[PATCH next v2 2/5] locking/osq_lock: Optimise the vcpu_is_preempted() check.

David Laight
2023-12-31T21:53:17Z

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@aculab.com>
---
kernel/locking/osq_lock.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index e0bc74d85a76..eb8a6dfdb79d 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 *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; /* encoded CPU # + 1 value */
};

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;
@@ -110,9 +106,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
if (old == OSQ_UNLOCKED_VAL)
return true;

- node->locked = 0;
+ node->prev_cpu = old;
prev = decode_cpu(old);
node->prev = prev;
+ node->locked = 0;

/*
* osq_lock() unqueue
@@ -144,7 +141,7 @@ 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(READ_ONCE(node->prev_cpu) - 1)))
return true;

/* unqueue */
@@ -201,6 +198,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* it will wait in Step-A.
*/

+ WRITE_ONCE(next->prev_cpu, prev->cpu);
WRITE_ONCE(next->prev, prev);
WRITE_ONCE(prev->next, next);

--
2.17.1

-
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