Kernel-3.10.0-957.el7_mutex-design

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 :
  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 :
  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 hardware or software interrupt
  • contexts such as tasklets and timers

  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);
void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
int mutex_lock_interruptible_nested(struct mutex *lock,
unsigned int subclass);
int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);