[Xen-devel] [PATCH 2/2] locking/mutex, rwsem: Reduce vcpu_is_preempted() calling frequency

[Waiman Long <longman@xxxxxxxxxx>]

As the vcpu_is_preempted() call is pretty costly compared with other
checks within mutex_spin_on_owner() and rwsem_spin_on_owner(), they
are done at a reduce frequency of once every 256 iterations.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/locking/mutex.c      | 5 ++++-
 kernel/locking/rwsem-xadd.c | 6 ++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index ad2d9e2..2ece0c4 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -423,6 +423,7 @@ bool mutex_spin_on_owner(struct mutex *lock, struct 
task_struct *owner,
                         struct ww_acquire_ctx *ww_ctx, struct mutex_waiter 
*waiter)
 {
        bool ret = true;
+       int loop = 0;
 
        rcu_read_lock();
        while (__mutex_owner(lock) == owner) {
@@ -436,9 +437,11 @@ bool mutex_spin_on_owner(struct mutex *lock, struct 
task_struct *owner,
 
                /*
                 * Use vcpu_is_preempted to detect lock holder preemption issue.
+                * As vcpu_is_preempted is more costly to use, it is called at
+                * a reduced frequencey (once every 256 iterations).
                 */
                if (!owner->on_cpu || need_resched() ||
-                               vcpu_is_preempted(task_cpu(owner))) {
+                  (!(++loop & 0xff) && vcpu_is_preempted(task_cpu(owner)))) {
                        ret = false;
                        break;
                }
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2ad8d8d..7a884a6 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -351,6 +351,7 @@ static inline bool rwsem_can_spin_on_owner(struct 
rw_semaphore *sem)
 static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
 {
        struct task_struct *owner = READ_ONCE(sem->owner);
+       int loop = 0;
 
        if (!rwsem_owner_is_writer(owner))
                goto out;
@@ -367,10 +368,11 @@ static noinline bool rwsem_spin_on_owner(struct 
rw_semaphore *sem)
 
                /*
                 * abort spinning when need_resched or owner is not running or
-                * owner's cpu is preempted.
+                * owner's cpu is preempted. The preemption check is done at
+                * a lower frequencey because of its high cost.
                 */
                if (!owner->on_cpu || need_resched() ||
-                               vcpu_is_preempted(task_cpu(owner))) {
+                  (!(++loop & 0xff) && vcpu_is_preempted(task_cpu(owner)))) {
                        rcu_read_unlock();
                        return false;
                }
-- 
1.8.3.1


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