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TNeoKernel
v1.03
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TNeoKernel
v1.03
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If you have experience of using TNKernel, you really want to read this.
Original TNKernel code designed to be built together with main project only, there's no way to build as a separate library: at least, arrays for idle and timer task stacks are allocated statically, so size of them is defined at tnkernel compile time.
It's much better if we could pass these things to tnkernel at runtime, so, tn_sys_start() now takes pointers to stack arrays and their sizes. Refer to Starting the kernel section for the details.
In original TNKernel, one should give bottom address of the task stack to tn_task_create(), like this:
Alex Borisov implemented it more conveniently in his port: one should give just array address, like this:
TNeoKernel uses the second way (i.e. the way used in port by Alex Borisov), and it does so independently of architecture.
In original TNKernel, requesting non-sleeping task to wake up is quite legal and causes next call to tn_task_sleep() to not sleep. The same is with suspending/resuming tasks.
So, if you call tn_task_wakeup() on non-sleeping task first time, TERR_NO_ERR is returned. If you call it second time, before target task called tn_task_sleep(), TERR_OVERFLOW is returned.
All of this seems to me as a complete dirty hack, it probably might be used as a workaround to avoid race condition problems, or as a hacky replacement for semaphore.
It just encourages programmer to go with hacky approach, instead of creating straightforward semaphore and provide proper synchronization.
In TNeoKernel these "features" are removed, and if you try to wake up non-sleeping task, or try to resume non-suspended task, TN_RC_WSTATE is returned.
By the way, suspend_count is present in TCB structure, but is never used, so, it is just removed. And comments for wakeup_count, activate_count, suspend_count suggested that these fields are used for statistics, which is clearly not true.
In original TNKernel it's illegal to pass block_size that is less than sizeof(int). But, it is legal to pass some value that isn't multiple of sizeof(int): in this case, block_size is silently rounded up, and therefore block_cnt is silently decremented to fit as many blocks of newly calculated block_size as possible. If resulting block_cnt is at least 2, it is assumed that everything is fine and we can go on.
Why I don't like it: firstly, silent behavior like this is generally bad practice that leads to hard-to-catch bugs. Secondly, it is inconsistency again: why is it legal for block_size not to be multiple of sizeof(int), but it is illegal for it to be less than sizeof(int)? After all, the latter is the partucular case of the former.
So, TNeoKernel returns TN_RC_WPARAM in these cases. User must provide start_addr and block_size that are properly aligned.
TNeoKernel also provides convenience macro TN_FMEM_BUF_DEF() for buffer definition, so, as a generic rule, it is good practice to define buffers for memory pool like this:
And then, construct your my_fmem as follows:
In original TNKernel, TERR_WCONTEXT is returned in the following cases:
tn_task_terminate() for already terminated task;tn_task_delete() for non-terminated task;tn_task_change_priority() for terminated task;tn_task_wakeup()/tn_task_iwakeup() for terminated task;tn_task_release_wait()/tn_task_irelease_wait() for terminated task.The actual error is, of course, wrong state, not wrong context; so, TNeoKernel returns TN_RC_WSTATE in these cases.
In original TNKernel, a call to tn_task_release_wait() / tn_task_irelease_wait() causes waiting task to wake up, regardless of wait reason, and TERR_NO_ERR is returned as a wait result. Actually I believe it is bad idea to ever use tn_task_release_wait(), but if we have this service, error code surely should be distinguishable from normal wait completion, so, new code is added: TN_RC_FORCED, and it is returned when task wakes up because of tn_task_release_wait() call.
In original TNKernel, tn_task_sleep() always returns TERR_NO_ERR, independently of what actually happened. In TNeoKernel, there are three possible return codes:
TN_RC_TIMEOUT if timeout is actually in effect;TN_RC_OK if task was woken up by some other task with tn_task_wakeup();TN_RC_FORCED if task was woken up forcibly by some other task with tn_task_release_wait();In original TNKernel, I always found events API somewhat confusing. Why is this object named "event", but there are many flags inside, so that they can actually represent many events?
Meanwhile, attributes like TN_EVENT_ATTR_SINGLE, TN_EVENT_ATTR_CLR imply that "event" object is really just a single event, since it makes no sense to clear just all event bits when some particular event happened.
After all, when we call tn_event_clear(&my_event_obj, flags), we might expect that flags argument actually specifies flags to clear. But in fact, we must invert it, to make it work: ~flags. This is really confusing.
In TNeoKernel, there is no such event object. Instead, there is object events group. Attributes like ...SINGLE, ...MULTI, ...CLR are removed, since they make no sense for events group. I have plans to offer a way to connect events group to queue and probably other kernel objects as well, so that queue will set and clear particular flag in the events group automatically, depending on whether a queue is empty. By means of that, it is quite easy to wait for data from multiple queues with just a single call to tn_eventgrp_wait().
For detailed API reference, refer to the tn_eventgrp.h.
In original TNKernel, system functions refused to perform job and returned TERR_WRONG_PARAM if timeout is 0, but it is actually neither convenient nor intuitive: it is much better if the function behaves just like ...polling() version of the function. All TNeoKernel system functions allows timeout to be zero: in this case, function doesn't wait.
Support of timers was added since TNeoKernel v1.02.
Timer is a kernel object that is used to ask the kernel to call some user-provided function at a particular time in the future, based on the system timer tick.
If you need to repeatedly wake up particular task, you can create semaphore which you should wait for in the task, and signal in the timer callback.
If you need to perform rather fast action (such as toggle some pin, or the like), consider doing that right in the timer callback, in order to avoid context switch overhead.
The timer callback approach provides ultimate flexibility.
For details, refer to the timers documentation.
