ipc/sem: add hysteresis

sysv sem has two lock modes: One with per-semaphore locks, one lock mode
with a single global lock for the whole array.
When switching from the per-semaphore locks to the global lock, all
per-semaphore locks must be scanned for ongoing operations.

The patch adds a hysteresis for switching from the global lock to the
per semaphore locks. This reduces how often the per-semaphore locks
must be scanned.

Compared to the initial patch, this is a simplified solution:
Setting USE_GLOBAL_LOCK_HYSTERESIS to 1 restores the current behavior.

In theory, a workload with exactly 10 simple sops and then
one complex op now scales a bit worse, but this is pure theory:
If there is concurrency, the it won't be exactly 10:1:10:1:10:1:...
If there is no concurrency, then there is no need for scalability.

Link: http://lkml.kernel.org/r/1476851896-3590-3-git-send-email-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: <1vier1@web.de>
Cc: kernel test robot <xiaolong.ye@intel.com>
Cc: <felixh@informatik.uni-bremen.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions