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-\begindata{text,748928}
-\textdsversion{12}
-\template{default}
-\center{\underline{\bigger{\bigger{\bigger{An Implementation Of 
-Multiprocessor Linux
-
-}}}}}
-\italic{
-}\indent{\indent{This document describes the implementation of a simple SMP 
-Linux kernel extension and how to use this to develop SMP Linux kernels for 
-architectures other than the Intel MP v1.1 architecture for Pentium and 486 
-processors.}\italic{
-
-}}\italic{
-
-
-Alan Cox, 1995
-
-
-The author wishes to thank Caldera Inc whose donation of an ASUS dual 
-pentium board made this project possible, and Thomas Radke, whose initial 
-work on multiprocessor Linux formed the backbone of this project.
-
-\begindata{bp,941568}
-\enddata{bp,941568}
-\view{bpv,941568,0,0,0}
-}
-\heading{Background: The Intel MP specification.
-
-}
-	Most IBM PC style multiprocessor motherboards combine Intel 486 or Pentium 
-processors and glue chipsets with a hardware/software specification. The 
-specification places much of the onus for hard work on the chipset and 
-hardware rather than the operating system.
-
-
-	The Intel pentium processors have a wide variety of inbuilt facilities for 
-supporting multiprocessing, including hardware cache coherency, built in 
-interprocessor interrupt handling and a set of atomic test and set, 
-exchange and similar operations. The cache coherency in particular makes the 
-operating systems job far easier.
-
-
-	The specification defines a detailed configuration structure in ROM that 
-the boot up processor can read to find the full configuration of the 
-processors and busses. It also defines a procedure for starting up the 
-other processors.
-
-
-\heading{Mutual Exclusion Within A Single Processor Linux Kernel
-
-}
-	For any kernel to function in a sane manner it has to provide internal 
-locking and protection of its own tables to prevent two processes updating 
-them at once and for example allocating the same memory block. There are 
-two strategies for this within current Unix and Unixlike kernels. 
-Traditional unix systems from the earliest of days use a scheme of 'Coarse 
-Grained Locking' where the entire kernel is protected as a small number of 
-locks only. Some modern systems use fine grained locking. Because fine 
-grained locking has more overhead it is normally used only on 
-multiprocessor kernels and real time kernels. In a real time kernel the 
-fine grained locking reduces the amount of time locks are held and reduces 
-the critical (to real time programming at least) latency times.
-
-
-	Within the Linux kernel certain guarantees are made. No process running in 
-kernel mode will be pre-empted by another kernel mode process unless it 
-voluntarily sleeps.  This ensures that blocks of kernel code are 
-effectively atomic with respect to other processes and greatly simplifies 
-many operation. Secondly interrupts may pre-empt a kernel running process, 
-but will always return to that process. A process in kernel mode may 
-disable interrupts on the processor and guarantee such an interruption will 
-not occur. The final guarantee is that an interrupt will not bne pre-empted 
-by a kernel task. That is interrupts will run to completion or be 
-pre-empted by other interrupts only.
-
-
-	The SMP kernel chooses to continue these basic guarantees in order to make 
-initial implementation and deployment easier.  A single lock is maintained 
-across all processors. This lock is required to access the kernel space. 
-Any processor may hold it and once it is held may also re-enter the kernel 
-for interrupts and other services whenever it likes until the lock is 
-relinquished. This lock ensures that a kernel mode process will not be 
-pre-empted and ensures that blocking interrupts in kernel mode behaves 
-correctly. This is guaranteed because only the processor holding the lock 
-can be in kernel mode, only kernel mode processes can disable interrupts 
-and only the processor holding the lock may handle an interrupt.
-
-
-	Such a choice is however poor for performance. In the longer term it is 
-necessary to move to finer grained parallelism in order to get the best 
-system performance. This can be done hierarchically by gradually refining 
-the locks to cover smaller areas. With the current kernel highly CPU bound 
-process sets perform well but I/O bound task sets can easily degenerate to 
-near single processor performance levels. This refinement will be needed to 
-get the best from Linux/SMP.
