On 12/29/23 22:13, Waiman Long wrote:
On 12/29/23 15:58, David Laight wrote:
The vcpu_is_preempted() test stops osq_lock() spinning if a virtual
cpu is no longer running.
Although patched out for bare-metal the code still needs the cpu number.
Reading this from 'prev->cpu' is a pretty much guaranteed have a
cache miss
when osq_unlock() is waking up the next cpu.
Instead save 'prev->cpu' in 'node->prev_cpu' and use that value instead.
Update in the osq_lock() 'unqueue' path when 'node->prev' is changed.
This is simpler than checking for 'node->prev' changing and caching
'prev->cpu'.
Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
---
kernel/locking/osq_lock.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index b60b0add0161..89be63627434 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -14,8 +14,9 @@
struct optimistic_spin_node {
struct optimistic_spin_node *self, *next, *prev;
- int locked; /* 1 if lock acquired */
- int cpu; /* encoded CPU # + 1 value */
+ int locked; /* 1 if lock acquired */
+ int cpu; /* encoded CPU # + 1 value */
+ int prev_cpu; /* actual CPU # for vpcu_is_preempted() */
};
static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node,
osq_node);
@@ -29,11 +30,6 @@ static inline int encode_cpu(int cpu_nr)
return cpu_nr + 1;
}
-static inline int node_cpu(struct optimistic_spin_node *node)
-{
- return node->cpu - 1;
-}
-
static inline struct optimistic_spin_node *decode_cpu(int
encoded_cpu_val)
{
int cpu_nr = encoded_cpu_val - 1;
@@ -114,6 +110,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
if (old == OSQ_UNLOCKED_VAL)
return true;
+ node->prev_cpu = old - 1;
prev = decode_cpu(old);
node->prev = prev;
node->locked = 0;
@@ -148,7 +145,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* polling, be careful.
*/
if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
- vcpu_is_preempted(node_cpu(node->prev))))
+ vcpu_is_preempted(node->prev_cpu)))
On second thought, I believe it is more correct to use READ_ONCE() to
access "node->prev_cpu" as this field is subjected to change by a
WRITE_ONCE().
Cheers,
Longman
