1Generic Mutex Subsystem
2
3started by Ingo Molnar <mingo@redhat.com>
4
5  "Why on earth do we need a new mutex subsystem, and what's wrong
6   with semaphores?"
7
8firstly, there's nothing wrong with semaphores. But if the simpler
9mutex semantics are sufficient for your code, then there are a couple
10of advantages of mutexes:
11
12 - 'struct mutex' is smaller on most architectures: .e.g on x86,
13   'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
14   A smaller structure size means less RAM footprint, and better
15   CPU-cache utilization.
16
17 - tighter code. On x86 i get the following .text sizes when
18   switching all mutex-alike semaphores in the kernel to the mutex
19   subsystem:
20
21        text    data     bss     dec     hex filename
22     3280380  868188  396860 4545428  455b94 vmlinux-semaphore
23     3255329  865296  396732 4517357  44eded vmlinux-mutex
24
25   that's 25051 bytes of code saved, or a 0.76% win - off the hottest
26   codepaths of the kernel. (The .data savings are 2892 bytes, or 0.33%)
27   Smaller code means better icache footprint, which is one of the
28   major optimization goals in the Linux kernel currently.
29
30 - the mutex subsystem is slightly faster and has better scalability for
31   contended workloads. On an 8-way x86 system, running a mutex-based
32   kernel and testing creat+unlink+close (of separate, per-task files)
33   in /tmp with 16 parallel tasks, the average number of ops/sec is:
34
35    Semaphores:                        Mutexes:
36
37    $ ./test-mutex V 16 10             $ ./test-mutex V 16 10
38    8 CPUs, running 16 tasks.          8 CPUs, running 16 tasks.
39    checking VFS performance.          checking VFS performance.
40    avg loops/sec:      34713          avg loops/sec:      84153
41    CPU utilization:    63%            CPU utilization:    22%
42
43   i.e. in this workload, the mutex based kernel was 2.4 times faster
44   than the semaphore based kernel, _and_ it also had 2.8 times less CPU
45   utilization. (In terms of 'ops per CPU cycle', the semaphore kernel
46   performed 551 ops/sec per 1% of CPU time used, while the mutex kernel
47   performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times
48   more efficient.)
49
50   the scalability difference is visible even on a 2-way P4 HT box:
51
52    Semaphores:                        Mutexes:
53
54    $ ./test-mutex V 16 10             $ ./test-mutex V 16 10
55    4 CPUs, running 16 tasks.          8 CPUs, running 16 tasks.
56    checking VFS performance.          checking VFS performance.
57    avg loops/sec:      127659         avg loops/sec:      181082
58    CPU utilization:    100%           CPU utilization:    34%
59
60   (the straight performance advantage of mutexes is 41%, the per-cycle
61    efficiency of mutexes is 4.1 times better.)
62
63 - there are no fastpath tradeoffs, the mutex fastpath is just as tight
64   as the semaphore fastpath. On x86, the locking fastpath is 2
65   instructions:
66
67    c0377ccb <mutex_lock>:
68    c0377ccb:       f0 ff 08                lock decl (%eax)
69    c0377cce:       78 0e                   js     c0377cde <.text.lock.mutex>
70    c0377cd0:       c3                      ret
71
72   the unlocking fastpath is equally tight:
73
74    c0377cd1 <mutex_unlock>:
75    c0377cd1:       f0 ff 00                lock incl (%eax)
76    c0377cd4:       7e 0f                   jle    c0377ce5 <.text.lock.mutex+0x7>
77    c0377cd6:       c3                      ret
78
79 - 'struct mutex' semantics are well-defined and are enforced if
80   CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have
81   virtually no debugging code or instrumentation. The mutex subsystem
82   checks and enforces the following rules:
83
84   * - only one task can hold the mutex at a time
85   * - only the owner can unlock the mutex
86   * - multiple unlocks are not permitted
87   * - recursive locking is not permitted
88   * - a mutex object must be initialized via the API
89   * - a mutex object must not be initialized via memset or copying
90   * - task may not exit with mutex held
91   * - memory areas where held locks reside must not be freed
92   * - held mutexes must not be reinitialized
93   * - mutexes may not be used in irq contexts
94
95   furthermore, there are also convenience features in the debugging
96   code:
97
98   * - uses symbolic names of mutexes, whenever they are printed in debug output
99   * - point-of-acquire tracking, symbolic lookup of function names
100   * - list of all locks held in the system, printout of them
101   * - owner tracking
102   * - detects self-recursing locks and prints out all relevant info
103   * - detects multi-task circular deadlocks and prints out all affected
104   *   locks and tasks (and only those tasks)
105
106Disadvantages
107-------------
108
109The stricter mutex API means you cannot use mutexes the same way you
110can use semaphores: e.g. they cannot be used from an interrupt context,
111nor can they be unlocked from a different context that which acquired
112it. [ I'm not aware of any other (e.g. performance) disadvantages from
113using mutexes at the moment, please let me know if you find any. ]
114
115Implementation of mutexes
116-------------------------
117
118'struct mutex' is the new mutex type, defined in include/linux/mutex.h
119and implemented in kernel/mutex.c. It is a counter-based mutex with a
120spinlock and a wait-list. The counter has 3 states: 1 for "unlocked",
1210 for "locked" and negative numbers (usually -1) for "locked, potential
122waiters queued".
123
124the APIs of 'struct mutex' have been streamlined:
125
126 DEFINE_MUTEX(name);
127
128 mutex_init(mutex);
129
130 void mutex_lock(struct mutex *lock);
131 int  mutex_lock_interruptible(struct mutex *lock);
132 int  mutex_trylock(struct mutex *lock);
133 void mutex_unlock(struct mutex *lock);
134 int  mutex_is_locked(struct mutex *lock);
135