-
-
-\subheading{\heading{Changes To The Portable Kernel Components
-
-
-}}	The kernel changes are split into generic SMP support changes and 
-architecture specific changes necessary to accommodate each different 
-processor type Linux is ported to.
-
-
-\subsection{Initialisation}
-
-
-	The first problem with a multiprocessor kernel is starting the other 
-processors up. Linux/SMP defines that a single processor enters the normal 
-kernel entry point start_kernel(). Other processors are assumed not to be 
-started or to have been captured elsewhere. The first processor begins the 
-normal Linux initialisation sequences and sets up paging, interrupts and 
-trap handlers. After it has obtained the processor information about the 
-boot CPU, the architecture specific function \
-
-
-\description{
-\leftindent{\bold{void smp_store_cpu_info(int processor_id)
-
-}}}
-is called to store any information about the processor into a per processor 
-array. This includes things like the bogomips speed ratings.
-
-
-	Having completed the kernel initialisation the architecture specific 
-function
-
-
-\description{\leftindent{\bold{void smp_boot_cpus(void)
-
-}}}
-is called and is expected to start up each other processor and cause it to 
-enter start_kernel() with its paging registers and other control 
-information correctly loaded. Each other processor skips the setup except 
-for calling the trap and irq initialisation functions that are needed on 
-some processors to set each CPU up correctly.  These functions will 
-probably need to be modified in existing kernels to cope with this.
-
-
-	Each additional CPU the calls the architecture specific function
-
-
-\description{\leftindent{\bold{void smp_callin(void)
-
-
-}}}which does any final setup and then spins the processor while the boot 
-up processor forks off enough idle threads for each processor. This is 
-necessary because the scheduler assumes there is always something to run. 
- Having generated these threads and forked init the architecture specific \
-
-
-
-\bold{\description{\leftindent{void smp_commence(void)}}}
-
-
-function is invoked. This does any final setup and indicates to the system 
-that multiprocessor mode is now active. All the processors spinning in the 
-smp_callin() function are now released to run the idle processes, which 
-they will run when they have no real work to process.
-
-
-\subsection{Scheduling
-
-}
-	The kernel scheduler implements a simple but very and effective task 
-scheduler. The basic structure of this scheduler is unchanged in the 
-multiprocessor kernel. A processor field is added to each task, and this 
-maintains the number of the processor executing a given task, or a magic 
-constant (NO_PROC_ID)  indicating the job is not allocated to a processor. \
-
-
-	\
-
-
-	Each processor executes the scheduler itself and will select the next task 
-to run from all runnable processes not allocated to a different processor. 
-The algorithm used by the selection is otherwise unchanged. This is 
-actually inadequate for the final system because there are advantages to 
-keeping a process on the same CPU, especially on processor boards with per 
-processor second level caches.
-
-
-	Throughout the kernel the variable 'current' is used as a global for the 
-current process. In Linux/SMP this becomes a macro which expands to 
-current_set[smp_processor_id()]. This enables almost the entire kernel to 
-be unaware of the array of running processors, but still allows the SMP 
-aware kernel modules to see all of the running processes.
-
-
-	The fork system call is modified to generate multiple processes with a 
-process id of zero until the SMP kernel starts up properly. This is 
-necessary because process number 1 must be init, and it is desirable that 
-all the system threads are process 0. \
-
-
-
-	The final area within the scheduling of processes that does cause problems 
-is the fact the uniprocessor kernel hard codes tests for the idle threads 
-as task[0] and the init process as task[1]. Because there are multiple idle 
-threads it is necessary to replace these with tests that the process id is 
-0 and a search for process ID 1, respectively.
-
-\subheading{
-}\subsection{Memory Management}\heading{
-
-
-}	The memory management core of the existing Linux system functions 
-adequately within the multiprocessor framework providing the locking is 
-used. Certain processor specific areas do need changing, in particular 
-invalidate() must invalidate the TLB's of all processors before it returns.
