When a process is running on a CPU, the operating system is not running in the background as a single core CPU can execute only 1 instruction at a time. Then how does the operating system preempt a process, is it done by the hardware?
I couldn't find an answer anywhere
To understand how the OS regains control of a process, the concept of interrupts must be understood. An interrupt is a signal sent to the CPU that signifies that the current processes must be stopped (i.e. interrupted) so that another process can begin. In some sense, this is accomplished at the hardware level as there are dedicated registers in the CPU that interrupting bits are placed in.
When an interrupt occurs, the contents of the CPU's registers are stored, the current stack pointer is saved, and the program counter is then pointed to the next instruction set forth by the scheduler that decides which process to begin next - usually the interrupting one. Barring deadlock, in which no progress on any processes can be made - the scheduler will make its way back to the original process, and that process's executing context will be reloaded into the machine (since we saved it prior). This concept of saving the state of the machine, executing a new process, and returning to the original process is known as a context switch. More on that here
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I have been studying the subsystems of the Linux kernel. There, it is written that Linux kernel is responsible for context switching(letting another process to use the CPU). Here are the steps the kernel goes through to do context switching:
The CPU (the actual hardware) interrupts the current process based
on an internal timer, switches into kernel mode, and hands control
back to the kernel.
The kernel records the current state of the CPU and memory, which
will be essential to resuming the process that was just interrupted.
The kernel performs any tasks that might have come up during the
preceding time slice (such as collecting data from input and output,
or I/O, operations).
The kernel is now ready to let another process run. The kernel analyzes
the list of processes that are ready to run and chooses one.
The kernel prepares the memory for this new process, and then prepares the CPU.
The kernel tells the CPU how long the time slice for the new process
will last.
The kernel switches the CPU into user mode and hands control of the
CPU to the process.
Problem with me is that I can't understand the 3rd step above. Can someone please shed some light on that sentence? Thanks!
The CPU (the actual hardware) interrupts the current process based on an internal timer, switches into kernel mode, and hands control back to the kernel.
Most task switches are caused by tasks blocking (because they have to wait for a mutex, disk IO, user IO, end-user action, etc).
It's better (more accurate) to say that "something" (IRQ, system call) causes a switch to the kernel's code before the kernel decides it wants to do a task switch, and that this "something" is not part of the task switch itself.
The kernel records the current state of the CPU and memory, which will be essential to resuming the process that was just interrupted.
Sort of. Because "something" (IRQ, system call) causes a switch to the kernel's code before the kernel decides it wants to do a task switch; all task switches only every happen between kernel's code (for one task) and kernel's code (for another task). Because task switches only ever switch from kernel's code to kernel's code; the task switch itself doesn't need to care about user-space memory (which isn't so important for kernel's code) or kernel's memory (which is global/shared by all CPUs and all virtual address spaces). More; because some registers are "callee preserved" (by C calling conventions) and some are "constant as far as kernel is concerned" (e.g. segment registers) the task switch code doesn't need to care about various parts of the CPU state either.
The kernel performs any tasks that might have come up during the preceding time slice (such as collecting data from input and output, or I/O, operations).
Also not part of the task switch (more "things that happen before or after kernel decides to do a task switch").
The kernel is now ready to let another process run. The kernel analyzes the list of processes that are ready to run and chooses one.
Sort of; but it's no as simple as a list, and sometimes (e.g. high-priority real time thread unblocks and preempts a less important task) the kernel knows which task it needs to switch to without doing anything extra.
The kernel prepares the memory for this new process, and then prepares the CPU.
For memory; kernel mostly just loads a new "reference for new task's virtual address space" (e.g. a single mov cr3, ... instruction on 80x86). For CPU state, it's the reverse of "2. The kernel records the current state of the CPU ..." above (loading what was previously saved, where some CPU state isn't loaded and isn't saved).
The kernel tells the CPU how long the time slice for the new process will last.
Yes.
The kernel switches the CPU into user mode and hands control of the CPU to the process.
Not really. It's better (more accurate) to say after the kernel has finished doing a task switch, the kernel code for the new task does whatever it wants (and eventually may return to user-space); and that "things that happen after task switch finished" are not part of the task switch.
There is a program which makes a variety of system calls, some of them do IO, some others don't; let's pick a few examples:
gettimeofday
fork
write
read
ioctl
What happens in operating system kernel (Linux 2+/FreeBSD7+/OSX) when a process is being killed (SIGKILL) while it is executing a system call? In particular: does every single system call know how to clean-up itself when process termination is imminent?
What happens in operating system kernel (Linux 2+/FreeBSD7+/OSX) when
a process is being killed (SIGKILL) while it is executing a system
call?
A process executes a system call in the context of kernel mode. At this time if kernel receives any interrupt(signal comes under software interrupt), it would check out for the priority of that particular interrupt. Software interrupt has least priority among all type of interrupt and hence it would continue to execute until it finished the critical path execution. At this point kernel would store its context and would determine the reason and priority of interrupt and service the interrupt(which is in this case is to terminate/kill the program).
