In Linux at Kernel level we have threads/tasks (belonging to Kernel and user), e.g.,
swapper: is a kernel thread (process 0), the ancestor of all processes, created from scratch during the initialization phase of Linux by the start_kernel() function. Also
init: an additional kernel thread, process 1 (init process)
HelloWorld: A thread for user program
My question is about the Kernel scheduler, that performs the following jobs:
-Schedule tasks within a fixed amount of time (i.e. context-switching)
-Calculate the timeslices dynamically (short/long vs priority based)
-Assigns process priorities dynamically (when needed)
-Monitoring the processes to its jobs
(does this include any further?)
More specifically my questions becomes: Which thread/task(s) at Kernel level correspond to the scheduler? Should it be 'scheduler' etc or Does any other task from kernel do its job?
P.S.:
"swapper" in kernel is an idle thread/task with lowest priority (halt) [1]. Does this do anything other than "step-down"?
Does Linux create a dedicated instance of scheduler for each core in multi-core system? If no, then how it does on multicore?
[1] Why do we need a swapper task in linux?
The Linux scheduler does not have a task or thread corresponding to it. The Linux scheduler code, mainly the schedule() function, is run when the timer used for scheduling issues an interrupt or when it is explicitly called in the kernel code (e.g. as part of a system call).
On multicore, the scheduler code is run independently on each core. The timer interrupt received on Core 0 is usually broadcast by using IPIs (Inter-Processor Interrupts) to the other cores. If the platform has per CPU timers, then Linux usually uses these to issue the interrupt required for scheduling instead of using IPIs.
I don't think there is a separate thread that runs the scheduler. schedule() is the function which does the scheduling. Some of the places where schedule() function may be called from:
1) A process(A user space or kernel space) which is executing the kernel code wants to wait for some event (calls any of the wait functions)
2) A process which calls schedule() function to yield the processor for itself for sometime.
3) Every time when a timer interrupt is generated.
Like this there may be many places where the schedule() function may be invoked.
Related
I looked up the CPU scheduler source code built into the kernel.
https://github.com/torvalds/linux/tree/master/kernel/sched
But I have a question.
There are mixed opinions on the cpu scheduler on the Internet.
I saw an opinion that CPU scheduler is a process.
Question: If so, when ps-ef on Linux, the scheduler process should be visible. It was difficult to find the PID and name of the scheduler process.
The PID for the CPU scheduler process is not on the internet either. However, the PID 0 SWAPPER process is called SCHED, but in Linux, PID 0 functions as an idle process.
I saw an opinion that CPU scheduler is not a process.
CPU scheduler is a passive source code built into the kernel, and user processes frequently enter the kernel and rotate the source code.
Question: How does the user process execute the kernel's scheduler source code on its own?
What if you created a user program without adding a system call using the scheduler of the kernel?
How does the user process self-rotate the scheduler in the kernel without such code?
You have 2 similar questions (The opinion that the scheduler built into the kernel is the program and the opinion that it is the process and I want to know how to implement the cpu scheduling process in Linux operating system) so I'll answer for both of these here.
The answer is that it doesn't work that way at all. The scheduler is not called by user mode processes by using system calls. The scheduler isn't a system call. There are timers that are programmed to throw interrupts after some time has elapsed. Timers are accessed using registers that are memory in RAM often called memory mapped IO (MMIO). You write to some position in RAM specified by the ACPI tables (https://wiki.osdev.org/ACPI) and it will allow to control the chips in the CPU or external PCI devices (PCI is everything nowadays).
When the timer reaches 0, it will trigger an interrupt. Interrupts are thrown by hardware (the CPU). The CPU thus includes special mechanism to let the OS determine the position at which it will jump on interrupt (https://wiki.osdev.org/Interrupt_Descriptor_Table). Interrupts are used by the CPU to notify the OS that an event happened. Without interrupts, the OS would have to reserve at least one core of the processor for a special kernel process that would constantly poll the registers of peripherals and other things. It would be impossible to implement. Also, if user mode processes did the scheduler system call by themselves, the kernel would be slave to user mode because it wouldn't be able to tell if a process is finished and processes could be selfish over CPU time.
I didn't look at the source code but I think the scheduler is also often called on some IO completion (also on interrupt but not always on timer interrupt). I am quite sure that the scheduler must not be preempted. That is interrupts (and other things) will be disabled while the schedule() function runs.
I don't think you can call the scheduler a process (not even a kernel thread). The scheduler can be called by kernel threads that are created by interrupts due to bottom half processing. In bottom half processing, the top "half" of the interrupt handler runs fast and efficiently while the bottom "half" is added to the queue of processes and runs when the scheduler decides it should be scheduled. This has the effect of creating some kernel threads. The scheduler can thus be called from kernel threads but not always from bottom half of interrupts. There has to be a mechanism to call the scheduler without the scheduler having to schedule the task itself. Otherwise, the kernel will stop functioning.
Why we can sleep in software interrupt case while it is not allowed in case of hardware interrupt?
e.g. System calls can sleep while ISR cannot sleep.
