Re: [PATCH v3 next 4/5] Optimise decode_cpu() and per_cpu_ptr()

[David Laight]

On Fri,  6 Mar 2026 22:51:49 +0000
david.laight.linux@xxxxxxxxx wrote:

> From: David Laight <david.laight.linux@xxxxxxxxx>
> 
> Changing the 'cpu number' variables to 'unsigned int' generates
> slightly better code (and the values can never be negative).
> 
> More specifically gcc knows that decrementing the 'encoded' value
> zeros the high 32bits (on sane 64bit architectures) so that it doesn't
> need to zero/sign extend the value to index __per_cpu_offset[].
> 
> Not massive but saves two instructions.
> 
> Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>
> ---
> 
> Proposed by Linus.
> Part of a discussion from v1 about whether removing the offset would help.
> 
>  kernel/locking/osq_lock.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 5dd7e08d4fda..0619691e2756 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -15,7 +15,7 @@
>  struct optimistic_spin_node {
>  	struct optimistic_spin_node *next;
>  	int locked; /* 1 if lock acquired */
> -	int prev_cpu; /* encoded CPU # + 1 value */
> +	unsigned int prev_cpu; /* encoded CPU # + 1 value */
>  };
>  
>  static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
> @@ -24,19 +24,19 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
>   * We use the value 0 to represent "no CPU", thus the encoded value
>   * will be the CPU number incremented by 1.
>   */
> -static inline int encode_cpu(int cpu_nr)
> +static inline unsigned int encode_cpu(unsigned int cpu_nr)
>  {
>  	return cpu_nr + 1;
>  }
>  
> -static inline int prev_cpu_nr(struct optimistic_spin_node *node)
> +static inline unsigned int prev_cpu_nr(struct optimistic_spin_node *node)
>  {
>  	return READ_ONCE(node->prev_cpu) - 1;
>  }
>  
> -static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
> +static inline struct optimistic_spin_node *decode_cpu(unsigned int encoded_cpu_val)
>  {
> -	int cpu_nr = encoded_cpu_val - 1;
> +	unsigned int cpu_nr = encoded_cpu_val - 1;
>  
>  	return per_cpu_ptr(&osq_node, cpu_nr);
>  }
> @@ -53,9 +53,9 @@ static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
>  static inline struct optimistic_spin_node *
>  osq_wait_next(struct optimistic_spin_queue *lock,
>  	      struct optimistic_spin_node *node,
> -	      int old_cpu)
> +	      unsigned int old_cpu)
>  {
> -	int curr = encode_cpu(smp_processor_id());
> +	unsigned int curr = encode_cpu(smp_processor_id());
>  
>  	for (;;) {
>  		if (atomic_read(&lock->tail) == curr &&
> @@ -94,8 +94,8 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  {
>  	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
>  	struct optimistic_spin_node *prev, *next;
> -	int curr = encode_cpu(smp_processor_id());
> -	int prev_cpu;
> +	unsigned int curr = encode_cpu(smp_processor_id());
> +	unsigned int prev_cpu;
>  
>  	node->next = NULL;
>