sent? (i.e. when is a consumer considered to be active? when it starts or when it processes its first event)
ii) likewise, if the consumer is processing an event and the code that waits on the next notification hasn't yet begun its wait (I'm assuming a Wait -> Process -> Loop to Wait code structure) then should it know that another event occurred whilst it was looping around?
I'd assume that i) is a "not really" as it's a race between process start up and being "ready" and ii) is "yes"; that is notifications are, effectively, queued per consumer once the consumer is present and each consumer gets to consume all events that are produced whilst it's active and doesn't get to skip any.
So, what you're after is the ability to send a stream of notifications to a set of consumers where a consumer is guaranteed to act on all notifications in that stream from the point where it acts on the first to the point where it shuts down. i.e. if the producer produces the following stream of notifications
1 2 3 4 5 6 7 8 9 0
and consumer a) starts up and processes 3, it should also process 4-0
if consumer b) starts up and processes 5 but is shut down after 9 then it should have processed 5,6,7,8,9
if consumer c) was running when the notifications began it should have processed 1-0
etc.
Simply pulsing an event wont work. If a consumer is not actively waiting on the event when the event is pulsed then it will miss the event so we will fail if events are produced faster than we can loop around to wait on the event again.
Using a semaphore also wont work as if one consumer runs faster than another consumer to such an extent that it can loop around to the semaphore call before the other completes processing and if there's another notification within that time then one consumer could process an event more than once and one could miss one. That is you may well release 3 threads (if the producer knows there are 3 consumers) but you cant ensure that each consumer is released just the once.
A ring buffer of events (tick counts) in shared memory with each consumer knowing the value of the event it last processed and with consumers alerted via a pulsed event should work at the expense of some of the consumers being out of sync with the ticks sometimes; that is if they miss one they will catch up next time they get pulsed. As long as the ring buffer is big enough so that all consumers can process the events before the producer loops in the buffer you should be OK.
With the example above, if consumer d misses the pulse for event 4 because it wasn't waiting on its event at the time and it then settles into a wait it will be woken when event 5 is produced and since it's last processed counted is 3 it will process 4 and 5 and then loop back to the event...
If this isn't good enough then I'd suggest something like PGM via sockets to give you a reliable multicast; the advantage of this would be that you could move your consumers off onto different machines...
The reason PulseEvent is "unreliable" is not so much because of anything wrong in the function itself, just that if your consumer doesn't happen to be waiting on the event at the exact moment that PulseEvent is called, it'll miss it.
In your scenario, I think the best solution is to manually keep the counter yourself. So the producer thread keeps a count of the current "clock tick" and when a consumer thread starts up, it reads the current value of that counter. Then, instead of using PulseEvent, increment the "clock ticks" counter and use SetEvent to wake all threads waiting on the tick. When the consumer thread wakes up, it checks it's "clock tick" value against the producer's "clock ticks" and it'll know how many ticks have elapsed. Just before it waits on the event again, it can check to see if another tick has occurred.
I'm not sure if I described the above very well, but hopefully that gives you an idea :)
There are two inherent problems with PulseEvent:
if it's used with auto-reset events, it releases one waiter only.
threads might never be awaken if they happen to be removed from the waiting queue due to APC at the moment of the PulseEvent.
An alternative is to broadcast a window message and have any listener have a top-level message -only window that listens to this particular message.
The main advantage of this approach is that you don't have to block your thread explicitly. The disadvantage of this approach is that your listeners have to be STA (can't have a message queue on an MTA thread).
The biggest problem with that approach would be that the processing of the event by the listener will be delayed with the amount of time it takes the queue to get to that message.
You can also make sure you use manual-reset events (so that all waiting threads are awaken) and do SetEvent/ResetEvent with some small delay (say 150ms) to give a bigger chance for threads temporarily woken by APC to pick up your event.
Of course, whether any of these alternative approaches will work for you depends on how often you need to fire your events and whether you need the listeners to process each event or just the last one they get.
If I understand your question correctly, it seems like you can simply use SetEvent. It will release one thread. Just make sure it is an auto-reset event.
If you need to allow multiple threads, you could use a named semaphore with CreateSemaphore. Each call to ReleaseSemaphore increases the count. If the count is 3, for example, and 3 threads wait on it, they will all run.