When you enter in the kernel code through a process (i.e., a syscall) the kernel is said to be in process context. This means that the kernel is executed on behalf of a process. The execution of the kernel is synchronous with the user-level, and therefore it is possible to access user-level. It is also possible to call sleeping functions, because the scheduler is capable of schedule a new process.
When you enter in the kernel from a hardware source (i.e., an interrupt), then the kernel is said to be in interrupt context. The execution of the kernel is asynchronous with respect to the user-level, and you cannot do any ssumption of what is being executed at user-level. For example, some resources may be in some unconsistent state. For this reason, the code cannot block because the scheduler cannot schedule a new process.
This difference is well explained in Rubini's book Linux Device Drivers, 3rd edition which is freely available on the web.
Normally, ISRs run with interrupt disabled, so if sleeped in ISR we have no chance to wake up.
Interrupt handler uses interrupted process's kernel stack. If we switched to other process in ISR, the kernel stack will be changed to other process's.
I have some doubts regarding some linux kernel scheduling.
1) Does linux kernel(schedular to be specific) always runs on CPU-0?
2) One Scenario:
One kernel thread running on CPU - 0, goes into sleep with interrupts disabled.
In this case, will the schedular run on other CPU?
if Yes, how is the selection made out of the remaining core so as to which will run
the schedular, is this decision made while disabling interrupts on CPU - 0?
The scheduler is just a piece of code (in particular, the schedule() function).
Like most other parts of the kernel, it runs on whatever CPU it is called.
The scheduler gets called when some thread wants to sleep or after an interrupt has been handled; this can happen on all CPUs.
1) Does linux kernel(schedular to be specific) always runs on CPU-0?
(No, scheduler can runs on any CPU cores.)
2) One Scenario:
One kernel thread running on CPU - 0, goes into sleep with interrupts disabled.
In this case, will the schedular run on other CPU?
(A thread running on CPU -0, goes into sleep. Which means the thread
quits the CPU voluntarily.The sleep code will call linux scheduler, and the scheduler will choose another thread/process to run.This is noting to do with the interrupts.Disabling the interrupts(eg. timer interrupt), can stop the thread being interrupted and scheduled out to the CPU against its will.)
if Yes, how is the selection made out of the remaining core so as to which will run
the schedular, is this decision made while disabling interrupts on CPU - 0?
(Hope this helps!)
Could someone help clarify my understanding of kernel threads. I heard that, on Linux/Unix, kernel threads(such as those of system calls) get executed faster than user threads. But, aren't those user threads scheduled by kernel and executed using kernel threads? could someone please tell me what is the difference between a kernel thread and a user thread other than the fact that they have access to different address spaces. what are other difference between them? Is it true that on a single processor box, when user thread is running, kernel will be suspended?
Thanks in advance,
Alex
I heard that, on Linux/Unix, kernel threads(such as those of system calls) get executed faster than user threads.
This is a largely inaccurate statement.
Kernel threads are used for "background" tasks internal to the kernel, such as handling interrupts and flushing data to disk. The bulk of system calls are processed by the kernel within the context of the process that called them.
Kernel threads are scheduled more or less the same way as user processes. Some kernel threads have higher than default priority (up to realtime priority in some cases), but saying that they are "executed faster" is misleading.
Is it true that on a single processor box, when user thread is running, kernel will be suspended?
Of course. Only one process can be running at a time on a single CPU core.
That being said, there are a number of situations where the kernel can interrupt a running task and switch to another one (which may be a kernel thread):
When the timer interrupt fires. By default, this occurs 100 times every second.
When the task makes a blocking system call (such as select() or read()).
When a CPU exception occurs in the task (e.g, a memory access fault).
What is a reentrant kernel?
Much simpler answer:
Kernel Re-Entrance
If the kernel is not re-entrant, a process can only be suspended while it is in user mode. Although it could be suspended in kernel mode, that would still block kernel mode execution on all other processes. The reason for this is that all kernel threads share the same memory. If execution would jump between them arbitrarily, corruption might occur.
A re-entrant kernel enables processes (or, to be more precise, their corresponding kernel threads) to give away the CPU while in kernel mode. They do not hinder other processes from also entering kernel mode. A typical use case is IO wait. The process wants to read a file. It calls a kernel function for this. Inside the kernel function, the disk controller is asked for the data. Getting the data will take some time and the function is blocked during that time.
With a re-entrant kernel, the scheduler will assign the CPU to another process (kernel thread) until an interrupt from the disk controller indicates that the data is available and our thread can be resumed. This process can still access IO (which needs kernel functions), like user input. The system stays responsive and CPU time waste due to IO wait is reduced.
This is pretty much standard for today's desktop operating systems.
Kernel pre-emption
Kernel pre-emption does not help in the overall throughput of the system. Instead, it seeks for better responsiveness.