-
-
-\subsection{Miscellaneous Functions}
-
-\heading{
-}	The portable SMP code rests on a small set of functions and variables 
-that are provided by the processor specification functionality. These are
-
-
-\display{\bold{int smp_processor_id(void)
-
-
-}}which returns the identity of the process the call is executed upon. This 
-call is assumed to be valid at all times. This may mean additional tests 
-are needed during initialisation.
-
-
-	\display{\bold{int smp_num_cpus;
-
-}}
-	This is the number of processors in the system. \
-
-
-
-	\bold{void smp_message_pass(int target, int msg, unsigned long data,
-
-		int wait)
-
-}
-	This function passes messages between processors. At the moment it is not 
-sufficiently defined to sensibly document and needs cleaning up and further 
-work. Refer to the processor specific code documentation for more details.
-
-
-\heading{Architecture Specific Code For the Intel MP Port
-
-}
-	The architecture specific code for the intel port splits fairly cleanly 
-into four sections. Firstly the initialisation code used to boot the 
-system, secondly the message handling and support code, thirdly the 
-interrupt and kernel syscall entry function handling and finally the 
-extensions to standard kernel facilities to cope with multiple processors.
-
-
-\subsection{Initialisation
-
-
-}	The intel MP architecture captures all the processors except for a single 
-processor known as the 'boot processor' in the BIOS at boot time. Thus a 
-single processor enters the kernel bootup code. The first processor 
-executes the bootstrap code, loads and uncompresses the kernel. Having 
-unpacked the kernel it sets up the paging and control registers then enters 
-the C kernel startup.
-
-
-	The assembler startup code for the kernel is modified so that it can be 
-used by the other processors to do the processor identification and various 
-other low level configurations but does not execute those parts of the 
-startup code that would damage the running system (such as clearing the BSS 
-segment). \
-
-
-
-	In the initialisation done by the first processor the arch/i386/mm/init 
-code is modified to scan the low page, top page and BIOS for intel MP 
-signature blocks. This is necessary because the MP signature blocks must 
-be read and processed before the kernel is allowed to allocate and destroy 
-the page at the top of low memory. Having established the number of 
-processors it reserves a set of pages to provide a stack come boot up area 
-for each processor in the system. These must be allocated at startup to 
-ensure they fall below the 1Mb boundary.
-
-
-	Further processors are started up in smp_boot_cpus() by programming the 
-APIC controller registers and sending an inter-processor interrupt (IPI) to 
-the processor. This message causes the target processor to begin executing 
-code at the start of any page of memory within the lowest 1Mb, in 16bit 
-real mode. The kernel uses the single page it allocated for each processor 
-to use as stack. Before booting a given CPU the relocatable code from 
-trampoline.S and trampoline32.S is copied to the bottom of its stack page 
-and used as the target for the startup. \
-
-
-
-	The trampoline code calculates the desired stack base from the code 
-segment (since the code segment on startup is the bottom of the stack), 
- enters 32bit mode and jumps to the kernel entry assembler. This as 
-described above is modified to only execute the parts necessary for each 
-processor, and then to enter start_kernel(). On entering the kernel the 
-processor initialises its trap and interrupt handlers before entering 
-smp_callin(), where it reports its status and sets a flag that causes the 
-boot processor to continue and look for further processors. The processor 
-then spins until smp_commence() is invoked.
-
-
-	Having started each processor up the smp_commence( ) function flips a 
-flag. Each processor spinning in smp_callin() then loads the task register 
-with the task state segment (TSS) of its idle thread as is needed for task 
-switching.
-
-
-\subsection{Message Handling and Support Code}
-
-
-	The architecture specific code implements the smp_processor_id() function 
-by querying the APIC logical identity register. Because the APIC isn't 
-mapped into the kernel address space at boot, the initial value returned is 
-rigged by setting the APIC base pointer to point at a suitable constant. 
-Once the system starts doing the SMP setup (in smp_boot_cpus()), the APIC 
-is mapped with a vremap() call and the apic pointer is adjusted 
-appropriately. From then on the real APIC logical identity register is 
-read.
