From 90631822c5d307b5410500806e8ac3e63928aa3e Mon Sep 17 00:00:00 2001 From: Jason Low Date: Mon, 14 Jul 2014 10:27:49 -0700 Subject: locking/spinlocks/mcs: Convert osq lock to atomic_t to reduce overhead The cancellable MCS spinlock is currently used to queue threads that are doing optimistic spinning. It uses per-cpu nodes, where a thread obtaining the lock would access and queue the local node corresponding to the CPU that it's running on. Currently, the cancellable MCS lock is implemented by using pointers to these nodes. In this patch, instead of operating on pointers to the per-cpu nodes, we store the CPU numbers in which the per-cpu nodes correspond to in atomic_t. A similar concept is used with the qspinlock. By operating on the CPU # of the nodes using atomic_t instead of pointers to those nodes, this can reduce the overhead of the cancellable MCS spinlock by 32 bits (on 64 bit systems). Signed-off-by: Jason Low Signed-off-by: Peter Zijlstra Cc: Scott Norton Cc: "Paul E. McKenney" Cc: Dave Chinner Cc: Waiman Long Cc: Davidlohr Bueso Cc: Rik van Riel Cc: Andrew Morton Cc: "H. Peter Anvin" Cc: Steven Rostedt Cc: Tim Chen Cc: Konrad Rzeszutek Wilk Cc: Aswin Chandramouleeswaran Cc: Linus Torvalds Cc: Chris Mason Cc: Heiko Carstens Cc: Josef Bacik Link: http://lkml.kernel.org/r/1405358872-3732-3-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar --- include/linux/osq_lock.h | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) create mode 100644 include/linux/osq_lock.h (limited to 'include/linux/osq_lock.h') diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h new file mode 100644 index 000000000000..b001682bf7cb --- /dev/null +++ b/include/linux/osq_lock.h @@ -0,0 +1,19 @@ +#ifndef __LINUX_OSQ_LOCK_H +#define __LINUX_OSQ_LOCK_H + +/* + * An MCS like lock especially tailored for optimistic spinning for sleeping + * lock implementations (mutex, rwsem, etc). + */ + +#define OSQ_UNLOCKED_VAL (0) + +struct optimistic_spin_queue { + /* + * Stores an encoded value of the CPU # of the tail node in the queue. + * If the queue is empty, then it's set to OSQ_UNLOCKED_VAL. + */ + atomic_t tail; +}; + +#endif -- cgit v1.2.3