The idea here is that normally kernel functions are only interrupted by hardware causes: Either external interrupts, or IO wait cases, where it voluntarily gives away control to the scheduler. A pre-emptive kernel instead also interrupts and suspends kernel functions just like it would interrupt processes in user mode. The system is more responsive, as processes e.g. handling mouse input, are woken up even while heavy work is done inside the kernel.
Pre-emption on kernel level makes things harder for the kernel developer: The kernel function cannot be suspended only voluntarily or by interrupt handlers (which are somewhat a controlled environment), but also by any other process due to the scheduler. Care has to be taken to e.g. avoid deadlocks: A thread locks resource A but needing resource B is interrupted by another thread which locks resource B, but then needs resource A.
Take my explanation of pre-emption with a grain of salt. I'm happy for any corrections.
All Unix kernels are reentrant. This means that several processes may be executing in Kernel Mode at the same time. Of course, on uniprocessor systems, only one process can progress, but many can be blocked in Kernel Mode when waiting for the CPU or the completion of some I/O operation. For instance, after issuing a read to a disk on behalf of a process, the kernel lets the disk controller handle it and resumes executing other processes. An interrupt notifies the kernel when the device has satisfied the read, so the former process can resume the execution.
One way to provide reentrancy is to write functions so that they modify only local variables and do not alter global data structures. Such functions are called reentrant functions . But a reentrant kernel is not limited only to such reentrant functions (although that is how some real-time kernels are implemented). Instead, the kernel can include nonreentrant functions and use locking mechanisms to ensure that only one process can execute a nonreentrant function at a time.
If a hardware interrupt occurs, a reentrant kernel is able to suspend the current running process even if that process is in Kernel Mode. This capability is very important, because it improves the throughput of the device controllers that issue interrupts. Once a device has issued an interrupt, it waits until the CPU acknowledges it. If the kernel is able to answer quickly, the device controller will be able to perform other tasks while the CPU handles the interrupt.
Now let's look at kernel reentrancy and its impact on the organization of the kernel. A kernel control path denotes the sequence of instructions executed by the kernel to handle a system call, an exception, or an interrupt.
In the simplest case, the CPU executes a kernel control path sequentially from the first instruction to the last. When one of the following events occurs, however, the CPU interleaves the kernel control paths :
A process executing in User Mode invokes a system call, and the corresponding kernel control path verifies that the request cannot be satisfied immediately; it then invokes the scheduler to select a new process to run. As a result, a process switch occurs. The first kernel control path is left unfinished, and the CPU resumes the execution of some other kernel control path. In this case, the two control paths are executed on behalf of two different processes.
The CPU detects an exception-for example, access to a page not present in RAM-while running a kernel control path. The first control path is suspended, and the CPU starts the execution of a suitable procedure. In our example, this type of procedure can allocate a new page for the process and read its contents from disk. When the procedure terminates, the first control path can be resumed. In this case, the two control paths are executed on behalf of the same process.
A hardware interrupt occurs while the CPU is running a kernel control path with the interrupts enabled. The first kernel control path is left unfinished, and the CPU starts processing another kernel control path to handle the interrupt. The first kernel control path resumes when the interrupt handler terminates. In this case, the two kernel control paths run in the execution context of the same process, and the total system CPU time is accounted to it. However, the interrupt handler doesn't necessarily operate on behalf of the process.
An interrupt occurs while the CPU is running with kernel preemption enabled, and a higher priority process is runnable. In this case, the first kernel control path is left unfinished, and the CPU resumes executing another kernel control path on behalf of the higher priority process. This occurs only if the kernel has been compiled with kernel preemption support.
These information available on http://jno.glas.net/data/prog_books/lin_kern_2.6/0596005652/understandlk-CHP-1-SECT-6.html
More On http://linux.omnipotent.net/article.php?article_id=12496&page=-1
The kernel is the core part of an operating system that interfaces directly with the hardware and schedules processes to run.
Processes call kernel functions to perform tasks such as accessing hardware or starting new processes. For certain periods of time, therefore, a process will be executing kernel code. A kernel is called reentrant if more than one process can be executing kernel code at the same time. "At the same time" can mean either that two processes are actually executing kernel code concurrently (on a multiprocessor system) or that one process has been interrupted while it is executing kernel code (because it is waiting for hardware to respond, for instance) and that another process that has been scheduled to run has also called into the kernel.
A reentrant kernel provides better performance because there is no contention for the kernel. A kernel that is not reentrant needs to use a lock to make sure that no two processes are executing kernel code at the same time.
A reentrant function is one that can be used by more than one task concurrently without fear of data corruption. Conversely, a non-reentrant function is one that cannot be shared by more than one task unless mutual exclusion to the function is ensured either by using a semaphore or by disabling interrupts during critical sections of code. A reentrant function can be interrupted at any time and resumed at a later time without loss of data. Reentrant functions either use local variables or protect their data when global variables are used.
A reentrant function:
Does not hold static data over successive calls
Does not return a pointer to static data; all data is provided by the caller of the function
Uses local data or ensures protection of global data by making a local copy of it
Must not call any non-reentrant functions