Operating System Design Operating System Design LINUX KERNEL DESIGN (2.6/3.X) Dr. C.C. Lee Ref: Linux Kernel Development by R. Love Ref: Operating System Concepts by Silberschatz…
Dec 16, 2015
Operating System DesignOperating System Design
LINUX KERNEL DESIGN (2.6/3.X)
Dr. C.C. Lee
Ref: Linux Kernel Development by R. Love
Ref: Operating System Concepts by Silberschatz…
IntroductionIntroduction
Monolithic & dynamically loadable kernel module
SMP support (run queue per CPU, load balance)
Kernel preemptive, schedulable, thread support
CPU (soft & hard) affinity
Kernel memory not pageable
Source in GNU C (not ANSI C) with extension, in-line for efficiency,
Kernel source tree – architecture indep/dep. part
Portable to different architecture
CPU AffinityCPU Affinity
CPU affinity: less overhead, in cache
Soft affinity means that processes do not frequently migrate between processors.
Hard affinity means that processes run on processors you specify
Reason 1: You have a hunch – computations
Reason 2: Testing complex applications – linear scalability?
Reason 3: Running time-sensitive, deterministic processes
sched_setaffinity (…) set CPU affinity mask
Process (Task) BasicsProcess (Task) Basics
Process States
TASK_RUNNING (run or ready)
TASK_INTERRUPTIBLE (sleeping or blocked, may be waken by signal)
TASK_UNTERRUPTIBLE (sleeping/blocked, only event can wake this task)
TASK_STOPPED (SIGSTOP, SIGTTIN, SIGTTOU signals)
TASK_ZOMBIE (pending for parent task to issue wait)
Process (Task) Basics - ContinueProcess (Task) Basics - Continue
Context Process context – user code or kernel (from system calls)
Interrupt context – kernel interrupt handling
Task (Process) Creation Fork (may be implemented by: COW i.e.Copy On Write)
Vfork :same as fork (but shared page table, parent wait for child)
Clone system call is used to implement fork and vfork
Threads are created the same as normal tasks except that the clone system call is passed with spec. resources shared
Task (Process) Termination Memory/files/timers/semaphores released, notify parent
Process (Task) Process (Task) SchedulingScheduling
Preemptive Scheduler Classes (priority for classes)
Real-time: FIFO and RR (timeslice), fixed priority Normal (SCHED_NORMAL)
SMP (Run queue/structure per CPU, why?)
Processor Affinity (Soft & Hard)
Load balancing
Process (Task) Process (Task) Scheduling Cont.Scheduling Cont.
Two process-scheduling Classes: Normal time-sharing (dynamic)
(Nice value: 19 to -20, with default 0 = 120)
Real-time algorithm (FIFO/RR) - Soft
Absolute priorities (static): 0-99
FIFO run till Exit , Yield, or Block
RR run with time slice
Preemption possible with priority
Normal Processes: to be studied here
Early Kernel 2.6 - O(1) SchedulerEarly Kernel 2.6 - O(1) Scheduler
O(1) Scheduler (Early Kernel 2.6)
Improved scheduler with O(1) operations
using bit map operations to search highest
priority queue
Active and Expired Array (Run Queues per CPU)
Scalable
Heuristics for CPU/IO bound, Interactivities
21.9 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
O(1) Scheduler Priority ArrayO(1) Scheduler Priority Array
O(1) Scheduler SummaryO(1) Scheduler Summary
Implements a priority-based array of task entries that enables the highest-priority task to be found quickly (by using a priority bitmap with a fast instruction).
