[Xen-devel] [PATCHv1] arm: reduce power use by contented spin locks with WFE/SEV

[David Vrabel <david.vrabel@xxxxxxxxxx>]

Instead of cpu_relax() while spinning and observing the ticket head,
introduce spin_relax() which executes a WFE instruction.  After the
ticket head is changed call spin_signal() to execute an SVE
instruction to wake any spinners.

This should improve power consumption when locks are contented and
spinning.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
I've not tested this but it looks straight-forward...
---
 xen/common/spinlock.c          | 5 +++--
 xen/include/asm-arm/spinlock.h | 3 ++-
 xen/include/asm-x86/spinlock.h | 3 +++
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
index 29149d1..fc3e8e7 100644
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -141,7 +141,7 @@ void _spin_lock(spinlock_t *lock)
     while ( tickets.tail != observe_head(&lock->tickets) )
     {
         LOCK_PROFILE_BLOCK;
-        cpu_relax();
+        spin_relax();
     }
     LOCK_PROFILE_GOT;
     preempt_disable();
@@ -170,6 +170,7 @@ void _spin_unlock(spinlock_t *lock)
     preempt_enable();
     LOCK_PROFILE_REL;
     add_sized(&lock->tickets.head, 1);
+    spin_signal();
 }
 
 void _spin_unlock_irq(spinlock_t *lock)
@@ -228,7 +229,7 @@ void _spin_barrier(spinlock_t *lock)
     if ( sample.head != sample.tail )
     {
         while ( observe_head(&lock->tickets) == sample.head )
-            cpu_relax();
+            spin_relax();
 #ifdef LOCK_PROFILE
         if ( lock->profile )
         {
diff --git a/xen/include/asm-arm/spinlock.h b/xen/include/asm-arm/spinlock.h
index 81955d1..311764b 100644
--- a/xen/include/asm-arm/spinlock.h
+++ b/xen/include/asm-arm/spinlock.h
@@ -1,6 +1,7 @@
 #ifndef __ASM_SPINLOCK_H
 #define __ASM_SPINLOCK_H
 
-/* Nothing ARM specific. */
+#define spin_relax() asm volatile("wfe" ::: "memory");
+#define spin_signal() asm volatile("sev" ::: "memory");
 
 #endif /* __ASM_SPINLOCK_H */
diff --git a/xen/include/asm-x86/spinlock.h b/xen/include/asm-x86/spinlock.h
index 7d69e75..0c04a4e 100644
--- a/xen/include/asm-x86/spinlock.h
+++ b/xen/include/asm-x86/spinlock.h
@@ -4,4 +4,7 @@
 #define _raw_read_unlock(l) \
     asm volatile ( "lock; dec%z0 %0" : "+m" ((l)->lock) :: "memory" )
 
+#define spin_relax() cpu_relax()
+#define spin_signal()
+
 #endif /* __ASM_SPINLOCK_H */
-- 
2.1.4


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