-
-
-	Message passing is accomplished using a pair of IPI's on interrupt 13 
-(unused by the 80486 FPU's in SMP mode) and interrupt 16. Two are used in 
-order to separate messages that cannot be processed until the receiver 
-obtains the kernel spinlock from messages that can be processed 
-immediately. In effect IRQ 13 is a fast IRQ handler that does not obtain 
-the locks, and cannot cause a reschedule, while IRQ 16 is a slow IRQ that 
-must acquire the kernel spinlocks and can cause a reschedule. This 
-interrupt is used for passing on slave timer messages from the processor 
-that receives the timer interrupt to the rest of the processors, so that 
-they can reschedule running tasks.
-
-
-\subsection{Entry And Exit Code}
-
-
-	A single spinlock protects the entire kernel. The interrupt handlers, the 
-syscall entry code and the exception handlers all acquire the lock before 
-entering the kernel proper. When the processor is trying to acquire the 
-spinlock it spins continually on the lock with interrupts disabled. This 
-causes a specific deadlock problem. The lock owner may need to send an 
-invalidate request to the rest of the processors and wait for these to 
-complete before continuing. A processor spinning on the lock would not be 
-able to do thus. Thus the loop of the spinlock tests and handles invalidate 
-requests. If the invalidate bit for the spinning CPU is set the processor 
-invalidates its TLB and atomically clears the bit. When the spinlock is 
-obtained that processor will take an IPI and in the IPI test the bit and 
-skip the invalidate as the bit is clear.
-
-
-	One complexity of the spinlock is that a process running in kernel mode 
-can sleep voluntarily and be pre-empted. A switch from such a process to a 
-process executing in user space may reduce the lock count. To track this 
-the kernel uses a syscall_count and a per process lock_depth parameter to 
-track the kernel lock state. The switch_to() function is modified in SMP 
-mode to adjust the lock appropriately.
-
-
-	The final problem is the idle thread. In the single processor kernel the 
-idle thread executes 'hlt' instructions. This saves power and reduces the 
-running temperature of the processors when they are idle. However it means 
-the process spends all its time in kernel mode and would thus hold the 
-kernel spinlock. The SMP idle thread continually reschedules a new task and 
-returns to user mode. This is far from ideal and will be modified to use 
-'hlt' instructions and release the spinlock soon. Using 'hlt' is even more 
-beneficial on a multiprocessor system as it almost completely takes an idle 
-processor off the bus.
-
-
-	Interrupts are distributed by an i82489 APIC. This chip is set up to work 
-as an emulation of the traditional PC interrupt controllers when the 
-machine boots (so that an Intel MP machine boots one CPU and PC 
-compatible). The kernel has all the relevant locks but does not yet 
-reprogram the 82489 to deliver interrupts to arbitrary processors as it 
-should. This requires further modification of the standard Linux interrupt 
-handling code, and is particularly messy as the interrupt handler behaviour 
-has to change as soon as the 82489 is switched into SMP mode.
-
-
-\subsection{Extensions To Standard Facilities}
-
-
-	The kernel maintains a set of per processor control information such as 
-the speed of the processor for delay loops. These functions on the SMP 
-kernel look the values up in a per processor array that is set up from the 
-data generated at boot up by the smp_store_cpu_info() function. This 
-includes other facts such as whether there is an FPU on the processor. The 
-current kernel does not handle only some processors having floating point.
-
-
-	The highly useful atomic bit operations are prefixed with the 'lock' 
-prefix in the SMP kernel to maintain their atomic properties when used 
-outside of (and by) the spinlock and message code. Amongst other things 
-this is needed for the invalidate handler, as all  CPU's will invalidate at 
-the same time without any locks.
-
-
-	Interrupt 13 floating point error reporting is removed. This facility is 
-not usable on a multiprocessor board, nor relevant to the Intel MP 
-architecture which does not cover the 80386/80387 processor pair. \
-
-
-
-	The /proc filesystem support is changed so that the /proc/cpuinfo file 
-contains a column for each processor present. This information is extracted 
-from the data save by smp_store_cpu_info().
-
-\enddata{text,748928}
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