Re: [PATCH v4 next 2/9] locking/osq_lock: Save the cpu number for 'prev' not the node address

[Peter Zijlstra]

On Mon, Sep 07, 2026 at 09:41:26AM +0100, David Laight wrote:
> The cpu number of node->prev is needed for both the vcpu_is_preempted()
> test and to update lock->tail.
> This saves reading the cache line for the other cpu's per-cpu data.
> 
> The cpu member of optimistic_spin_node is no longer needed.
> 
> Merges patches 2 and 3 from v3.
> 
> Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>
> ---
>  kernel/locking/osq_lock.c | 33 ++++++++++++++-------------------
>  1 file changed, 14 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index b17aa704c449..01988d00c480 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -34,9 +34,9 @@
>   */
>  
>  struct optimistic_spin_node {
> -	struct optimistic_spin_node *next, *prev;
> +	struct optimistic_spin_node *next;
>  	int locked; /* 1 if lock acquired */
> -	int cpu; /* encoded CPU # + 1 value */
> +	int prev; /* CPU number offset by 1 */
>  };
>  
>  static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);

> @@ -114,13 +109,12 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>  bool osq_lock(struct optimistic_spin_queue *lock)
>  {
>  	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> -	struct optimistic_spin_node *prev, *next;
> +	struct optimistic_spin_node *prev_ptr, *next;
>  	int curr = encode_cpu(smp_processor_id());
> -	int old;
> +	int prev;

I'm not a fan in the asymmetry of the naming, that is very confusing at
best.