Re: [PATCH next 5/5] locking/osq_lock: Optimise vcpu_is_preempted() check.

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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






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