Recalculates the timeslice and priority of an expired task before it places it on the expired queue. When all the tasks expire, the scheduler simply needs to swap the active and expired queue pointers and schedule the next task. Long scans of runqueues are, thus, eliminated
This process takes the same amount of processing, irrespective of the number of tasks in the system. It no longer depends on the value of n, but is a fixed constant
O(1) Scheduler ProblemsO(1) Scheduler Problems
Although O(1) scheduler performed well and scaled effortlessly for large systems with many tens or hundreds of processors,
IT FAILS ON:
Slow response to latency-sensitive
applications i.e. interactive processes
for typical desktop systems
Not achieving Fair (Equal) CPU Allocation
Current: Completely Fair Scheduler Current: Completely Fair Scheduler (CFS) (CFS)
Since Kernel 2.6.23 CFS Aiming at
Giving each task a fair share (portion) of the processor time (Completely Fair)
Improving the interactive performance of O(1) scheduler for desktop. While O(1) scheduler is ideal for large server workloads
Introduces simple/efficient algorithmic approach (red-black tree) with O(log N). While O(1) scheduler uses heuristics and the code is large and lacks algorithm substance.
CFS – Processor Time AllocationCFS – Processor Time Allocation
Select next that has run the least. Rather than assign each process a time slice, CFS calculates how long a process should run as a function of the total number of runnable processes and its niceness (default: 1 ms as minimum granularity)
Nice values are used to weight the portion of processor a process is to receive (not by additive increases, but by geometric differences). Each process will run for a “timeslice” proportional to its weight divided by total weight of all runnable processes. Assume TARGETED_LANTENCY = 20ms:
Two threads: the niceness are 0(10), and 5(15),
CFS assigns relative weight 3 : 1 (approx.) – *particular algorithm
Niceness 0(10) receives 15ms and Niceness 5(15) receives 5ms
Here, CPU portion is determined only by the relative value.
CFS – The Virtual Runtime (vruntime) CFS – The Virtual Runtime (vruntime)
The virtual runtime (vruntime) is the actual runtime (the amount of time spent) weighted by its niceness
nice=0, factor=1; vruntime is same as real run time spent by task
nice<0, factor< 1; vruntime is less than real run time spent. vruntime
grows slower than real run time used.
nice>0, factor> 1; vruntime is more than real run time spent. vruntime grows faster than real run time used.
(The virtual runtime is measured in nano seconds)
Every time a thread runs for t ns, vruntime += t
(weighted by task niceness i.e. priority)
The virtual runtime (vruntime) is used to account for how long a process has run. CFS will then pick up the process with the smallest vruntime.
CFS – Process SelectionCFS – Process Selection
CFS select the process with the minimum virtual runtime i.e. vruntime
CFS use a red-black tree (rbtree – a type of self-balancing binary search tree) to manage the list of runnable processes and efficiently (algorithm) find the process with the smallest vruntime
The selected process with the smallest vruntime is the leftmost node in the (rbtree) tree.
CFS – Process just Created or AwakenCFS – Process just Created or Awaken
A new process is created
The new process is assigned the current Minimum
Virtual Runtime (adjusted) and inserted into the
rbtree
A process is awakened from blocking vruntime = Maximum (old vruntime, current Min_vruntime
substracted by adjusted TARGETED_LANTENCY)
This can prevent a process that blocked for a long
time from monopolizing the CPU
CFS – Group SchedulingCFS – Group Scheduling
In plain CFS, if there are 25 runnable processes,
CFS will allocate 4% to each (assume same). If
20 belong to user A, and 5 belong to user B, then
user B is at an inherent disadvantage.
Group scheduling will first try to be fair to the
group and then individual in the group, i.e. 50% to
user A and 50% to user B.
Thus for A, the allocated 50% of A will be divided
fairly among A’s 20 tasks. For B, the allocated
50% will be divided fairly among B’s 5 tasks.
CFS – Run Queue (Red-Black Tree)CFS – Run Queue (Red-Black Tree)
Tasks are maintained in a time-ordered (i.e. vruntime) red-black tree for each CPU
Red-Black Tree: Self-balancing binary search tree
Balancing is preserved by painting each node with one of two colors in a way to satisfy certain properties. When the tree is modified , the new tree is rearranged and repainted to restore the coloring properties.