Events are more suitable for communications between the treads inside one process (unnamed events). As you have described, you have zero ore more clients that need to read something interested. I understand that the number of clients changes dynamically. In this case, the best chose will be a named pipe.
Named Pipe is King
If you need to just send data to multiple processes, it’s better to use named pipes, not the events. Unlike auto-reset events, you don't need own pipe for each of the client processes. Each named pipe has an associated server process and one or more associated client processes (and even zero). When there are many clients, many instances of the same named pipe are automatically created by the operating system for each of the clients. All instances of a named pipe share the same pipe name, but each instance has its own buffers and handles, and provides a separate conduit for client/server communication. The use of instances enables multiple pipe clients to use the same named pipe simultaneously. Any process can act as both a server for one pipe and a client for another pipe, and vice versa, making peer-to-peer communication possible.
If you will use a named pipe, there would be no need in the events at all in your scenario, and the data will have guaranteed delivery no matter what happens with the processes – each of the processes may get long delays (e.g. by a swap) but the data will be finally delivered ASAP without your special involvement.
On The Events
If you are still interested in the events -- the auto-reset event is king! ☺
The CreateEvent function has the bManualReset argument. If this parameter is TRUE, the function creates a manual-reset event object, which requires the use of the ResetEvent function to set the event state to non-signaled. This is not what you need. If this parameter is FALSE, the function creates an auto-reset event object, and system automatically resets the event state to non-signaled after a single waiting thread has been released.
These auto-reset events are very reliable and easy to use.
If you wait for an auto-reset event object with WaitForMultipleObjects or WaitForSingleObject, it reliably resets the event upon exit from these wait functions.
So create events the following way:
EventHandle := CreateEvent(nil, FALSE, FALSE, nil);
Wait for the event from one thread and do SetEvent from another thread. This is very simple and very reliable.
Don’t' ever call ResetEvent (since it automatically reset) or PulseEvent (since it is not reliable and deprecated). Even Microsoft has admitted that PulseEvent should not be used. See https://msdn.microsoft.com/en-us/library/windows/desktop/ms684914(v=vs.85).aspx
This function is unreliable and should not be used, because only those threads will be notified that are in the "wait" state at the moment PulseEvent is called. If they are in any other state, they will not be notified, and you may never know for sure what the thread state is. A thread waiting on a synchronization object can be momentarily removed from the wait state by a kernel-mode Asynchronous Procedure Call, and then returned to the wait state after the APC is complete. If the call to PulseEvent occurs during the time when the thread has been removed from the wait state, the thread will not be released because PulseEvent releases only those threads that are waiting at the moment it is called.
You can find out more about the kernel-mode Asynchronous Procedure Calls at the following links:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms681951(v=vs.85).aspx
http://www.drdobbs.com/inside-nts-asynchronous-procedure-call/184416590
http://www.osronline.com/article.cfm?id=75
We have never used PulseEvent in our applications. As about auto-reset events, we are using them since Windows NT 3.51 (although they appeared in the first 32-bit version of NT - 3.1) and they work very well.
Your Inter-Process Scenario
Unfortunately, your case is a little bit more complicated. You have multiple threads in multiple processes waiting for an event, and you have to make sure that all the threads did in fact receive the notification. There is no other reliable way other than to create own event for each consumer. So, you will need to have as many events as are the consumers. Besides that, you will need to keep a list of registered consumers, where each consumer has an associated event name. So, to notify all the consumers, you will have to do SetEvent in a loop for all the consumer events. This is a very fast, reliable and cheap way. Since you are using cross-process communication, the consumers will have to register and de-register its events via other means of inter-process communication, like SendMessage. For example, when a consumer process registers itself at your main notifier process, it sends SendMessage to your process to request a unique event name. You just increment the counter and return something like Event1, Event2, etc, and creating events with that name, so the consumers will open existing events. When the consumer de-registers – it closes the event handle that it opened before, and sends another SendMessage, to let you know that you should CloseHandle too on your side to finally release this event object. If the consumer process crashes, you will end up with a dummy event, since you will not know that you should do CloseHandle, but this should not be a problem - the events are very fast and very cheap, and there is virtually no limit on the kernel objects - the per-process limit on kernel handles is 2^24. If you are still concerned, you may to the opposite – the clients create the events but you open them. If they won’t open – then the client has crashed and you just remove it from the list.