However these concepts are bit complicated and may vary on different system. You may want to refer the great book "The Design Of UNIX Operating System" By Maurice J Bach which describes these concepts and the implementation in detailed way.
In particular: does every single system call know how to clean-up
itself when process termination is imminent?
Yes, As system calls gets executed in the context of kernel mode on behalf of a process.
So they always do have the logic to clean-up while process has started to abort/terminate. If user mode program terminates due to unexpected reason,kernel ensures that it clean up all the resources of that particular process. This is the great thing about kernel.
Consider the process keventd. It spends all it's lifetime in kernel mode.
Now, as far as I know, Linux checks if a context switch is due, while the process is switching from kernel mode to user mode, and as far as I know, keventd will never switch from kernel mode to user mode, so, how will the Linux kernel know when to switch it off?
If the kernel were to do as you say, and only check whether a process is due to be switched out on an explicit user-to-kernel-mode transition, then the following loop would lock up a core of your computer:
while (1);
Obviously, this does not happen on normal desktop operating systems. The reason why is preemption, where after a process has run for its time slice the kernel gets an alarm, steps in, and forcibly switches contexts as necessary.
Preemption could in principle work for kernel processes too. However, I'm not sure that's what keventd does - it's more likely that it voluntarily relinquishes its time slice on a regular basis (see sched_yield, a userspace call for the same effect), especially since the kernel can be configured to be non-preemptible. That is a kernel process' prerogative.
What happens (in detail) when a thread makes a system call by raising interrupt 80? What work does Linux do to the thread's stack and other state? What changes are done to the processor to put it into kernel mode? After running the interrupt handler, how is control restored back to the calling process?
What if the system call can't be completed quickly: e.g. a read from disk. How does the interrupt handler relinquish control so that the processor can do other stuff while data is being loaded and how does it then obtain control again?
A crash course in kernel mode in one stack overflow answer
Good questions! (Interview questions?)
What happens (in detail) when a
thread makes a system call by raising
interrupt 80?
The int $80 operation is vaguely like a function call. The CPU "takes a trap" and restarts at a known address in kernel mode, typically with a different MMU mode as well. The kernel will save many of the registers, though it doesn't have to save the registers that a program would not expect an ordinary function call to save.
What work does Linux do to the
thread's stack and other state?
Typically an OS will save registers that the ABI promises not to change during procedure calls. The stack will stay the same; the kernel will run on a per-thread kernel stack rather than the per-thread user stack. Naturally some state will change, otherwise there would be no reason to do the system call.
What changes are done to the
processor to put it into kernel mode?
This is usually entirely automatic. The CPU has, generically, a software-interrupt instruction that is a bit like a functional-call operation. It will cause the switch to kernel mode under controlled conditions. Typically, the CPU will change some sort of PSW protection bit, save the old PSW and PC, start at a well-known trap vector address, and may also switch to a different memory management protection and mapping arrangement.
After running the interrupt handler,
how is control restored back to the
calling process?
There will be some sort of "return from interrupt" or "return from trap" instruction, typically, that will act a bit like a complicated function-return instruction. Some RISC processors did very little automatically and required specific code to do the return and some CISC processors like x86 have (never-really-used) instructions that would execute dozens of operations documented in pages of architecture-manual pseudo-code for capability adjustments.
What if the system call can't be
completed quickly: e.g. a read from
disk. How does the interrupt handler
relinquish control so that the
processor can do other stuff while
data is being loaded and how does it
then obtain control again?
The kernel itself is threaded much like a threaded user program is. It just switches stacks (threads) and works on someone else's process for a while.
To answer the last part of the question - what does the kernel do if the system call needs to sleep -
After a system call, the kernel is still logically running in the context of the same task that made the system call - it's just in kernel mode rather than user mode - it is NOT a separate thread and most system calls do not invoke logic from another task/thread. What happens is that the system call calls wait_event, or wait_event_timeout or some other wait function, which adds the task to a list of tasks waiting for something, then puts the task to sleep, which changes its state, and calls schedule() to relinquish the current CPU.
After this the task cannot be run again until it gets woken up, typically by another task (kernel task, etc) or interrupt handler calling a wake* function which will wake up the task(s) sleeping waiting for that particular event, which means the scheduler will soon schedule them again.
It's worth noting that userspace tasks (i.e. threads) are only one type of task and there are a few others internal to the kernel which can do work as well - these are kernel threads and bottom half handlers / tasklets / task queues etc. Work which doesn't belong to any particular userspace process (for example network handling e.g. responding to pings) gets done in these. These tasks are allowed to go to sleep, unlike interrupts (which should not invoke the scheduler)
http://tldp.org/LDP/khg/HyperNews/get/syscall/syscall86.html
This should help people who seek for answers to what happens when the syscall instruction is executed which transfers the control to the kernel (user mode to kernel mode). This is based upon x86_64 architecture.
https://0xax.gitbooks.io/linux-insides/content/SysCall/syscall-2.html
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