The balancing of the tree can guarantee that no leaf can be more than twice as deep as others and the tree operations (searching/insertion/deletion/recoloring) can be performed in O(log N) time
CFS will switch to the leftmost task in the tree, that is, the one with the lowest virtual runtime (most need for CPU) to maintain fairness.
CFS – Red-Black TreeCFS – Red-Black Tree(www.ibm.com/developerworks/linux/library/l-completely-fair-scheduler/)(www.ibm.com/developerworks/linux/library/l-completely-fair-scheduler/)
Interrupt HandlingInterrupt Handling
Interrupts (Hardware) Asynchronous
Dev.->Interrupt Controller->CPU-->Interrupt Handlers
Device has unique value for each interrupt line: IRQ
(Interrupt ReQuest number)
On PC, IRQ 0 = timer interrupt, IRQ 1 is keyboard interrupt
Exceptions (Soft Interrupt) Synchronous
Fault (segment fault, page fault,…)
Trap (system call)
Programming exception
Top Halves and Bottom HalvesTop Halves and Bottom Halves
Top Half Interrupts disabled (Line, local)
Run (immediately)
ACK & reset hardware, copy data from hardware buffer
Bottom Half Interrupt enabled
Run (deferred)
Detailed work processing
Example of Network Card Top half: alert the kernel to optimize network throughput, copy
packets to memory, ACK network hardware and ready network card for more packets
The rest will be left to bottom half
Top-HalfTop-Half
Writing an Interrupt Handlers (for vectored interrupt table)
Registering an Interrupt Handler
int request_irq (irq#, *handler, irqflags, *devname, *dev_id)
When kernel receives interrupt
From interrupt table (IRQ number)
invokes sequentially each registered
handler on the line (till device is found)
Bottom Halves and Deferring WorkBottom Halves and Deferring Work
Softirqs – interrupt context routine(can not block)
Handling those with time-critical and high concurrency.
Handling routines run right after top-half that raised softirq.
Tasklets: Special softirqs, intended for those with
less time-critical/concurrency/locking requirements
It has simpler interface and implementation
Work Queues – A different form of deferring work Work queues run by kernel threads in process context –
thus schedulable. Therefore, If the deferred work needs to
sleep (allocate a lot of memory, obtain semaphores…),
work queues should be used. Otherwise, softirqs/tasklets
are used.
Bottom Halves - KsoftirqdBottom Halves - Ksoftirqd
When the system is overwhelmed with softirqs
activities, low-priority user processes can not
run and may become starved. Thus
A per-CPU kernel thread Ksoftirqd (run with the
lowest priority i.e. nice value=19) will be awakened.
With this low-level priority Ksoftirqd to
handle softirqs under the busy situation, user
processes can be relieved from starvation.
Which Bottom Half to Use Which Bottom Half to Use Bottom Half Context Inherent Serialization
Softirq Interrupt None
Tasklet Interrupt Against the same tasklet
Work Queues Process None
If the deferred work needs to run in process context: work queue The highest overhead: work queue (kernel thread, context switch) Ease of use: work queue The fastest, highly threaded, timing critical use: softirq Same as softirq, but simple interface and ease of use: tasklets
Normal driver writers have two choices:
Need a schedulable entity to perform the work (sleep for events?)
If so, work queue is the only choice. Otherwise, tasklets are preferred,
unless scalability is a concern which will use softirq (highly threaded)
Kernel SynchronizationKernel Synchronization
Kernel has concurrency (threads) and need synchronization
Code safe from concurrent access - Terminology
Interrupt safe (from interrupt handler)
SMP safe
Preempt safe (kernel preemption)
Spinlock, R/W spinlock, semaphore, R/W semaphore, sequential lock, completion variables
Spin LocksSpin Locks
Spin locks: Lightweight For short durations to save context switch overhead
Spin Locks and Top-Half Kernel must disable local interrupts before obtaining
the spin locks. Otherwise the Interrupt Handler (IH) may interrupt kernel and attempts to acquire the same lock while the lock is held by the kernel – spin?
