Download Kernel Control Path

Kernel Synchronization in Linux
Uni-processor and Multi-processor
Environment
By Kathryn Bean and Wafa’ Jaffal (Group A3)
Topics
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Linux History
Kernel Control Path
Synchronization Technique
SMP Architecture
Hardware Support for Synchronization
(Pentium-based Architecture)
Linux/SMP kernel
Conclusion
Linux History
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University of Helsinki (1997), Master’s thesis –
“Linux, a Portable Operating System” by L.
Torvalds
OS for IBM-compatible personal computers
(Intel 80386 microprocessor).
Source code under GNU General Public
License
Kernel Control Path
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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:
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System calls
Exceptions
Interrupts
Synchronization Technique
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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
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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
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Two kinds of locking:
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Kernel semaphores, used by both uni-processor
and multiprocessor systems
Spin Locks, used by only multiprocessor systems
Kernel Semaphore Implementation
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Kernel semaphore – is object of type structure
semaphore, see include/asm/semaphore.h file
Fields
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count – integer number
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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
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wait – the address of a wait queue.
waking – integer.
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The releasing process increments waking field(s).
Each of awakened process PI then enters a critical region
of the down() function
Is PI’s waking <> 0, if waking > 0 – 1. acquire the resource
2. other PK’s waking-if waking < 0 – go back to sleep
Kernel Semaphore Implementation,
Continued
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Function
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down() – called, if process wishes to acquire a semaphore.
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up() – called, if process releases a semaphore
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count -count <> 0, if count  0 – process enter the critical section
if count < 0 – process is suspended
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
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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
CPU 1
CPU n
system bus
Graphical card
Memory
• CPUs share the same memory unit
• application processing and kernel processing are spread
amongst all CPUs.
Other Multiprocessor Architectures
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Asymmetric Multiprocessing
Master CPU executes the operating system code
and application programs run on the remaining
CPUs.
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Massively Parallel Processing (MPP)
Assemble hundreds or thousand of CPUs, each with
own system memory
Hardware Support for
Synchronization
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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.
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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
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SMP Atomic Operation
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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
CPU 1
CPU n
Local APIC
Local APIC
ICC bus
I/O APIC
APIC – Advanced Programmable Interrupt
Controller
ICC – Interrupt Controller Communication
IRQ lines
I/O APIC - router
Linux/SMP Kernel
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Process Descriptor Modification
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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
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Modern versions of Linux are available
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Compaq Alpha
SPARC
PowerPC
Motorola MC680x0
IBM System/390
Multiprocessor operating system
Supports up to 32 CPUs
Bibliography
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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. Eranian
IA-64 Linux Kernel
Prentice Hall PTR, 2002
Thank You
Any Questions?