[PATCH] mutex subsystem, documentation

Add mutex design related documentation.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@infradead.org>

1 files changed, 135 insertions, 0 deletions

diff --git a/Documentation/mutex-design.txt b/Documentation/mutex-design.txt
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+Generic Mutex Subsystem
+
+started by Ingo Molnar <mingo@redhat.com>
+
+  "Why on earth do we need a new mutex subsystem, and what's wrong
+   with semaphores?"
+
+firstly, there's nothing wrong with semaphores. But if the simpler
+mutex semantics are sufficient for your code, then there are a couple
+of advantages of mutexes:
+
+ - 'struct mutex' is smaller on most architectures: .e.g on x86,
+   'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
+   A smaller structure size means less RAM footprint, and better
+   CPU-cache utilization.
+
+ - tighter code. On x86 i get the following .text sizes when
+   switching all mutex-alike semaphores in the kernel to the mutex
+   subsystem:
+
+        text    data     bss     dec     hex filename
+     3280380  868188  396860 4545428  455b94 vmlinux-semaphore
+     3255329  865296  396732 4517357  44eded vmlinux-mutex
+
+   that's 25051 bytes of code saved, or a 0.76% win - off the hottest
+   codepaths of the kernel. (The .data savings are 2892 bytes, or 0.33%)
+   Smaller code means better icache footprint, which is one of the
+   major optimization goals in the Linux kernel currently.
+
+ - the mutex subsystem is slightly faster and has better scalability for
+   contended workloads. On an 8-way x86 system, running a mutex-based
+   kernel and testing creat+unlink+close (of separate, per-task files)
+   in /tmp with 16 parallel tasks, the average number of ops/sec is:
+
+    Semaphores:                        Mutexes:
+
+    $ ./test-mutex V 16 10             $ ./test-mutex V 16 10
+    8 CPUs, running 16 tasks.          8 CPUs, running 16 tasks.
+    checking VFS performance.          checking VFS performance.
+    avg loops/sec:      34713          avg loops/sec:      84153
+    CPU utilization:    63%            CPU utilization:    22%
+
+   i.e. in this workload, the mutex based kernel was 2.4 times faster
+   than the semaphore based kernel, _and_ it also had 2.8 times less CPU
+   utilization. (In terms of 'ops per CPU cycle', the semaphore kernel
+   performed 551 ops/sec per 1% of CPU time used, while the mutex kernel
+   performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times
+   more efficient.)
+
+   the scalability difference is visible even on a 2-way P4 HT box:
+
+    Semaphores:                        Mutexes:
+
+    $ ./test-mutex V 16 10             $ ./test-mutex V 16 10
+    4 CPUs, running 16 tasks.          8 CPUs, running 16 tasks.
+    checking VFS performance.          checking VFS performance.
+    avg loops/sec:      127659         avg loops/sec:      181082
+    CPU utilization:    100%           CPU utilization:    34%
+
+   (the straight performance advantage of mutexes is 41%, the per-cycle
+    efficiency of mutexes is 4.1 times better.)
+
+ - there are no fastpath tradeoffs, the mutex fastpath is just as tight
+   as the semaphore fastpath. On x86, the locking fastpath is 2
+   instructions:
+
+    c0377ccb <mutex_lock>:
+    c0377ccb:       f0 ff 08                lock decl (%eax)
+    c0377cce:       78 0e                   js     c0377cde <.text.lock.mutex>
+    c0377cd0:       c3                      ret
+
+   the unlocking fastpath is equally tight:
+
+    c0377cd1 <mutex_unlock>:
+    c0377cd1:       f0 ff 00                lock incl (%eax)
+    c0377cd4:       7e 0f                   jle    c0377ce5 <.text.lock.mutex+0x7>
+    c0377cd6:       c3                      ret
+
+ - 'struct mutex' semantics are well-defined and are enforced if
+   CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have
+   virtually no debugging code or instrumentation. The mutex subsystem
+   checks and enforces the following rules:
+
+   * - only one task can hold the mutex at a time
+   * - only the owner can unlock the mutex
+   * - multiple unlocks are not permitted
+   * - recursive locking is not permitted
+   * - a mutex object must be initialized via the API
+   * - a mutex object must not be initialized via memset or copying
+   * - task may not exit with mutex held
+   * - memory areas where held locks reside must not be freed
+   * - held mutexes must not be reinitialized
+   * - mutexes may not be used in irq contexts
+
+   furthermore, there are also convenience features in the debugging
+   code:
+
+   * - uses symbolic names of mutexes, whenever they are printed in debug output
+   * - point-of-acquire tracking, symbolic lookup of function names
+   * - list of all locks held in the system, printout of them
+   * - owner tracking
+   * - detects self-recursing locks and prints out all relevant info
+   * - detects multi-task circular deadlocks and prints out all affected
+   *   locks and tasks (and only those tasks)
+
+Disadvantages
+-------------
+
+The stricter mutex API means you cannot use mutexes the same way you
+can use semaphores: e.g. they cannot be used from an interrupt context,
+nor can they be unlocked from a different context that which acquired
+it. [ I'm not aware of any other (e.g. performance) disadvantages from
+using mutexes at the moment, please let me know if you find any. ]
+
+Implementation of mutexes
+-------------------------
+
+'struct mutex' is the new mutex type, defined in include/linux/mutex.h
+and implemented in kernel/mutex.c. It is a counter-based mutex with a
+spinlock and a wait-list. The counter has 3 states: 1 for "unlocked",
+0 for "locked" and negative numbers (usually -1) for "locked, potential
+waiters queued".
+
+the APIs of 'struct mutex' have been streamlined:
+
+ DEFINE_MUTEX(name);
+
+ mutex_init(mutex);
+
+ void mutex_lock(struct mutex *lock);
+ int  mutex_lock_interruptible(struct mutex *lock);
+ int  mutex_trylock(struct mutex *lock);
+ void mutex_unlock(struct mutex *lock);
+ int  mutex_is_locked(struct mutex *lock);
+