Spin Locks and Bottom Halves Kernel must disable bottom-half before obtaining the
spin locks. Otherwise, the bottom-half may preempt kernel code and attempts to acquire this same lock while the lock is held by the kernel – spin?
Reader-Writer Spin LocksReader-Writer Spin Locks
Shared/Exclusive Locks
Reader and Writer Path
read_lock(&my_rwlock) write_lock(…)
CR CR
read_unlock(…) write_unlock(…)
Linux 2.6 favors readers over writers (starvation of writers) for Reader-Writer Spin Locks
SemaphoresSemaphores
Semaphores for long wait
Semaphores are for process context (can sleep)
Can not hold a spin lock while acquiring a semaphore (may sleep)
Kernel code holding semaphore can be interrupted or preempted
Using Semaphores: down, up
Reader-Writer SemaphoreReader-Writer Semaphore
Reader-Writer flavor of semaphores
Reader-Writer Semaphores are mutexes
Reader-Writer Semaphores : locks use uninterruptible sleep
As with semaphores, the following are provided:
down_read_trylock(), down_write_trylock()
down_read, down_write, up_read, up_write
Completion variablesCompletion variables
A task signals other task for an event
One task waits on the completion variable while other task performs work. When it completes, it uses a completion variable to wake up the other task
init_completion(struct completion *) or
DECLARE_COMPLETION (mr_comp)
wait_for_completion (struct completion *)
complete (struct completion *)
Sequential LocksSequential Locks
Simple mechanism for reading and writing shared data by maintaining a sequence counter
write lock obtained seq# incr; unlock -> seq# incr.
Prior to and after read: the sequence number is read
The sequence number must be even (prior read) and equal at end
Writer always succeed (if no other writers), Readers never block
Favors writers over readers
Readers does not affect writer’s locking
Seq locks provide very light weight and scalable lock for use with many readers and a few writers
Sequential Locks (Cont.)Sequential Locks (Cont.)
Example:
seqlock_t mr_seq_lock *s1
WRITE:
write_seq_lock (s1); {spin_lock(s1->lock); ++s1->sequence; SMP_wmb();}
/* Write Data */
write_sequnlock (s1); {SMP_wmb(); s1->sequence++; spin_unlock(s1-> lock);}
READ:
do {
seq = read_seqbegin (s1); {ret = s1->sequence; SMP_rmb(); return ret;}
/* read data */
} while (read_seqretry (s1, seq)); {SMP_rmb(); return (seq&1) | s1->sequence^seq) }
Pending writers continually cause read loop to repeat until writers are done.
Ordering and BarriersOrdering and Barriers
Both compiler and CPU can reorder reads/writes:
Compiler: optimization, CPU: performance i.e. pipeline
Instruct CPU not to reorder R/W
Barrier() call to instruct compiler not to reorder R/W
Memory Barrier and Compiler Barrier Methods
barrier() // compiler barrier - load/store
smp_rmb(), wmb(), mb()
Intel X86 processors: do not ever reorder writes
Memory ManagementMemory Management
Main Memory : Three (3) parts kernel memory (never paged out),
kernel memory for memory map (never paged out)
pageable page frames (user pages, paging cache, etc.)
Memory Map : mem_map Array of page descriptor for each page frame in system
with pointers to address space they belong to (if not free) or
with linked list for free frames
Memory ManagementMemory Management
Physical Memory For kernel (never paged out)
For memory map table (never paged out)
For page frame to virtual page mapping
For maintaining free page list
For pageable page frames
User pages and paging caches
Arbitrary size, contiguous kernel memory
Kmalloc(…)
Memory Allocation Mechanisms Memory Allocation Mechanisms
Page allocator - buddy algorithm (2**i split or combined) 65 page chunk->ask for 128 page chunk
Slab allocator: carves chunk (from buddy algorithm) into slabs - one or more physically contiguous pages
A cache (for each kernel data structure): one or more slabs and is populated with kernel objects (TCBs, semaphores)
Example: To allocate a new task_struct, Kernel looks in the object cache. Try: partially full slab?, empty slab?, then a new slab?
kmalloc(): Similar to user-space malloc. It returns a pointer to a region of (physically contiguous) memory that is at least requested ‘size’ bytes in length.
