Kernel Synchronization in Linux Uni-processor and Multi-processor Environment By Kathryn Bean and Wafa’ Jaffal (Group A3)

Topics Linux History Kernel Control Path Synchronization Technique SMP Architecture Hardware Support for Synchronization (Pentium-based Architecture) Linux/SMP kernel Conclusion

Linux History University of Helsinki (1997), Master’s thesis – “Linux, a Portable Operating System” by L. Torvalds OS for IBM-compatible personal computers (Intel microprocessor). Source code under GNU General Public License

Kernel Control Path Kernel control path is the sequence of instructions executed in Kernel Mode to handle a kernel request. Kernel control path executes due to the following reasons: – System calls – Exceptions – Interrupts

Synchronization Technique Nonpreemptability Atomic Operations Interrupt Disabling Locks

Condition to be Preempted Kernel control path can preempt a running process; however, when an interrupt handle terminates, the process resumes. Only kernel control path can interrupt another kernel control path.

Atomic Operation An atomic operation - performed by executing a single assembly language instruction Linux kernel provides special functions such as: atomic_int(v)  v++

Interrupt Disabling Because of its simplicity, interrupt disabling is used by kernel functions for implementing a critical region. This technique does not always prevent kernel control path interleaving. Critical section should be short because any communication between CPU and I/O is blocked while a kernel control path is running in this section.

Locking Two kinds of locking: – Kernel semaphores, used by both uni-processor and multiprocessor systems – Spin Locks, used by only multiprocessor systems

Kernel Semaphore Implementation Kernel semaphore – is object of type structure semaphore, see include/asm/semaphore.h file Fields – count – integer number count > 0 – semaphore is available count  0 – semaphore is busy, |count| - number of processes waiting for resource. Count = 0 – one use, nothing is waiting The count field is decremented when a process acquires the lock and is incremented when the same process releases it.

Kernel Semaphore Implementation, Continued – wait – the address of a wait queue. – waking – integer. The releasing process increments waking field(s). Each of awakened process P I then enters a critical region of the down() function Is P I ’s waking <> 0, if waking > 0 – 1. acquire the resource 2. other P K ’s waking-- if waking < 0 – go back to sleep

Kernel Semaphore Implementation, Continued Function – down() – called, if process wishes to acquire a semaphore. count -- count <> 0, if count  0 – process enter the critical section if count < 0 – process is suspended – up() – called, if process releases a semaphore count ++ count <> 0 – if count > 0 – up() terminates if count < 0 – wake up other processes Deadlock – semaphore requests are performed in the address order.

SMP Architecture Scalability of Linux - supports multiprocessing through Shared Memory Symmetric Multiprocessors (SMM) architecture. scalability is the capability of a system to adapt to an ever-increasing work load.

SMP Architecture, Continued system bus CPU 1CPU n Graphical card Memory CPUs share the same memory unit application processing and kernel processing are spread amongst all CPUs.

Other Multiprocessor Architectures Asymmetric Multiprocessing Master CPU executes the operating system code and application programs run on the remaining CPUs. Massively Parallel Processing (MPP) Assemble hundreds or thousand of CPUs, each with own system memory

Hardware Support for Synchronization Shared Memory Memory arbiter – (chip between bus and every RAM chip) grants access to a CPU if the chip is free and delays access if the chip is busy. Cache Synchronization Hardware cache is utilized using the locality principle. In multiprocessor environment, each CPU has its own cache. Process of updating cache - cache snooping

Hardware Support for Synchronization, Continued SMP Atomic Operation – Lock instruction prefixes for atomic operations were introduced. – If control unit detects them  lock the memory bus, no other processes can access this memory location

Hardware Support for Synchronization, Continued Distributed Interrupt Handling ICC bus Local APIC I/O APIC CPU 1CPU n IRQ lines APIC – Advanced Programmable Interrupt Controller ICC – Interrupt Controller Communication I/O APIC - router

Linux/SMP Kernel Process Descriptor Modification – has_cpu: if has_cpu > 0 – process is running – Processor – logical number of its CPU Spin Locks Blocked process keeps its own CPU by spinning while waiting for a resource.

Conclusion Modern versions of Linux are available – Compaq Alpha – SPARC – PowerPC – Motorola MC680x0 – IBM System/390 Multiprocessor operating system Supports up to 32 CPUs

Bibliography D. P. Bovet, M. Cesati. Understanding the Linux Kernel. O’Reilly, 2000 Linus Torvalds. Linux: a Portable Operating System.Master of Science Thesis, University of Helsinki, Finland, 1997 D. Mosberg, S. EranianIA-64 Linux Kernel Prentice Hall PTR, 2002

Thank You Any Questions?