Sometimes I feel lack of mutexes that allow recursive locking. I know there are developers who believe that recursive locking leads to the code of lower quality, and I understand it. Even Linux kernel doesn't have recursive mutexes.
Sometimes they are really useful though (say, if you want to use some third-party library that requires locking primitives to be recursive), so I decided to implement an option for that: TN_MUTEX_REC. If it is non-zero, mutexes allow recursive locking; otherwise you get TN_RC_ILLEGAL_USE when you try to lock mutex that is already locked by this task. Default value: 1.
TNeoKernel can notify you by calling your callback function if deadlock occurs. See:
TN_MUTEX_DEADLOCK_DETECT,tn_callback_deadlock_set(),TN_CBDeadlock.Several system services were added:
tn_cur_task_get() : return a pointer to the struct TN_Task of the currently running task;tn_cur_task_body_get() : return pointer to the currently running task body function;tn_task_state_get() : get state of the task.There is a terrible mess with MAKE_ALIG() macro: TNKernel docs specify that the argument of it should be the size to align, but almost all ports, including original one, defined it so that it takes type, not size.
But the port by AlexB implemented it differently (i.e. accordingly to the docs) : it takes size as an argument.
When I was moving from the port by AlexB to another one, do you have any idea how much time it took me to figure out why do I have rare weird bug? :)
By the way, additional strange thing: why doesn't this macro have any prefix like TN_?
TNeoKernel provides macro TN_MAKE_ALIG_SIZE() whose argument is size, so, its usage is as follows: TN_MAKE_ALIG_SIZE(sizeof(struct MyStruct)). This macro is preferred.
But for compatibility with messy MAKE_ALIG() from original TNKernel, there is an option TN_API_MAKE_ALIG_ARG with two possible values;
TN_API_MAKE_ALIG_ARG__SIZE - default value, use macro like this: MAKE_ALIG(sizeof(struct my_struct)), like in the port by Alex.TN_API_MAKE_ALIG_ARG__TYPE - use macro like this: MAKE_ALIG(struct my_struct), like in any other port.By the way, I wrote to the author of TNKernel (Yuri Tiomkin) about this mess, but he didn't answer anything. It's a pity of course, but we have what we have.
You can still use "manual" definition of stack arrays, like that:
Although it is recommended to use convenience macro for that: TN_STACK_ARR_DEF(). See tn_task_create() for the usage example.
Similarly to the previous section, you can still use "manual" definition of the buffer for fixed memory block pool, it is recommended to use convenience macro for that: TN_FMEM_BUF_DEF(). See tn_fmem_create() for usage example.
There is a lot of inconsistency with naming stuff in original TNKernel:
tn_queue_send_polling() / tn_queue_isend_polling() (notice the i letter before the verb, not before polling), but tn_fmem_get_polling() / tn_fmem_get_ipolling() (notice the i letter before polling)?tn_<noun>_<verb>[_<adjustment>](), but the tn_start_system() is special, for some strange reason. To make it consistent, it should be named tn_system_start() or tn_sys_start();TN_ prefix;So, a lot of things (functions, macros, etc) has renamed. Old names are also available through tn_oldsymbols.h, which is included automatically if TN_OLD_TNKERNEL_NAMES option is non-zero.
One of the renamings deserves special mentioning: tn_sem_acquire() and friends are renamed to tn_sem_wait() and friends. That's because names acquire/release are actually misleading for the semaphore: semaphore is a signaling mechanism, and not the locking mechanism.
Actually, there's a lot of confusion about usage of mutexes/semaphores, so it's quite recommended to read small article by Michael Barr: Mutexes and Semaphores Demystified.
Old names (tn_sem_acquire() and friends) are still available through tn_oldsymbols.h.
Yes, timer task's job is important: it manages tn_wait_timeout_list, i.e. it wakes up tasks whose timeout is expired. But it's actually better to do it right in tn_tick_int_processing() that is called from timer ISR, because presence of the special task provides significant overhead. Look at what happens when timer interrupt is fired (assume we don't use shadow register set for that, which is almost always the case):
(measurements were made at PIC32 port)
tn_tick_int_processing() is called;tn_tick_int_processing() disables interrupts, manages round-robin (if needed), then it wakes up tn_timer_task, sets tn_next_task_to_run, and enables interrupts back;tn_tick_int_processing() finishes, so ISR macro checks that tn_next_task_to_run is different from tn_curr_run_task, and sets CS0 interrupt bit, so that context should be switched as soon as possible;CS0 ISR is immediately called, so full context (32 words) gets saved on task's stack, and context of tn_timer_task is restored;tn_timer_task disables interrupts, performs its not so big job (manages tn_wait_timeout_list), puts itself to wait, enables interrupts and pends context switching again;CS0 ISR is immediately called, so full context of tn_timer_task gets saved in its stack, and then, after all, context of my own interrupted task gets restored and my task continues to run.I've measured with MPLABX's stopwatch how much time it takes: with just three tasks (idle task, timer task, my own task with priority 6), i.e. without any sleeping tasks, all this routine takes 682 cycles. So I tried to get rid of tn_timer_task and perform its job right in the tn_tick_int_processing().
Previously, application callback was called from timer task; since it is removed now, startup routine has changed, refer to Starting the kernel for details.
Now, the following steps are performed when timer interrupt is fired:
tn_tick_int_processing() is called;tn_tick_int_processing() disables interrupts, manages round-robin (if needed), manages tn_wait_timeout_list, and enables interrupts back;tn_tick_int_processing() finishes, ISR macro checks that tn_next_task_to_run is the same as tn_curr_run_taskThat's all. It takes 251 cycles: 2.7 times less.
So, we need to make sure that interrupt stack size is enough for this (not big) job. As a result, RAM is saved (since you don't need to allocate stack for timer task) and things work much faster. Win-win.
1.8.8