summaryrefslogtreecommitdiffstats
path: root/kernel/locking/rwsem.h
diff options
context:
space:
mode:
authorWaiman Long <Waiman.Long@hpe.com>2016-05-17 21:26:19 -0400
committerIngo Molnar <mingo@kernel.org>2016-06-08 15:16:59 +0200
commit19c5d690e41697fcdd19379ab9d10d8d37818414 (patch)
tree6212929f726d890a738b80932903f690c6faad4b /kernel/locking/rwsem.h
parentd157bd860f1c828593730dca594d0ce51956833b (diff)
downloadlinux-stable-19c5d690e41697fcdd19379ab9d10d8d37818414.tar.gz
linux-stable-19c5d690e41697fcdd19379ab9d10d8d37818414.tar.bz2
linux-stable-19c5d690e41697fcdd19379ab9d10d8d37818414.zip
locking/rwsem: Add reader-owned state to the owner field
Currently, it is not possible to determine for sure if a reader owns a rwsem by looking at the content of the rwsem data structure. This patch adds a new state RWSEM_READER_OWNED to the owner field to indicate that readers currently own the lock. This enables us to address the following 2 issues in the rwsem optimistic spinning code: 1) rwsem_can_spin_on_owner() will disallow optimistic spinning if the owner field is NULL which can mean either the readers own the lock or the owning writer hasn't set the owner field yet. In the latter case, we miss the chance to do optimistic spinning. 2) While a writer is waiting in the OSQ and a reader takes the lock, the writer will continue to spin when out of the OSQ in the main rwsem_optimistic_spin() loop as the owner field is NULL wasting CPU cycles if some of readers are sleeping. Adding the new state will allow optimistic spinning to go forward as long as the owner field is not RWSEM_READER_OWNED and the owner is running, if set, but stop immediately when that state has been reached. On a 4-socket Haswell machine running on a 4.6-rc1 based kernel, the fio test with multithreaded randrw and randwrite tests on the same file on a XFS partition on top of a NVDIMM were run, the aggregated bandwidths before and after the patch were as follows: Test BW before patch BW after patch % change ---- --------------- -------------- -------- randrw 988 MB/s 1192 MB/s +21% randwrite 1513 MB/s 1623 MB/s +7.3% The perf profile of the rwsem_down_write_failed() function in randrw before and after the patch were: 19.95% 5.88% fio [kernel.vmlinux] [k] rwsem_down_write_failed 14.20% 1.52% fio [kernel.vmlinux] [k] rwsem_down_write_failed The actual CPU cycles spend in rwsem_down_write_failed() dropped from 5.88% to 1.52% after the patch. The xfstests was also run and no regression was observed. Signed-off-by: Waiman Long <Waiman.Long@hpe.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Jason Low <jason.low2@hp.com> Acked-by: Davidlohr Bueso <dave@stgolabs.net> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Dave Chinner <david@fromorbit.com> Cc: Douglas Hatch <doug.hatch@hpe.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Scott J Norton <scott.norton@hpe.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1463534783-38814-2-git-send-email-Waiman.Long@hpe.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking/rwsem.h')
-rw-r--r--kernel/locking/rwsem.h41
1 files changed, 41 insertions, 0 deletions
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 870ed9a5b426..8f43ba234787 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -1,3 +1,20 @@
+/*
+ * The owner field of the rw_semaphore structure will be set to
+ * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
+ * the owner field when it unlocks. A reader, on the other hand, will
+ * not touch the owner field when it unlocks.
+ *
+ * In essence, the owner field now has the following 3 states:
+ * 1) 0
+ * - lock is free or the owner hasn't set the field yet
+ * 2) RWSEM_READER_OWNED
+ * - lock is currently or previously owned by readers (lock is free
+ * or not set by owner yet)
+ * 3) Other non-zero value
+ * - a writer owns the lock
+ */
+#define RWSEM_READER_OWNED ((struct task_struct *)1UL)
+
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
@@ -9,6 +26,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
sem->owner = NULL;
}
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+ /*
+ * We check the owner value first to make sure that we will only
+ * do a write to the rwsem cacheline when it is really necessary
+ * to minimize cacheline contention.
+ */
+ if (sem->owner != RWSEM_READER_OWNED)
+ sem->owner = RWSEM_READER_OWNED;
+}
+
+static inline bool rwsem_owner_is_writer(struct task_struct *owner)
+{
+ return owner && owner != RWSEM_READER_OWNED;
+}
+
+static inline bool rwsem_owner_is_reader(struct task_struct *owner)
+{
+ return owner == RWSEM_READER_OWNED;
+}
#else
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
@@ -17,4 +54,8 @@ static inline void rwsem_set_owner(struct rw_semaphore *sem)
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
}
+
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+}
#endif