Vmalloc(): allocates chunk of physical memory (that may not be contiguous) and fix up the page tables to map the memory into a contiguous chunk of logical address space.
Virtual MemoryVirtual Memory
Virtual Address Space Homogeneous, contiguous, page-aligned areas (text, mapped files)
Page size: 4KB (Pentium), 8KB (Alpha) – Linux also support 4MB
Memory Descriptor A process address space is represented by mm_struct (pointed to by mm field of task_struct)
struct mm_struct {
struct vm_area_struct *mmap; // list of memory areas – text, data,…
pgd_t *pgd; // page global directory
atomic_t mm_users // addr. space users – 2 for 2 threads
atomic_t mm_count; // primary reference count
struct list_head mmlist; // list of all mm_struct
… // lock, semaphore…
…. // start/end addr. Of code, data, heap, stack
}
Virtual Memory - PagingVirtual Memory - Paging
Four-level paging (for 64 bit architectures)
global/upper/middle directory, and page table
Pentium using two-level paging (global directory points to page table)
Demand paging (no pre-paging)
With only user structure (PCB), and page tables need to be in memory
Page daemon (process 2): awaken (periodically or demand) – check ‘free’
Page ReplacementPage Replacement
Modified Version of LRU Scheme
One particular failure of the LRU strategy (besides its cost of implementation) is that many files are accessed once and then never again. Putting them at the top of the LRU list is thus not optimal.
In general, the kernel has no way of knowing that a file is going to be accessed only once.
However, it does know how many times it has been accessed in the past. This leads to a modified version of LRU i.e. Two-List Strategy as follows:
Page Replacement (Cont.)Page Replacement (Cont.)
Two-list strategy (modified version of LRU)
Active list (hot) and Inactive list (reclaim candidate)
Pages when first allocated are placed on inactive list
If referenced while on that list, it will be placed on active list
Both lists are maintained in a pseudo-LRU manner: items are added to the tail and remove from the head as a queue.
Lists balanced: if active list becomes larger, items will be moved from the active list back to the inactive list for potential eviction. The action starts from the head item:
The reference bit is checked. If it was set, it will be reset, the item
is moved back to the list, and the next page is checked. Otherwise
it will be moved to the inactive list (resembles a Clock algorithm)
Page Replacement (Cont.)Page Replacement (Cont.)
A Global Policy
All reclaimable pages are contained in just two lists and pages belonging to any process may be reclaimed, rather than just those belonging to a faulting process
The two-list strategy enables simpler, pseudo-LRU semantics to perform well
Solves the only-used-once failure in a classical LRU scheme
The FilesystemThe Filesystem
To the user, Linux’s file system appears as a hierarchical directory tree obeying UNIX semantics
Internally, the kernel hides implementation details and manages the multiple different file systems via an abstraction layer, that is, the virtual file system (VFS)
The Linux VFS is designed around object-oriented principles: Write -> sys_write() // VFS
Then --> filesystem’s write method --> physical media
VFS Objects Primary: superblock, inode(cached), dentry (cached), and file objects
An operation object is contained within each primary object: super_operations, inode_operation, dentry_operation, file_operations
Other VFS Objects: file_system_type, vfsmount, and three per-process structures such as file_struct, fs_struct and namespace structures
File System and Device DriversFile System and Device Drivers
User mode
Kernel mode
Libraries
User applications
File subsystem
Buffer/page cache
Hardware control
Block device driverCharacter device driver